bootstrap unable to copy `rust-llvm-dwp` when cross compiling rustc

[jam1garner]

Similar to #85590, this is an issue with copying files when cross compiling (Linux -> Windows), however a bit more complicated so I didn't proceed with a fix (don't mind implementing one but would need guidance).

thread 'main' panicked at 'src.symlink_metadata() failed with No such file or directory (os error 2)', src/bootstrap/lib.rs:1196:24
stack backtrace:

   0: rust_begin_unwind
             at /rustc/215738137bcbef2c3637a5bd290ef612cffe6ba5/library/std/src/panicking.rs:493:5
   1: std::panicking::begin_panic_fmt
             at /rustc/215738137bcbef2c3637a5bd290ef612cffe6ba5/library/std/src/panicking.rs:435:5
   2: bootstrap::Build::copy
             at ./src/bootstrap/lib.rs:1196:24
   3: <bootstrap::compile::Assemble as bootstrap::builder::Step>::run
             at ./src/bootstrap/compile.rs:1116:17
   4: bootstrap::builder::Builder::ensure
             at ./src/bootstrap/builder.rs:1518:23
   5: bootstrap::builder::Builder::compiler
             at ./src/bootstrap/builder.rs:585:9
   6: <bootstrap::tool::Rustdoc as bootstrap::builder::Step>::make_run
             at ./src/bootstrap/tool.rs:500:23
   7: bootstrap::builder::StepDescription::maybe_run
             at ./src/bootstrap/builder.rs:179:13
   8: bootstrap::builder::StepDescription::run
             at ./src/bootstrap/builder.rs:220:21
   9: bootstrap::builder::Builder::run_step_descriptions
             at ./src/bootstrap/builder.rs:577:9
  10: bootstrap::builder::Builder::execute_cli
             at ./src/bootstrap/builder.rs:569:9
  11: bootstrap::Build::build
             at ./src/bootstrap/lib.rs:510:13
  12: bootstrap::main
             at ./src/bootstrap/bin/main.rs:33:5
  13: core::ops::function::FnOnce::call_once
             at /rustc/215738137bcbef2c3637a5bd290ef612cffe6ba5/library/core/src/ops/function.rs:227:5

From adding debug prints it attempts to copy

.../rust/build/x86_64-unknown-linux-gnu/llvm/build/bin/llvm-dwp.exe

to

.../rust/build/x86_64-pc-windows-gnu/stage2/lib/rustlib/x86_64-pc-windows-gnu/bin/rust-llvm-dwp.exe

Looking at the Relevant code it appears to be running llvm-config from the host in order to find the copy of llvm-dwp to copy, however when cross compiling you can't just copy over the host's llvm-dwp. This results in an error due to the fact it does properly respect the exe suffix, which means the file won't exist when cross compiling as llvm-dwp.exe won't exist in the host copy of llvm's bin directory.

[jam1garner]

A potential fix could look something like this:

if builder.config.rust_codegen_backends.contains(&INTERNER.intern_str("llvm")) {
    let src_exe = exe("llvm-dwp", target_compiler.host);
    let dst_exe = exe("rust-llvm-dwp", target_compiler.host);
    let llvm_config_bin = builder.ensure(native::Llvm { target: target_compiler.host });
    if !builder.config.dry_run {
        if target_compiler.host == build_compiler.host {
            let llvm_bin_dir = output(Command::new(llvm_config_bin).arg("--bindir"));
            let llvm_bin_dir = Path::new(llvm_bin_dir.trim());
            builder.copy(&llvm_bin_dir.join(&src_exe), &libdir_bin.join(&dst_exe));
        } else {
            let llvm_bin_dir = format!("build/{}/llvm/build/bin", target_compiler.host);
            let llvm_bin_dir = Path::new(&llvm_bin_dir);
            builder.copy(&llvm_bin_dir.join(&src_exe), &libdir_bin.join(&dst_exe));
        }
    }
}

however, notably, I don't think format! is the proper way to do the above, I'd appreciate guidance on the best way to "properly" get the expected llvm build directory given the target triple.

(Note that the above is tested and appears to work, just isn't the best way to do it and I imagine breaks under some configuration of LLVM/config.toml/etc)

[jyn514]

@jam1garner do you want to make a PR with that fix? :) I think as long as it passes in CI it's no worse than the existing situation, and it sounds like it's fixed your problem.

[jam1garner]

I'll try and put together a PR, been a while though so I'll need to refamiliarize myself first