diff --git a/docs/release-notes/.FSharp.Compiler.Service/9.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/9.0.100.md
index 91b91f3c3eb..4fdf50a224a 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/9.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/9.0.100.md
@@ -52,7 +52,7 @@
 * Better CE error reporting when using `use!` with `and!` ([PR #17671](https://github.com/dotnet/fsharp/pull/17671))
 * Better error reporting for let bindings. ([PR #17601](https://github.com/dotnet/fsharp/pull/17601))
 * Optimize ILTypeDef interface impls reading from metadata. ([PR #17382](https://github.com/dotnet/fsharp/pull/17382))
+* Make ILTypeDef interface impls calculation lazy. ([PR #17392](https://github.com/dotnet/fsharp/pull/17392))
 * Better error reporting for active patterns. ([PR #17666](https://github.com/dotnet/fsharp/pull/17666))
 
-
 ### Breaking Changes
diff --git a/src/Compiler/AbstractIL/il.fs b/src/Compiler/AbstractIL/il.fs
index 9ed1822676b..b10a119fced 100644
--- a/src/Compiler/AbstractIL/il.fs
+++ b/src/Compiler/AbstractIL/il.fs
@@ -1909,6 +1909,23 @@ let inline conditionalAdd condition flagToAdd source =
 
 let NoMetadataIdx = -1
 
+type InterfaceImpl =
+    { Idx: int; Type: ILType; mutable CustomAttrsStored: ILAttributesStored }
+
+    member x.CustomAttrs =
+        match x.CustomAttrsStored with
+        | ILAttributesStored.Reader f ->
+            let res = ILAttributes(f x.Idx)
+            x.CustomAttrsStored <- ILAttributesStored.Given res
+            res
+        | ILAttributesStored.Given attrs -> attrs
+        
+    static member Create(ilType: ILType, customAttrsStored: ILAttributesStored) =
+        { Idx = NoMetadataIdx; Type = ilType; CustomAttrsStored = customAttrsStored }
+
+    static member Create(ilType: ILType) = InterfaceImpl.Create(ilType, emptyILCustomAttrsStored)
+
+
 [<NoComparison; NoEquality; StructuredFormatDisplay("{DebugText}")>]
 type ILMethodDef
     (
@@ -2635,8 +2652,7 @@ type ILTypeDef
         name: string,
         attributes: TypeAttributes,
         layout: ILTypeDefLayout,
-        implements: ILTypes,
-        implementsCustomAttrs: (ILAttributesStored * int) list option,
+        implements: InterruptibleLazy<InterfaceImpl list>,
         genericParams: ILGenericParameterDefs,
         extends: ILType option,
         methods: ILMethodDefs,
@@ -2659,7 +2675,6 @@ type ILTypeDef
         attributes,
         layout,
         implements,
-        implementsCustomAttrs,
         genericParams,
         extends,
         methods,
@@ -2676,7 +2691,6 @@ type ILTypeDef
             attributes,
             layout,
             implements,
-            implementsCustomAttrs,
             genericParams,
             extends,
             methods,
@@ -2703,8 +2717,6 @@ type ILTypeDef
 
     member _.Implements = implements
 
-    member _.ImplementsCustomAttrs = implementsCustomAttrs
-
     member _.Extends = extends
 
     member _.Methods = methods
@@ -2744,8 +2756,7 @@ type ILTypeDef
             ?properties,
             ?newAdditionalFlags,
             ?customAttrs,
-            ?securityDecls,
-            ?implementsCustomAttrs
+            ?securityDecls
         ) =
         ILTypeDef(
             name = defaultArg name x.Name,
@@ -2754,7 +2765,6 @@ type ILTypeDef
             genericParams = defaultArg genericParams x.GenericParams,
             nestedTypes = defaultArg nestedTypes x.NestedTypes,
             implements = defaultArg implements x.Implements,
-            implementsCustomAttrs = defaultArg implementsCustomAttrs x.ImplementsCustomAttrs,
             extends = defaultArg extends x.Extends,
             methods = defaultArg methods x.Methods,
             securityDecls = defaultArg securityDecls x.SecurityDecls,
@@ -3333,6 +3343,8 @@ let mkILTypeDefs l = mkILTypeDefsFromArray (Array.ofList l)
 let mkILTypeDefsComputed f = ILTypeDefs f
 let emptyILTypeDefs = mkILTypeDefsFromArray [||]
 
+let emptyILInterfaceImpls = InterruptibleLazy<InterfaceImpl list>.FromValue([])
+
 // --------------------------------------------------------------------
 // Operations on method tables.
 // --------------------------------------------------------------------
@@ -4240,7 +4252,7 @@ let mkILSimpleStorageCtor (baseTySpec, ty, extraParams, flds, access, tag, impor
 let mkILStorageCtor (preblock, ty, flds, access, tag, imports) =
     mkILStorageCtorWithParamNames (preblock, ty, [], addParamNames flds, access, tag, imports)
 
-let mkILGenericClass (nm, access, genparams, extends, impl, methods, fields, nestedTypes, props, events, attrs, init) =
+let mkILGenericClass (nm, access, genparams, extends, impls, methods, fields, nestedTypes, props, events, attrs, init) =
     let attributes =
         convertTypeAccessFlags access
         ||| TypeAttributes.AutoLayout
@@ -4254,8 +4266,7 @@ let mkILGenericClass (nm, access, genparams, extends, impl, methods, fields, nes
         name = nm,
         attributes = attributes,
         genericParams = genparams,
-        implements = impl,
-        implementsCustomAttrs = None,
+        implements = InterruptibleLazy.FromValue(impls),
         layout = ILTypeDefLayout.Auto,
         extends = Some extends,
         methods = methods,
@@ -4279,8 +4290,7 @@ let mkRawDataValueTypeDef (iltyp_ValueType: ILType) (nm, size, pack) =
              ||| TypeAttributes.ExplicitLayout
              ||| TypeAttributes.BeforeFieldInit
              ||| TypeAttributes.AnsiClass),
-        implements = [],
-        implementsCustomAttrs = None,
+        implements = emptyILInterfaceImpls,
         extends = Some iltyp_ValueType,
         layout = ILTypeDefLayout.Explicit { Size = Some size; Pack = Some pack },
         methods = emptyILMethods,
@@ -5586,7 +5596,7 @@ and refsOfILMethodImpl s m =
 and refsOfILTypeDef s (td: ILTypeDef) =
     refsOfILTypeDefs s td.NestedTypes
     refsOfILGenericParams s td.GenericParams
-    refsOfILTypes s td.Implements
+    refsOfILTypes s (td.Implements.Value |> List.map _.Type)
     Option.iter (refsOfILType s) td.Extends
     refsOfILMethodDefs s td.Methods
     refsOfILFieldDefs s (td.Fields.AsList())
diff --git a/src/Compiler/AbstractIL/il.fsi b/src/Compiler/AbstractIL/il.fsi
index e3ec95a40d7..1487830e8f0 100644
--- a/src/Compiler/AbstractIL/il.fsi
+++ b/src/Compiler/AbstractIL/il.fsi
@@ -327,6 +327,15 @@ type ILCallingSignature =
       ArgTypes: ILTypes
       ReturnType: ILType }
 
+type InterfaceImpl =
+    { Idx: int
+      Type: ILType
+      mutable CustomAttrsStored: ILAttributesStored }
+
+    member CustomAttrs: ILAttributes
+    static member Create: ilType: ILType * customAttrsStored: ILAttributesStored -> InterfaceImpl
+    static member Create: ilType: ILType -> InterfaceImpl
+
 /// Actual generic parameters are  always types.
 type ILGenericArgs = ILType list
 
@@ -1518,8 +1527,7 @@ type ILTypeDef =
         name: string *
         attributes: TypeAttributes *
         layout: ILTypeDefLayout *
-        implements: ILTypes *
-        implementsCustomAttrs: (ILAttributesStored * int) list option *
+        implements: InterruptibleLazy<InterfaceImpl list> *
         genericParams: ILGenericParameterDefs *
         extends: ILType option *
         methods: ILMethodDefs *
@@ -1539,8 +1547,7 @@ type ILTypeDef =
         name: string *
         attributes: TypeAttributes *
         layout: ILTypeDefLayout *
-        implements: ILTypes *
-        implementsCustomAttrs: (ILAttributesStored * int) list option *
+        implements: InterruptibleLazy<InterfaceImpl list> *
         genericParams: ILGenericParameterDefs *
         extends: ILType option *
         methods: ILMethodDefs *
@@ -1559,8 +1566,7 @@ type ILTypeDef =
     member GenericParams: ILGenericParameterDefs
     member Layout: ILTypeDefLayout
     member NestedTypes: ILTypeDefs
-    member Implements: ILTypes
-    member ImplementsCustomAttrs: (ILAttributesStored * int) list option
+    member Implements: InterruptibleLazy<InterfaceImpl list>
     member Extends: ILType option
     member Methods: ILMethodDefs
     member SecurityDecls: ILSecurityDecls
@@ -1609,7 +1615,7 @@ type ILTypeDef =
         ?name: string *
         ?attributes: TypeAttributes *
         ?layout: ILTypeDefLayout *
-        ?implements: ILTypes *
+        ?implements: InterruptibleLazy<InterfaceImpl list> *
         ?genericParams: ILGenericParameterDefs *
         ?extends: ILType option *
         ?methods: ILMethodDefs *
@@ -1620,8 +1626,7 @@ type ILTypeDef =
         ?properties: ILPropertyDefs *
         ?newAdditionalFlags: ILTypeDefAdditionalFlags *
         ?customAttrs: ILAttributesStored *
-        ?securityDecls: ILSecurityDecls *
-        ?implementsCustomAttrs: (ILAttributesStored * int) list option ->
+        ?securityDecls: ILSecurityDecls ->
             ILTypeDef
 
 /// Represents a prefix of information for ILTypeDef.
@@ -2161,7 +2166,7 @@ val internal mkILGenericClass:
     ILTypeDefAccess *
     ILGenericParameterDefs *
     ILType *
-    ILType list *
+    InterfaceImpl list *
     ILMethodDefs *
     ILFieldDefs *
     ILTypeDefs *
@@ -2245,6 +2250,8 @@ val internal mkCtorMethSpecForDelegate: ILGlobals -> ILType * bool -> ILMethodSp
 /// The toplevel "class" for a module or assembly.
 val internal mkILTypeForGlobalFunctions: ILScopeRef -> ILType
 
+val emptyILInterfaceImpls: InterruptibleLazy<InterfaceImpl list>
+
 /// Making tables of custom attributes, etc.
 val mkILCustomAttrs: ILAttribute list -> ILAttributes
 val mkILCustomAttrsFromArray: ILAttribute[] -> ILAttributes
diff --git a/src/Compiler/AbstractIL/ilmorph.fs b/src/Compiler/AbstractIL/ilmorph.fs
index 334ed93d212..9f2f3d0582f 100644
--- a/src/Compiler/AbstractIL/ilmorph.fs
+++ b/src/Compiler/AbstractIL/ilmorph.fs
@@ -368,8 +368,13 @@ let rec tdef_ty2ty_ilmbody2ilmbody_mdefs2mdefs isInKnownSet enc fs (tdef: ILType
     let mdefsR = fMethodDefs (enc, tdef) tdef.Methods
     let fdefsR = fdefs_ty2ty fTyInCtxtR tdef.Fields
 
+    let implements =
+        tdef.Implements.Value
+        |> List.map (fun x -> { x with Type = fTyInCtxtR x.Type })
+        |> InterruptibleLazy.FromValue
+
     tdef.With(
-        implements = List.map fTyInCtxtR tdef.Implements,
+        implements = implements,
         genericParams = gparams_ty2ty fTyInCtxtR tdef.GenericParams,
         extends = Option.map fTyInCtxtR tdef.Extends,
         methods = mdefsR,
diff --git a/src/Compiler/AbstractIL/ilprint.fs b/src/Compiler/AbstractIL/ilprint.fs
index 9d278dbe317..6ed8aec9286 100644
--- a/src/Compiler/AbstractIL/ilprint.fs
+++ b/src/Compiler/AbstractIL/ilprint.fs
@@ -752,8 +752,9 @@ let goutput_superclass env os =
         output_string os "extends "
         (goutput_typ_with_shortened_class_syntax env) os typ
 
-let goutput_implements env os (imp: ILTypes) =
+let goutput_implements env os (imp: InterfaceImpl list) =
     if not (List.isEmpty imp) then
+        let imp = imp |> Seq.map _.Type
         output_string os "implements "
         output_seq ", " (goutput_typ_with_shortened_class_syntax env) os imp
 
@@ -836,7 +837,7 @@ let rec goutput_tdef enc env contents os (cd: ILTypeDef) =
         output_string os "\n\t"
         goutput_superclass env os cd.Extends
         output_string os "\n\t"
-        goutput_implements env os cd.Implements
+        goutput_implements env os cd.Implements.Value
         output_string os "\n{\n "
 
         if contents then
diff --git a/src/Compiler/AbstractIL/ilread.fs b/src/Compiler/AbstractIL/ilread.fs
index 02696c53f0e..7ffa71feeee 100644
--- a/src/Compiler/AbstractIL/ilread.fs
+++ b/src/Compiler/AbstractIL/ilread.fs
@@ -2192,8 +2192,7 @@ and typeDefReader ctxtH : ILTypeDefStored =
         let fdefs = seekReadFields ctxt (numTypars, hasLayout) fieldsIdx endFieldsIdx
         let nested = seekReadNestedTypeDefs ctxt idx
 
-        let impls, intImplsAttrs =
-            seekReadInterfaceImpls ctxt mdv numTypars idx |> List.unzip
+        let impls = seekReadInterfaceImpls ctxt mdv numTypars idx
 
         let mimpls = seekReadMethodImpls ctxt numTypars idx
         let props = seekReadProperties ctxt numTypars idx
@@ -2206,7 +2205,6 @@ and typeDefReader ctxtH : ILTypeDefStored =
             layout = layout,
             nestedTypes = nested,
             implements = impls,
-            implementsCustomAttrs = Some intImplsAttrs,
             extends = super,
             methods = mdefs,
             securityDeclsStored = ctxt.securityDeclsReader_TypeDef,
@@ -2240,19 +2238,26 @@ and seekReadNestedTypeDefs (ctxt: ILMetadataReader) tidx =
         |])
 
 and seekReadInterfaceImpls (ctxt: ILMetadataReader) mdv numTypars tidx =
-    seekReadIndexedRows (
-        ctxt.getNumRows TableNames.InterfaceImpl,
-        id,
-        id,
-        (fun idx ->
-            let mutable addr = ctxt.rowAddr TableNames.InterfaceImpl idx
-            let _tidx = seekReadUntaggedIdx TableNames.TypeDef ctxt mdv &addr
-            simpleIndexCompare tidx _tidx),
-        isSorted ctxt TableNames.InterfaceImpl,
-        (fun idx ->
-            let intfIdx = seekReadInterfaceIdx ctxt mdv idx
-            seekReadTypeDefOrRef ctxt numTypars AsObject [] intfIdx, (ctxt.customAttrsReader_InterfaceImpl, idx))
-    )
+    InterruptibleLazy(fun () ->
+        seekReadIndexedRows (
+            ctxt.getNumRows TableNames.InterfaceImpl,
+            id,
+            id,
+            (fun idx ->
+                let mutable addr = ctxt.rowAddr TableNames.InterfaceImpl idx
+                let _tidx = seekReadUntaggedIdx TableNames.TypeDef ctxt mdv &addr
+                simpleIndexCompare tidx _tidx),
+            isSorted ctxt TableNames.InterfaceImpl,
+            (fun idx ->
+                let intfIdx = seekReadInterfaceIdx ctxt mdv idx
+                let ilType = seekReadTypeDefOrRef ctxt numTypars AsObject [] intfIdx
+
+                {
+                    Idx = idx
+                    Type = ilType
+                    CustomAttrsStored = ctxt.customAttrsReader_InterfaceImpl
+                })
+        ))
 
 and seekReadGenericParams ctxt numTypars (a, b) : ILGenericParameterDefs =
     ctxt.seekReadGenericParams (GenericParamsIdx(numTypars, a, b))
diff --git a/src/Compiler/AbstractIL/ilreflect.fs b/src/Compiler/AbstractIL/ilreflect.fs
index 9b0b7eddb9e..2a65eb8679b 100644
--- a/src/Compiler/AbstractIL/ilreflect.fs
+++ b/src/Compiler/AbstractIL/ilreflect.fs
@@ -2178,7 +2178,8 @@ let rec buildTypeDefPass2 cenv nesting emEnv (tdef: ILTypeDef) =
     let typB = envGetTypB emEnv tref
     let emEnv = envPushTyvars emEnv (getGenericArgumentsOfType typB)
     // add interface impls
-    tdef.Implements
+    tdef.Implements.Value
+    |> List.map _.Type
     |> convTypes cenv emEnv
     |> List.iter (fun implT -> typB.AddInterfaceImplementationAndLog implT)
     // add methods, properties
@@ -2339,7 +2340,8 @@ let createTypeRef (visited: Dictionary<_, _>, created: Dictionary<_, _>) emEnv t
         if verbose2 then
             dprintf "buildTypeDefPass4: Creating Interface Chain of %s\n" tdef.Name
 
-        tdef.Implements |> List.iter (traverseType CollectTypes.All)
+        tdef.Implements.Value
+        |> List.iter (fun x -> traverseType CollectTypes.All x.Type)
 
         if verbose2 then
             dprintf "buildTypeDefPass4: Do value types in fields of %s\n" tdef.Name
diff --git a/src/Compiler/AbstractIL/ilwrite.fs b/src/Compiler/AbstractIL/ilwrite.fs
index 832aa1c2810..3cbdd3c752b 100644
--- a/src/Compiler/AbstractIL/ilwrite.fs
+++ b/src/Compiler/AbstractIL/ilwrite.fs
@@ -1323,7 +1323,7 @@ and GenTypeDefPass2 pidx enc cenv (tdef: ILTypeDef) =
         // Now generate or assign index numbers for tables referenced by the maps.
         // Don't yet generate contents of these tables - leave that to pass3, as
         // code may need to embed these entries.
-        cenv.implementsIdxs[tidx] <- tdef.Implements |> List.map (GenImplementsPass2 cenv env tidx)            
+        cenv.implementsIdxs[tidx] <- tdef.Implements.Value |> List.map (fun x -> GenImplementsPass2 cenv env tidx x.Type)            
 
         tdef.Fields.AsList() |> List.iter (GenFieldDefPass2 tdef cenv tidx)
         tdef.Methods |> Seq.iter (GenMethodDefPass2 tdef cenv tidx)
@@ -2875,12 +2875,9 @@ let rec GenTypeDefPass3 enc cenv (tdef: ILTypeDef) =
         let env = envForTypeDef tdef
         let tidx = GetIdxForTypeDef cenv (TdKey(enc, tdef.Name))
 
-        match tdef.ImplementsCustomAttrs with
-        | None -> ()
-        | Some attrList ->
-            attrList
-            |> List.zip cenv.implementsIdxs[tidx]
-            |> List.iter (fun (impIdx,(attrs,metadataIdx)) -> GenCustomAttrsPass3Or4 cenv (hca_InterfaceImpl,impIdx) (attrs.GetCustomAttrs metadataIdx))
+        tdef.Implements.Value
+        |> List.zip cenv.implementsIdxs[tidx]
+        |> List.iter (fun (impIdx, impl) -> GenCustomAttrsPass3Or4 cenv (hca_InterfaceImpl,impIdx) impl.CustomAttrs)
 
         tdef.Properties.AsList() |> List.iter (GenPropertyPass3 cenv env)
         tdef.Events.AsList() |> List.iter (GenEventPass3 cenv env)
diff --git a/src/Compiler/Checking/TypeHierarchy.fs b/src/Compiler/Checking/TypeHierarchy.fs
index 102f36908fe..0805fcaf09b 100644
--- a/src/Compiler/Checking/TypeHierarchy.fs
+++ b/src/Compiler/Checking/TypeHierarchy.fs
@@ -117,17 +117,16 @@ let GetImmediateInterfacesOfMetadataType g amap m skipUnref ty (tcref: TyconRef)
             // succeeded with more reported. There are pathological corner cases where this
             // doesn't apply: e.g. for mscorlib interfaces like IComparable, but we can always
             // assume those are present.
-            match tdef.ImplementsCustomAttrs with
-            | Some attrsList when g.langFeatureNullness && g.checkNullness ->
-                for (attrs,attrsIdx),intfTy in tdef.Implements |> List.zip attrsList do
-                    if skipUnref = SkipUnrefInterfaces.No || CanRescopeAndImportILType scoref amap m intfTy then
+            let checkNullness = g.langFeatureNullness && g.checkNullness
+            for {Idx = attrsIdx; Type = intfTy; CustomAttrsStored = attrs} in tdef.Implements.Value do
+                if skipUnref = SkipUnrefInterfaces.No || CanRescopeAndImportILType scoref amap m intfTy then
+                    if checkNullness then
                         let typeAttrs = AttributesFromIL(attrsIdx,attrs)
                         let nullness = {DirectAttributes = typeAttrs; Fallback = FromClass typeAttrs}
                         RescopeAndImportILType scoref amap m tinst nullness intfTy
-            | _ ->
-                for intfTy in tdef.Implements do
-                    if skipUnref = SkipUnrefInterfaces.No || CanRescopeAndImportILType scoref amap m intfTy then
+                    else
                         RescopeAndImportILTypeSkipNullness scoref amap m tinst intfTy
+
         | FSharpOrArrayOrByrefOrTupleOrExnTypeMetadata ->
             for intfTy in tcref.ImmediateInterfaceTypesOfFSharpTycon do
                instType (mkInstForAppTy g ty) intfTy ]
diff --git a/src/Compiler/CodeGen/EraseClosures.fs b/src/Compiler/CodeGen/EraseClosures.fs
index ed32d83b369..df2604717ed 100644
--- a/src/Compiler/CodeGen/EraseClosures.fs
+++ b/src/Compiler/CodeGen/EraseClosures.fs
@@ -572,8 +572,7 @@ let rec convIlxClosureDef cenv encl (td: ILTypeDef) clo =
                         name = td.Name,
                         genericParams = td.GenericParams,
                         attributes = td.Attributes,
-                        implements = [],
-                        implementsCustomAttrs = None,
+                        implements = emptyILInterfaceImpls,
                         nestedTypes = emptyILTypeDefs,
                         layout = ILTypeDefLayout.Auto,
                         extends = Some cenv.mkILTyFuncTy,
@@ -707,8 +706,7 @@ let rec convIlxClosureDef cenv encl (td: ILTypeDef) clo =
                         name = td.Name,
                         genericParams = td.GenericParams,
                         attributes = td.Attributes,
-                        implements = [],
-                        implementsCustomAttrs = None,
+                        implements = emptyILInterfaceImpls,
                         layout = ILTypeDefLayout.Auto,
                         nestedTypes = emptyILTypeDefs,
                         extends = Some nowEnvParentClass,
diff --git a/src/Compiler/CodeGen/EraseUnions.fs b/src/Compiler/CodeGen/EraseUnions.fs
index 88336c057b2..79788107475 100644
--- a/src/Compiler/CodeGen/EraseUnions.fs
+++ b/src/Compiler/CodeGen/EraseUnions.fs
@@ -1571,8 +1571,7 @@ let mkClassUnionDef
                     genericParams = td.GenericParams,
                     attributes = enum 0,
                     layout = ILTypeDefLayout.Auto,
-                    implements = [],
-                    implementsCustomAttrs = None,
+                    implements = emptyILInterfaceImpls,
                     extends = Some g.ilg.typ_Object,
                     methods = emptyILMethods,
                     securityDecls = emptyILSecurityDecls,
diff --git a/src/Compiler/CodeGen/IlxGen.fs b/src/Compiler/CodeGen/IlxGen.fs
index 6758b6dcd94..82572ea888a 100644
--- a/src/Compiler/CodeGen/IlxGen.fs
+++ b/src/Compiler/CodeGen/IlxGen.fs
@@ -2227,7 +2227,8 @@ type AnonTypeGenerationTable() =
 
             let ilInterfaceTys =
                 [
-                    for intfTy, _, _ in tcaug.tcaug_interfaces -> GenType cenv m (TypeReprEnv.Empty.ForTypars tps) intfTy
+                    for intfTy, _, _ in tcaug.tcaug_interfaces ->
+                        GenType cenv m (TypeReprEnv.Empty.ForTypars tps) intfTy |> InterfaceImpl.Create
                 ]
 
             let ilTypeDef =
@@ -6023,7 +6024,8 @@ and GenStructStateMachine cenv cgbuf eenvouter (res: LoweredStateMachine) sequel
     let interfaceTys =
         GetImmediateInterfacesOfType SkipUnrefInterfaces.Yes g cenv.amap m templateStructTy
 
-    let ilInterfaceTys = List.map (GenType cenv m eenvinner.tyenv) interfaceTys
+    let ilInterfaceTys =
+        List.map (GenType cenv m eenvinner.tyenv >> InterfaceImpl.Create) interfaceTys
 
     let super = g.iltyp_ValueType
 
@@ -6254,8 +6256,7 @@ and GenStructStateMachine cenv cgbuf eenvouter (res: LoweredStateMachine) sequel
             methods = mkILMethods mdefs,
             methodImpls = mkILMethodImpls mimpls,
             nestedTypes = emptyILTypeDefs,
-            implements = ilInterfaceTys,
-            implementsCustomAttrs = None,
+            implements = InterruptibleLazy.FromValue(ilInterfaceTys),
             extends = Some super,
             additionalFlags = ILTypeDefAdditionalFlags.None,
             securityDecls = emptyILSecurityDecls
@@ -6381,7 +6382,8 @@ and GenObjectExpr cenv cgbuf eenvouter objExpr (baseType, baseValOpt, basecall,
     let mimpls = mimpls |> List.choose id // choose the ones that actually have method impls
 
     let interfaceTys =
-        interfaceImpls |> List.map (fst >> GenType cenv m eenvinner.tyenv)
+        interfaceImpls
+        |> List.map (fst >> GenType cenv m eenvinner.tyenv >> InterfaceImpl.Create)
 
     let super =
         (if isInterfaceTy g baseType then
@@ -6390,7 +6392,11 @@ and GenObjectExpr cenv cgbuf eenvouter objExpr (baseType, baseValOpt, basecall,
              ilCloRetTy)
 
     let interfaceTys =
-        interfaceTys @ (if isInterfaceTy g baseType then [ ilCloRetTy ] else [])
+        interfaceTys
+        @ (if isInterfaceTy g baseType then
+               [ InterfaceImpl.Create(ilCloRetTy) ]
+           else
+               [])
 
     let cloTypeDefs =
         GenClosureTypeDefs
@@ -6688,8 +6694,7 @@ and GenClosureTypeDefs
             methods = mkILMethods mdefs,
             methodImpls = mkILMethodImpls mimpls,
             nestedTypes = emptyILTypeDefs,
-            implements = ilIntfTys,
-            implementsCustomAttrs = None,
+            implements = InterruptibleLazy.FromValue(ilIntfTys),
             extends = Some ext,
             additionalFlags = ILTypeDefAdditionalFlags.None,
             securityDecls = emptyILSecurityDecls
@@ -10770,23 +10775,20 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
             let ilThisTy = GenType cenv m eenvinner.tyenv thisTy
             let tref = ilThisTy.TypeRef
             let ilGenParams = GenGenericParams cenv eenvinner tycon.TyparsNoRange
+            let checkNullness = g.langFeatureNullness && g.checkNullness
 
             let ilIntfTys =
                 tycon.ImmediateInterfaceTypesOfFSharpTycon
-                |> List.map (GenType cenv m eenvinner.tyenv)
-
-            let ilIntCustomAttrs =
-                if g.langFeatureNullness && g.checkNullness && not (isNil ilIntfTys) then
-                    tycon.ImmediateInterfaceTypesOfFSharpTycon
-                    |> List.map (
-                        GenAdditionalAttributesForTy g
-                        >> mkILCustomAttrs
-                        >> ILAttributesStored.Given
-                        >> (fun x -> x, 0)
-                    )
-                    |> Some
-                else
-                    None
+                |> List.map (fun x ->
+                    let ilType = GenType cenv m eenvinner.tyenv x
+
+                    let customAttrs =
+                        if checkNullness then
+                            GenAdditionalAttributesForTy g x |> mkILCustomAttrs |> ILAttributesStored.Given
+                        else
+                            emptyILCustomAttrsStored
+
+                    InterfaceImpl.Create(ilType, customAttrs))
 
             let ilTypeName = tref.Name
 
@@ -11447,11 +11449,7 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                             .WithSerializable(isSerializable)
                             .WithAbstract(isAbstract)
                             .WithImport(isComInteropTy g thisTy)
-                            .With(
-                                methodImpls = mkILMethodImpls methodImpls,
-                                newAdditionalFlags = additionalFlags,
-                                implementsCustomAttrs = ilIntCustomAttrs
-                            )
+                            .With(methodImpls = mkILMethodImpls methodImpls, newAdditionalFlags = additionalFlags)
 
                     let tdLayout, tdEncoding =
                         match TryFindFSharpAttribute g g.attrib_StructLayoutAttribute tycon.Attribs with
@@ -11613,8 +11611,7 @@ and GenTypeDef cenv mgbuf lazyInitInfo eenv m (tycon: Tycon) : ILTypeRef option
                             methods = mkILMethods ilMethods,
                             methodImpls = mkILMethodImpls methodImpls,
                             nestedTypes = emptyILTypeDefs,
-                            implements = ilIntfTys,
-                            implementsCustomAttrs = None,
+                            implements = InterruptibleLazy.FromValue(ilIntfTys),
                             extends =
                                 Some(
                                     if tycon.IsStructOrEnumTycon then
@@ -11837,7 +11834,7 @@ and GenExnDef cenv mgbuf eenv m (exnc: Tycon) : ILTypeRef option =
 
         let interfaces =
             exnc.ImmediateInterfaceTypesOfFSharpTycon
-            |> List.map (GenType cenv m eenv.tyenv)
+            |> List.map (GenType cenv m eenv.tyenv >> InterfaceImpl.Create)
 
         let tdef =
             mkILGenericClass (
diff --git a/src/Compiler/CodeGen/IlxGenSupport.fs b/src/Compiler/CodeGen/IlxGenSupport.fs
index 073b4473ea9..0e163db4069 100644
--- a/src/Compiler/CodeGen/IlxGenSupport.fs
+++ b/src/Compiler/CodeGen/IlxGenSupport.fs
@@ -46,7 +46,7 @@ let mkLocalPrivateAttributeWithDefaultConstructor (g: TcGlobals, name: string) =
         ILTypeDefAccess.Private,
         ILGenericParameterDefs.Empty,
         g.ilg.typ_Attribute,
-        ILTypes.Empty,
+        [],
         ilMethods,
         emptyILFields,
         emptyILTypeDefs,
@@ -140,7 +140,7 @@ let mkLocalPrivateAttributeWithPropertyConstructors
         ILTypeDefAccess.Private,
         ILGenericParameterDefs.Empty,
         g.ilg.typ_Attribute,
-        ILTypes.Empty,
+        [],
         mkILMethods (
             ilCtorDef
             :: (ilElements |> List.fold (fun acc (_, getter, _, _) -> getter @ acc) [])
@@ -205,7 +205,7 @@ let mkLocalPrivateAttributeWithByteAndByteArrayConstructors (g: TcGlobals, name:
         ILTypeDefAccess.Private,
         ILGenericParameterDefs.Empty,
         g.ilg.typ_Attribute,
-        ILTypes.Empty,
+        [],
         mkILMethods ([ ilScalarCtorDef; ilArrayCtorDef ]),
         mkILFields [ fieldDef ],
         emptyILTypeDefs,
@@ -233,7 +233,7 @@ let mkLocalPrivateInt32Enum (g: TcGlobals, tref: ILTypeRef, values: (string * in
         ILTypeDefAccess.Private,
         ILGenericParameterDefs.Empty,
         g.ilg.typ_Enum,
-        ILTypes.Empty,
+        [],
         mkILMethods [],
         mkILFields enumFields,
         emptyILTypeDefs,
diff --git a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl
index 3ae2414fc07..543c92ef5cc 100644
--- a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl
+++ b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.debug.bsl
@@ -1565,7 +1565,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs Properties
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs get_Properties()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls SecurityDecls
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls get_SecurityDecls()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributesStored], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]]])
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributesStored], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls])
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess Access
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess get_Access()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefLayout Layout
@@ -1574,20 +1574,18 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefs NestedTypes
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefs get_NestedTypes()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Int32 MetadataIndex
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Int32 get_MetadataIndex()
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] Implements
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] get_Implements()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef] GenericParams
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef] get_GenericParams()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType] Implements
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType] get_Implements()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType] Extends
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType] get_Extends()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]] ImplementsCustomAttrs
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]] get_ImplementsCustomAttrs()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.Reflection.TypeAttributes Attributes
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.Reflection.TypeAttributes get_Attributes()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String Name
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String ToString()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String get_Name()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, ILTypeDefAdditionalFlags, ILSecurityDecls, ILAttributesStored)
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, ILTypeDefAdditionalFlags, ILSecurityDecls, ILAttributesStored)
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess Item
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess get_Item()
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Tags: Int32 Nested
@@ -1773,6 +1771,20 @@ FSharp.Compiler.AbstractIL.IL+ILVersionInfo: UInt16 get_Major()
 FSharp.Compiler.AbstractIL.IL+ILVersionInfo: UInt16 get_Minor()
 FSharp.Compiler.AbstractIL.IL+ILVersionInfo: UInt16 get_Revision()
 FSharp.Compiler.AbstractIL.IL+ILVersionInfo: Void .ctor(UInt16, UInt16, UInt16, UInt16)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributes CustomAttrs
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributes get_CustomAttrs()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributesStored CustomAttrsStored
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributesStored CustomAttrsStored@
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributesStored get_CustomAttrsStored()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILType Type
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILType get_Type()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Int32 Idx
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Int32 get_Idx()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: InterfaceImpl Create(ILType)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: InterfaceImpl Create(ILType, ILAttributesStored)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: System.String ToString()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Void .ctor(Int32, ILType, ILAttributesStored)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Void set_CustomAttrsStored(ILAttributesStored)
 FSharp.Compiler.AbstractIL.IL+MethodBody+IL: Internal.Utilities.Library.InterruptibleLazy`1[FSharp.Compiler.AbstractIL.IL+ILMethodBody] Item
 FSharp.Compiler.AbstractIL.IL+MethodBody+IL: Internal.Utilities.Library.InterruptibleLazy`1[FSharp.Compiler.AbstractIL.IL+ILMethodBody] get_Item()
 FSharp.Compiler.AbstractIL.IL+MethodBody+PInvoke: System.Lazy`1[FSharp.Compiler.AbstractIL.IL+PInvokeMethod] Item
@@ -1909,6 +1921,7 @@ FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeInit
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeRef
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeSpec
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILVersionInfo
+FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+InterfaceImpl
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+MethodBody
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+PublicKey
 FSharp.Compiler.AbstractIL.IL: ILAttributes emptyILCustomAttrs
@@ -1957,6 +1970,8 @@ FSharp.Compiler.AbstractIL.IL: ILTypeDefs mkILTypeDefsComputed(Microsoft.FSharp.
 FSharp.Compiler.AbstractIL.IL: ILTypeDefs mkILTypeDefsFromArray(ILTypeDef[])
 FSharp.Compiler.AbstractIL.IL: Int32 NoMetadataIdx
 FSharp.Compiler.AbstractIL.IL: Int32 get_NoMetadataIdx()
+FSharp.Compiler.AbstractIL.IL: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] emptyILInterfaceImpls
+FSharp.Compiler.AbstractIL.IL: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] get_emptyILInterfaceImpls()
 FSharp.Compiler.AbstractIL.ILBinaryReader+ILModuleReader: ILModuleDef ILModuleDef
 FSharp.Compiler.AbstractIL.ILBinaryReader+ILModuleReader: ILModuleDef get_ILModuleDef()
 FSharp.Compiler.AbstractIL.ILBinaryReader+ILModuleReader: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILAssemblyRef] ILAssemblyRefs
diff --git a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl
index 3ae2414fc07..543c92ef5cc 100644
--- a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl
+++ b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.release.bsl
@@ -1565,7 +1565,7 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs Properties
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILPropertyDefs get_Properties()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls SecurityDecls
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILSecurityDecls get_SecurityDecls()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributesStored], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]]])
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDef With(Microsoft.FSharp.Core.FSharpOption`1[System.String], Microsoft.FSharp.Core.FSharpOption`1[System.Reflection.TypeAttributes], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefLayout], Microsoft.FSharp.Core.FSharpOption`1[Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef]], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType]], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILFieldDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILMethodImplDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILEventDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILPropertyDefs], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILTypeDefAdditionalFlags], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILAttributesStored], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILSecurityDecls])
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess Access
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefAccess get_Access()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefLayout Layout
@@ -1574,20 +1574,18 @@ FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefs NestedTypes
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: ILTypeDefs get_NestedTypes()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Int32 MetadataIndex
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Int32 get_MetadataIndex()
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] Implements
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] get_Implements()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef] GenericParams
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef] get_GenericParams()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType] Implements
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType] get_Implements()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType] Extends
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType] get_Extends()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]] ImplementsCustomAttrs
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]] get_ImplementsCustomAttrs()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.Reflection.TypeAttributes Attributes
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.Reflection.TypeAttributes get_Attributes()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String Name
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String ToString()
 FSharp.Compiler.AbstractIL.IL+ILTypeDef: System.String get_Name()
-FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILType], Microsoft.FSharp.Core.FSharpOption`1[Microsoft.FSharp.Collections.FSharpList`1[System.Tuple`2[FSharp.Compiler.AbstractIL.IL+ILAttributesStored,System.Int32]]], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, ILTypeDefAdditionalFlags, ILSecurityDecls, ILAttributesStored)
+FSharp.Compiler.AbstractIL.IL+ILTypeDef: Void .ctor(System.String, System.Reflection.TypeAttributes, ILTypeDefLayout, Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]], Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILGenericParameterDef], Microsoft.FSharp.Core.FSharpOption`1[FSharp.Compiler.AbstractIL.IL+ILType], ILMethodDefs, ILTypeDefs, ILFieldDefs, ILMethodImplDefs, ILEventDefs, ILPropertyDefs, ILTypeDefAdditionalFlags, ILSecurityDecls, ILAttributesStored)
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess Item
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Nested: ILMemberAccess get_Item()
 FSharp.Compiler.AbstractIL.IL+ILTypeDefAccess+Tags: Int32 Nested
@@ -1773,6 +1771,20 @@ FSharp.Compiler.AbstractIL.IL+ILVersionInfo: UInt16 get_Major()
 FSharp.Compiler.AbstractIL.IL+ILVersionInfo: UInt16 get_Minor()
 FSharp.Compiler.AbstractIL.IL+ILVersionInfo: UInt16 get_Revision()
 FSharp.Compiler.AbstractIL.IL+ILVersionInfo: Void .ctor(UInt16, UInt16, UInt16, UInt16)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributes CustomAttrs
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributes get_CustomAttrs()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributesStored CustomAttrsStored
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributesStored CustomAttrsStored@
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILAttributesStored get_CustomAttrsStored()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILType Type
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: ILType get_Type()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Int32 Idx
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Int32 get_Idx()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: InterfaceImpl Create(ILType)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: InterfaceImpl Create(ILType, ILAttributesStored)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: System.String ToString()
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Void .ctor(Int32, ILType, ILAttributesStored)
+FSharp.Compiler.AbstractIL.IL+InterfaceImpl: Void set_CustomAttrsStored(ILAttributesStored)
 FSharp.Compiler.AbstractIL.IL+MethodBody+IL: Internal.Utilities.Library.InterruptibleLazy`1[FSharp.Compiler.AbstractIL.IL+ILMethodBody] Item
 FSharp.Compiler.AbstractIL.IL+MethodBody+IL: Internal.Utilities.Library.InterruptibleLazy`1[FSharp.Compiler.AbstractIL.IL+ILMethodBody] get_Item()
 FSharp.Compiler.AbstractIL.IL+MethodBody+PInvoke: System.Lazy`1[FSharp.Compiler.AbstractIL.IL+PInvokeMethod] Item
@@ -1909,6 +1921,7 @@ FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeInit
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeRef
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILTypeSpec
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+ILVersionInfo
+FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+InterfaceImpl
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+MethodBody
 FSharp.Compiler.AbstractIL.IL: FSharp.Compiler.AbstractIL.IL+PublicKey
 FSharp.Compiler.AbstractIL.IL: ILAttributes emptyILCustomAttrs
@@ -1957,6 +1970,8 @@ FSharp.Compiler.AbstractIL.IL: ILTypeDefs mkILTypeDefsComputed(Microsoft.FSharp.
 FSharp.Compiler.AbstractIL.IL: ILTypeDefs mkILTypeDefsFromArray(ILTypeDef[])
 FSharp.Compiler.AbstractIL.IL: Int32 NoMetadataIdx
 FSharp.Compiler.AbstractIL.IL: Int32 get_NoMetadataIdx()
+FSharp.Compiler.AbstractIL.IL: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] emptyILInterfaceImpls
+FSharp.Compiler.AbstractIL.IL: Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+InterfaceImpl]] get_emptyILInterfaceImpls()
 FSharp.Compiler.AbstractIL.ILBinaryReader+ILModuleReader: ILModuleDef ILModuleDef
 FSharp.Compiler.AbstractIL.ILBinaryReader+ILModuleReader: ILModuleDef get_ILModuleDef()
 FSharp.Compiler.AbstractIL.ILBinaryReader+ILModuleReader: Microsoft.FSharp.Collections.FSharpList`1[FSharp.Compiler.AbstractIL.IL+ILAssemblyRef] ILAssemblyRefs
diff --git a/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs b/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs
index 9ce34ec9be7..ed245117916 100644
--- a/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs
+++ b/tests/FSharp.Compiler.Service.Tests/ModuleReaderCancellationTests.fs
@@ -115,7 +115,7 @@ type PreTypeDefData =
                 mkILMethods []
 
         let typeAttributes = TypeAttributes.Public
-        ILTypeDef(this.Name, typeAttributes, ILTypeDefLayout.Auto, [], None, [],
+        ILTypeDef(this.Name, typeAttributes, ILTypeDefLayout.Auto, emptyILInterfaceImpls, [],
             None, methodsDefs, mkILTypeDefs [], mkILFields [], emptyILMethodImpls, mkILEvents [], mkILProperties [], ILTypeDefAdditionalFlags.None,
             emptyILSecurityDecls, emptyILCustomAttrsStored)