Extend external_derive to cover proc-macro-generated impls - #2641
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Proc-macro crates such as serde do not emit #[automatically_derived] on the impls they generate. As a result, #[verifier::external_derive] had no effect on serde-derived Serialize/Deserialize impls (issues verus-lang#1575, verus-lang#1577, verus-lang#1956): the gate in get_attributes_for_automatic_derive returned None immediately for any impl lacking that attribute. The fix checks span.from_expansion() as a secondary signal. When an impl is macro-expanded but not #[automatically_derived], and the self type has #[verifier::external_derive] (with AllExternal or a matching trait name), the impl is now marked external. If the type has no external_derive annotation at all, macro-expanded impls fall through to the normal classification path unchanged, preserving existing behavior. Tests added in rust_verify_test/tests/std.rs simulate proc-macro derives with macro_rules! (which produces macro-expanded impls without #[automatically_derived]) and verify that external_derive AllExternal and SomeExternal(TraitName) both suppress verification of the generated impl.
The new is_macro_expanded path in get_attributes_for_automatic_derive runs for any macro-expanded impl, not just #[automatically_derived] ones. Unlike derive impls, macro-expanded impls can target primitive types (e.g. vstd's std_specs/ops.rs generates impl ... for u8/usize/... via macro_rules!), whose Res is PrimTy and has no DefId. Calling path.res.def_id() on those panics; switch to opt_def_id() and fall through to normal classification when there's no def_id, matching the existing fallback arms.
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Summary
#[verifier::external_derive]had no effect on impls generated by proc-macro crates (e.g., serde'sSerialize/Deserialize). Those crates do not emit#[automatically_derived]on the impls they generate, soget_attributes_for_automatic_deriveinexternal.rsreturnedNoneimmediately, leaving the generated impl subject to normal (failing) classification.Fixes #1575, #1577, #1956.
Changes
source/rust_verify/src/external.rsThe
get_attributes_for_automatic_derivefunction previously returnedNoneat the top if!is_automatically_derived(attrs). This PR adds a secondary check: when an impl span is from a macro expansion (span.from_expansion()) and the self type carries#[verifier::external_derive](eitherAllExternalorSomeExternalwith a matching trait name), the impl is now marked external.If the type has no
external_deriveannotation at all, macro-expanded impls fall through toNoneunchanged, preserving all existing behavior.source/rust_verify_test/tests/std.rsTwo new tests under
--no-external-by-default:external_derive_proc_macro_all: uses amacro_rules!that produces an impl without#[automatically_derived](simulating a proc-macro derive), verifies that#[verifier::external_derive](AllExternal) suppresses verification of the generated impl.external_derive_proc_macro_named: same, but with#[verifier::external_derive(FakeSerialize)]to test the named-trait path.Testing
The new tests simulate proc-macro derives with
macro_rules!, which produces macro-expanded impls without#[automatically_derived]-- exactly the shape serde emits. The existingexternal_derive_attrandexternal_derive_attr_listtests are unchanged and continue to pass.