Fix some unreachable!() with wasm components

This commit resolves a few panics in Wasmtime's compilation of
components with respect to tags and the exception-handling wasm
proposal. Despite exception-handling not being enabled in Wasmtime at
this time these panics were still reachable due to an issue in
wasm-tools's validation not being exhaustive enough for this proposal
feature combination: bytecodealliance/wasm-tools#2114.

This is not full support for exceptions since it can't be tested yet,
but this should be enough to ensure that the validator will get far
enough to flag components as otherwise invalid due to using tags that
can't be present (as core wasm tags can't be present in core modules
right now, the `EXCEPTIONS` feature is always disabled).

Author: alexcrichton

crates/environ/src/component/translate.rs

@@ -3,8 +3,8 @@ use crate::prelude::*;
 use crate::ScopeVec;
 use crate::{
     EngineOrModuleTypeIndex, EntityIndex, ModuleEnvironment, ModuleInternedTypeIndex,
-    ModuleTranslation, ModuleTypesBuilder, PrimaryMap, Tunables, TypeConvert, WasmHeapType,
-    WasmResult, WasmValType,
+    ModuleTranslation, ModuleTypesBuilder, PrimaryMap, TagIndex, Tunables, TypeConvert,
+    WasmHeapType, WasmResult, WasmValType,
 };
 use anyhow::anyhow;
 use anyhow::{bail, Result};
@@ -321,6 +321,7 @@ enum LocalInitializer<'data> {
     AliasExportTable(ModuleInstanceIndex, &'data str),
     AliasExportGlobal(ModuleInstanceIndex, &'data str),
     AliasExportMemory(ModuleInstanceIndex, &'data str),
+    AliasExportTag(ModuleInstanceIndex, &'data str),
     AliasComponentExport(ComponentInstanceIndex, &'data str),
     AliasModule(ClosedOverModule),
     AliasComponent(ClosedOverComponent),
@@ -1039,9 +1040,10 @@ impl<'a, 'data> Translator<'a, 'data> {
                     let index = GlobalIndex::from_u32(export.index);
                     EntityIndex::Global(index)
                 }
-
-                // doesn't get past validation
-                wasmparser::ExternalKind::Tag => unimplemented!("wasm exceptions"),
+                wasmparser::ExternalKind::Tag => {
+                    let index = TagIndex::from_u32(export.index);
+                    EntityIndex::Tag(index)
+                }
             };
             map.insert(export.name, idx);
         }
@@ -1123,9 +1125,7 @@ impl<'a, 'data> Translator<'a, 'data> {
             wasmparser::ExternalKind::Memory => LocalInitializer::AliasExportMemory(instance, name),
             wasmparser::ExternalKind::Table => LocalInitializer::AliasExportTable(instance, name),
             wasmparser::ExternalKind::Global => LocalInitializer::AliasExportGlobal(instance, name),
-            wasmparser::ExternalKind::Tag => {
-                unimplemented!("wasm exceptions");
-            }
+            wasmparser::ExternalKind::Tag => LocalInitializer::AliasExportTag(instance, name),
         }
     }
 

crates/environ/src/component/translate/inline.rs

@@ -204,6 +204,7 @@ struct InlinerFrame<'a> {
     memories: PrimaryMap<MemoryIndex, dfg::CoreExport<EntityIndex>>,
     tables: PrimaryMap<TableIndex, dfg::CoreExport<EntityIndex>>,
     globals: PrimaryMap<GlobalIndex, dfg::CoreExport<EntityIndex>>,
+    tags: PrimaryMap<GlobalIndex, dfg::CoreExport<EntityIndex>>,
     modules: PrimaryMap<ModuleIndex, ModuleDef<'a>>,
 
     // component model index spaces
@@ -1164,6 +1165,15 @@ impl<'a> Inliner<'a> {
                 );
             }
 
+            AliasExportTag(instance, name) => {
+                frame.tags.push(
+                    match self.core_def_of_module_instance_export(frame, *instance, *name) {
+                        dfg::CoreDef::Export(e) => e,
+                        _ => unreachable!(),
+                    },
+                );
+            }
+
             AliasComponentExport(instance, name) => {
                 match &frame.component_instances[*instance] {
                     // Aliasing an export from an imported instance means that
@@ -1480,6 +1490,7 @@ impl<'a> InlinerFrame<'a> {
             memories: Default::default(),
             tables: Default::default(),
             globals: Default::default(),
+            tags: Default::default(),
 
             component_instances: Default::default(),
             component_funcs: Default::default(),