bootstrap: support building rustc with monolithic lto

[matthiaskrgr]

I would like to be able to locally build rustc with monolithic lto enabled (-C lto) however it seems that using lto=true in src/Carto.toml does not have any effect.

RUSTFLAGS="-C lto" ./x.py build fails as well

Building stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compiling std v0.0.0 (file:///home/matthias/RUST/rust_build/src/libstd)
error: cannot prefer dynamic linking when performing LTO
note: only 'staticlib', 'bin', and 'cdylib' outputs are supported with LTO
error: aborting due to previous error
error: Could not compile `std`.

Edit: related: #49180

[hanna-kruppe]

rustc is primarily composed of dylibs, which as the error message states are incompatible with LTO. So an LTO'd rustc is fundamentally impossible, at least until we get rid of the dylibs (they're currently required for compiler plugins and proc macros).

[matthiaskrgr]

Hmm so forcing everything to be linked statically is not possible at the moment, right?
// cc #39998

[sanxiyn]

No, it is not possible.

[Mark-Simulacrum]

I'm going to close this as it's not feasible today and probably not something we'll want to do for quite some time if ever.

[matthiaskrgr]

@Mark-Simulacrum uhm, #49180 says that it would desirable to bootstrap with monolithic lto

[Mark-Simulacrum]

True, but I don't think a dedicated issue will help move this along at this point -- I don't particularly care either way though.