Auto merge of #137535 - Kobzol:split-metadata, r=petrochenkov

Introduce `-Zembed-metadata` to allow omitting full metadata from rlibs and dylibs

This is a continuation of https://github.com/rust-lang/rust/pull/120855 (I was mentored by `@bjorn3` to move it forward). Most of the original code was written by bjorn3, I tried to clean it up a bit and add some documentation and tests.

This PR introduces a new unstable compiler flag called `-Zembed-metadata=[no|yes]`, with the default being `yes` (see https://github.com/rust-lang/rust/issues/57076 for context). When set to `no`, rustc will only store a small metadata stub inside rlibs/dylibs instead of the full metadata, to keep their size smaller. It should be used in combination with `--emit=metadata`, so that the users of such a compiled library can still read the metadata from the corresponding `.rmeta` file. [This comment](https://github.com/rust-lang/rust/pull/120855#issuecomment-1937018169) shows an example of binary/artifact size wins that can be achieved using this approach.

Contrary to https://github.com/rust-lang/rust/pull/120855, this PR only introduces the new flag, along with a couple of run-make tests and documentation, but does not yet use it in bootstrap to actually compile rustc. I plan to do that as a follow-up step (along with integration in Cargo, which should ideally just always pass this flag to reduce the size of target directories).

Fixes https://github.com/rust-lang/rust/issues/23366
Closes https://github.com/rust-lang/rust/issues/29511
Fixes https://github.com/rust-lang/rust/issues/57076

Another attempt of https://github.com/rust-lang/rust/pull/93945 and https://github.com/rust-lang/rust/pull/120855.

r? `@petrochenkov`

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index b44be1710ed..4592e014438 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -787,6 +787,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(direct_access_external_data, Some(true));
     tracked!(dual_proc_macros, true);
     tracked!(dwarf_version, Some(5));
+    tracked!(embed_metadata, false);
     tracked!(embed_source, true);
     tracked!(emit_thin_lto, false);
     tracked!(emscripten_wasm_eh, true);