Use write-if-changed pattern in connectrpc-build#22
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Skip writing output files when content is already identical, preserving mtime so Cargo doesn't spuriously recompile downstream crates. Cargo's rebuild decision for include!-ed files is mtime-based (rustc dep-info lists the file, Cargo compares mtime vs fingerprint). Before this change, touching any single .proto file re-ran the build script and unconditionally rewrote every output .rs, bumping all their mtimes and cascading into a full recompile even when N-1 of N files were byte-identical. Mirrors anthropics/buffa#17 and prost-build's write_file_if_changed.
asacamano
approved these changes
Apr 1, 2026
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Skip writing output files when their content is already identical to what's on disk, preserving mtime so Cargo doesn't spuriously recompile downstream crates.
Why
Cargo's rebuild decision for
include!-ed files is mtime-based — rustc's dep-info (.d) file lists every included path, and Cargo compares each one's mtime against the stored fingerprint timestamp. Verified empirically:touch $OUT_DIR/gen.rs(content unchanged) →cargo build -vreportsDirty ...: the file 'gen.rs' has changed (mtime 1774896316s, 1m 14s after last build)and recompiles.Before this change, touching any single
.protofile re-ran the build script (correct, percargo:rerun-if-changed), which then unconditionally rewrote every output.rsfile — bumping all their mtimes and cascading into a full downstream recompile even when N-1 of N generated files were byte-identical.Change
Two call sites in
Config::compile()now go throughwrite_if_changed(read-compare-conditionally-write). The now-unuseduse std::io::Writeis removed. Three tests cover create/skip-identical/overwrite. Uses edition-2024 let-chain syntax for the nested condition.Prior art
buffa-buildprost-buildconfig.rs:1179—write_file_if_changedwith the identical implementation