Rollup merge of #142069 - nnethercote:Zmacro-stats, r=petrochenkov

Introduce `-Zmacro-stats`

Introduce `-Zmacro-stats`.

It collects data about macro expansions and prints them in a table after expansion finishes. It's very useful for detecting macro bloat, especially for proc macros.

r? `@petrochenkov`

1 files changed, 24 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/compiler-flags/macro-stats.md b/src/doc/unstable-book/src/compiler-flags/macro-stats.md
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+# `macro-stats`
+
+This feature is perma-unstable and has no tracking issue.
+
+----
+
+Some macros, especially procedural macros, can generate a surprising amount of
+code, which can slow down compile times. This is hard to detect because the
+generated code is normally invisible to the programmer.
+
+This flag helps identify such cases. When enabled, the compiler measures the
+effect on code size of all used macros and prints a table summarizing that
+effect. For each distinct macro, it counts how many times it is used, and the
+net effect on code size (in terms of lines of code, and bytes of code). The
+code size evaluation uses the compiler's internal pretty-printing, and so will
+be independent of the formatting in the original code.
+
+Note that the net effect of a macro may be negative. E.g. the `cfg!` and
+`#[test]` macros often strip out code.
+
+If a macro is identified as causing a large increase in code size, it is worth
+using `cargo expand` to inspect the post-expansion code, which includes the
+code produced by all macros. It may be possible to optimize the macro to
+produce smaller code, or it may be possible to avoid using it altogether.