fix(derive-encode): emit friendly compile errors instead of panicking - #307
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krisztianfekete merged 2 commits intoJul 28, 2026
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Hey, thanks for the PR, left a few comments!
Use syn::Error::new_spanned(...).to_compile_error() for all invalid input shapes so diagnostics point at the offending type or attribute value (e.g. the unknown ident in #[prometheus(unknown)]), rather than the derive site or a proc-macro panic. This also removes the double-parse + unwrap for #[prometheus(...)] by parsing the attribute once during field processing. Signed-off-by: Lohit Kolluri <[email protected]>
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@lohitkolluri, can you please rebase this?
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@krisztianfekete Done! |
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Thanks! |
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Summary
Replace
panic!calls in theEncodeLabelSetandEncodeLabelValueproc-macro derives withcompile_error!so unsupported shapes surface a direct diagnostic that points to the#[derive(...)]line the user wrote, instead of the currenterror: proc-macro derive panickedmessage that points into the proc-macro internals.Full changes
panic!sites inderive-encode/src/lib.rsnow emitcompile_error!tokens.#[prometheus(...)]attributes (which previously panicked inside a closure) are validated in a pre-pass so the derive can return acompile_error!token stream before generating the impl.compile_failtest inderive-encode/tests/build/friendly-compilation-error-msg.rscovers unnamed struct, unit struct, enum, union, and unknown-attribute paths. The expected.stderris committed alongside it.Testing
cargo test -p prometheus-client-derive-encode --test libpasses deterministically (with and withoutTRYBUILD=overwrite).cargo clippy -p prometheus-client-derive-encode --all-targets -- -D warningsclean.cargo fmt --check -p prometheus-client-derive-encodeclean.Notes
For valid inputs the derive behavior is unchanged. The new diagnostics are user-facing; no downstream tooling currently parses these derive errors, so this is not a breaking change.
Issues
Closes #269