[WIP] Use weak linkage instead of compiler generated shims

[bjorn3]

#86844 (unstably) made it possible to avoid the allocator shim when using #[global_allocator]. This PR makes it possible to also avoid the allocator shim when using the default allocator in libstd by making use of weak linkage. Eventual stabilization of avoiding the allocator shim may be blocked on it working with the default allocator too.

This is still keeping the __rust_no_alloc_shim_is_unstable symbol for now until #123015 gets stabilized.

TODO: Update comments everywhere, test on macOS and Windows and write a better PR description why we want this. Also prevent codegen of weak symbols when there is a non-weak definition of the same symbol in the same codegen unit.

try-job: x86_64-msvc
try-zzz-job: x86_64-apple-1
try-zzz-job: x86_64-apple-2

[rustbot]

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[bjorn3]

@bors try

[bors]

⌛ Trying commit abd406c with merge 25f4036...

[bors]

💔 Test failed - checks-actions

[bjorn3]

@bors try

[bors]

⌛ Trying commit 3e25d5a with merge 3659c42...

[bors]

💔 Test failed - checks-actions

[bjorn3]

@bors try

[bors]

⌛ Trying commit 3e25d5a with merge f0b0345...

[bors]

☔ The latest upstream changes (presumably #134604) made this pull request unmergeable. Please resolve the merge conflicts.

[bjorn3]

@bors try

[bors]

⌛ Trying commit 7952352 with merge d5915d9...

[bors]

☀️ Try build successful - checks-actions
Build commit: d5915d9 (d5915d9567206920e25d38421ce09513599a9d53)

[bjorn3]

And for windows.

@bors try

[bors]

⌛ Trying commit 7952352 with merge 0b44463...

[bors]

💔 Test failed - checks-actions

[bors]

☔ The latest upstream changes (presumably #136035) made this pull request unmergeable. Please resolve the merge conflicts.

[ChrisDenton]

I don't think anyone has worked on Rust's support for Windows weak linkage yet? Maybe extern_weak would work on Windows but I'm not even sure that's hooked up properly yet.

IIRC #[linkage] is just forwarding straight to LLVM and not always correctly.

[bors]

☔ The latest upstream changes (presumably #136751) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-log-analyzer]

The job mingw-check-2 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error[E0308]: mismatched types
   --> src/tools/miri/src/shims/foreign_items.rs:54:69
    |
54  |         match this.emulate_foreign_item_inner(link_name, abi, args, &dest)? {
    |                    --------------------------                       ^^^^^ expected `&MPlaceTy<'_, Provenance>`, found `&&PlaceTy<'_, Provenance>`
    |                    |
    |                    arguments to this method are incorrect
    |
    = note: expected reference `&rustc_const_eval::interpret::MPlaceTy<'_, machine::Provenance>`
               found reference `&&rustc_const_eval::interpret::PlaceTy<'tcx, machine::Provenance>`
---
    |
130 |                     if symbol_name == link_name {
    |                                    ^^ no implementation for `&str == rustc_span::Symbol`
    |
    = help: the trait `std::cmp::PartialEq<rustc_span::Symbol>` is not implemented for `&str`
    = help: the following other types implement trait `std::cmp::PartialEq<Rhs>`:
              `&str` implements `std::cmp::PartialEq<ByteString>`
              `&str` implements `std::cmp::PartialEq<Cow<'_, str>>`
              `&str` implements `std::cmp::PartialEq<OsString>`
              `&str` implements `std::cmp::PartialEq<bstr::bstr::BStr>`
              `&str` implements `std::cmp::PartialEq<bstr::bstring::BString>`
              `&str` implements `std::cmp::PartialEq<rustc_target::json::Json>`
              `&str` implements `std::cmp::PartialEq<std::bstr::ByteStr>`
              `&str` implements `std::cmp::PartialEq<std::string::String>`
            and 13 others

error[E0433]: failed to resolve: use of undeclared type `Conv`
   --> src/tools/miri/src/shims/foreign_items.rs:509:61
    |
509 |                     let [size, align] = ecx.check_shim(abi, Conv::Rust, link_name, args)?;
    |                                                             ^^^^ use of undeclared type `Conv`

error[E0433]: failed to resolve: use of undeclared type `Conv`
   --> src/tools/miri/src/shims/foreign_items.rs:544:62
    |
544 |                     let [size, align] = this.check_shim(abi, Conv::Rust, link_name, args)?;
    |                                                              ^^^^ use of undeclared type `Conv`

error[E0433]: failed to resolve: use of undeclared type `Conv`
   --> src/tools/miri/src/shims/foreign_items.rs:571:45
    |
571 |                         ecx.check_shim(abi, Conv::Rust, link_name, args)?;
    |                                             ^^^^ use of undeclared type `Conv`

error[E0433]: failed to resolve: use of undeclared type `Conv`
   --> src/tools/miri/src/shims/foreign_items.rs:606:46
    |
606 |                         this.check_shim(abi, Conv::Rust, link_name, args)?;
    |                                              ^^^^ use of undeclared type `Conv`

error[E0277]: can't compare `&str` with `rustc_span::Symbol`
   --> src/tools/miri/src/shims/foreign_items.rs:130:36
    |
130 |                     if symbol_name == link_name {
    |                                    ^^ no implementation for `&str == rustc_span::Symbol`
    |
    = help: the trait `std::cmp::PartialEq<rustc_span::Symbol>` is not implemented for `&str`
    = help: the following other types implement trait `std::cmp::PartialEq<Rhs>`:
              `&str` implements `std::cmp::PartialEq<ByteString>`
              `&str` implements `std::cmp::PartialEq<Cow<'_, str>>`
              `&str` implements `std::cmp::PartialEq<OsString>`
              `&str` implements `std::cmp::PartialEq<bstr::BStr>`
              `&str` implements `std::cmp::PartialEq<bstr::BString>`
              `&str` implements `std::cmp::PartialEq<bstr::bstr::BStr>`
              `&str` implements `std::cmp::PartialEq<bstr::bstring::BString>`
              `&str` implements `std::cmp::PartialEq<camino::Utf8Path>`
            and 24 others

Some errors have detailed explanations: E0277, E0308, E0433.
For more information about an error, try `rustc --explain E0277`.
[RUSTC-TIMING] miri test:false 3.888