fix(compiler/rustc_target): set correct linker flags for wasm32v1-none

[StackOverflowExcept1on]

Resolves #145491

r? @alexcrichton

https://github.com/llvm/llvm-project/blob/llvmorg-20.1.8/lld/wasm/Writer.cpp#L573-L691

https://releases.llvm.org/20.1.0/docs/ReleaseNotes.html#changes-to-the-webassembly-backend

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[bjorn3]

Disabling feature checking doesn't seem right to me. Why is that necessary?

[StackOverflowExcept1on]

https://github.com/llvm/llvm-project/blob/llvmorg-20.1.8/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp#L249-L257
because if there are 0 functions in the LLVM IR module, then AnyDefinedFuncs = false and this will result in all features from wasm32-unknown-unknown being enabled

[StackOverflowExcept1on]

hmm, you're right in sense. because it will break -C target-feature=+...

[StackOverflowExcept1on]

it seems that the correct fix is to somehow set the fallback to cpu="mvp" instead of cpu="" in wasm-ld

[bjorn3]

Maybe adding a --mllvm=-mcpu=<target_cpu> argument at

rust/compiler/rustc_codegen_ssa/src/back/linker.rs

Line 1489 in 425a9c0

self.link_arg(&format!("--lto-{opt_level}"));

would work? I don't know if that is the correct way to pass the target cpu to wasm-ld though.

[StackOverflowExcept1on]

it works

[StackOverflowExcept1on]

although it seems that this is problem with -C target-feature=+...

[bjorn3]

Doing it at the place I indicated (the WasmLd implementation) has the advantage that it works for all wasm targets and also when the user passes -Ctarget-cpu to override the target default.

[StackOverflowExcept1on]

Yes, that's probably good idea, but we need WasmTM->CPU to be set from the command line.

[bjorn3]

wasm-ld can't notice any difference between setting this cli arg in the target spec and setting it in the WasmLd implementation I linked to.

[StackOverflowExcept1on]

Yes, you're right. I checked in the debugger and it's also worth adding --mllvm=-mattr=+mutable-globals to change WasmTM->TargetFS (features set).