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Document -Zcache-proc-macros #151227

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3 changes: 3 additions & 0 deletions src/doc/unstable-book/src/compiler-flags/cache-proc-macros.md
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## `cache-proc-macros`

This option instructs `rustc` to cache (derive) proc-macro invocations using the incremental system. Note that this can be unsouned - we currently do not check if the macro invocation is actually "cacheable" or not.
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@lqd lqd Jan 17, 2026

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This option instructs `rustc` to cache (derive) proc-macro invocations using the incremental system. Note that this can be unsouned - we currently do not check if the macro invocation is actually "cacheable" or not.
This option instructs `rustc` to cache (derive) proc-macro invocations using the incremental system. Note that this can be unsound - we currently do not check if the macro invocation is actually "cacheable" or not.

also, about the "we currently do not check" part, is the plan is to infer whether the proc-macro is cacheable (seems unlikely), or rather respect what the user promises about its behavior? This could just mention that enabling the flag for proc-macros that are not pure functions is bound to reuse stale extensions.

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