autobox feature

Author: daniel-levin

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "thiserror"
-version = "2.0.12"
+version = "2.0.13"
 authors = ["David Tolnay <[email protected]>"]
 categories = ["rust-patterns"]
 description = "derive(Error)"
@@ -13,6 +13,7 @@ rust-version = "1.61"
 
 [features]
 default = ["std"]
+autobox = ["std", "thiserror-impl/autobox"]
 
 # Std feature enables support for formatting std::path::{Path, PathBuf}
 # conveniently in an error message.
@@ -28,7 +29,7 @@ default = ["std"]
 std = []
 
 [dependencies]
-thiserror-impl = { version = "=2.0.12", path = "impl" }
+thiserror-impl = { version = "=2.0.13", path = "impl" }
 
 [dev-dependencies]
 anyhow = "1.0.73"

impl/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "thiserror-impl"
-version = "2.0.12"
+version = "2.0.13"
 authors = ["David Tolnay <[email protected]>"]
 description = "Implementation detail of the `thiserror` crate"
 edition = "2021"
@@ -11,6 +11,10 @@ rust-version = "1.61"
 [lib]
 proc-macro = true
 
+[features]
+default = []
+autobox = []
+
 [dependencies]
 proc-macro2 = "1.0.74"
 quote = "1.0.35"

impl/src/expand.rs

@@ -449,22 +449,45 @@ fn impl_enum(input: Enum) -> TokenStream {
                 #ty::#variant #body
             }
         };
+
         let from_impl = quote_spanned! {span=>
             #[automatically_derived]
             impl #impl_generics ::core::convert::From<#from> for #ty #ty_generics #where_clause {
                 #from_function
             }
+
         };
-        Some(quote! {
-            #[allow(
-                deprecated,
-                unused_qualifications,
-                clippy::elidable_lifetime_names,
-                clippy::needless_lifetimes,
-            )]
-            #from_impl
-        })
-    });
+
+        #[cfg(feature = "autobox")]
+        let from_impl_for_box = quote_spanned! {span=>
+            #[automatically_derived]
+            impl #impl_generics ::core::convert::From<#from> for ::std::boxed::Box< #ty #ty_generics > #where_clause {
+                fn from(#source_var: #from) -> Self {
+                    ::std::boxed::Box::new(#ty::#variant #body)
+                }
+            }
+        };
+
+        Some([quote! {
+              #[allow(
+                  deprecated,
+                  unused_qualifications,
+                  clippy::elidable_lifetime_names,
+                  clippy::needless_lifetimes,
+              )]
+              #from_impl
+            },
+            #[cfg(feature = "autobox")]
+            quote! {
+              #[allow(
+                  deprecated,
+                  unused_qualifications,
+                  clippy::elidable_lifetime_names,
+                  clippy::needless_lifetimes,
+              )]
+              #from_impl_for_box
+        }])
+    }).flatten();
 
     if input.generics.type_params().next().is_some() {
         let self_token = <Token![Self]>::default();

src/lib.rs

@@ -257,7 +257,7 @@
 //!   [`anyhow`]: https://github.com/dtolnay/anyhow
 
 #![no_std]
-#![doc(html_root_url = "https://docs.rs/thiserror/2.0.12")]
+#![doc(html_root_url = "https://docs.rs/thiserror/2.0.13")]
 #![allow(
     clippy::elidable_lifetime_names,
     clippy::module_name_repetitions,

tests/test_autobox.rs

@@ -0,0 +1,41 @@
+#![cfg(feature = "autobox")]
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+#[error("not great")]
+struct VeryLargeError {
+    a: [u8; 2048],
+}
+
+impl VeryLargeError {
+    fn new() -> Self {
+        Self { a: [0; 2048] }
+    }
+}
+
+#[derive(Debug, Error)]
+enum ErrorEnum {
+    #[error("bad")]
+    Any(#[from] anyhow::Error),
+
+    #[error("worse")]
+    Big(#[from] VeryLargeError),
+}
+
+/// External code may still return large errors...
+fn do_something() -> Result<(), VeryLargeError> {
+    Err(VeryLargeError::new())
+}
+
+/// But we should be able to box them automatically!
+fn do_something_else() -> Result<(), Box<ErrorEnum>> {
+    do_something()?;
+
+    Ok(())
+}
+
+#[test]
+fn autobox() {
+    let _ = do_something_else().unwrap_err();
+}