Auto merge of #51936 - japaric:rust-lld, r=alexcrichton

rename rustc's lld to rust-lld

to not shadow the system installed LLD when linking with LLD.

Before:

- `-C linker=lld -Z linker-flavor=ld.lld` uses rustc's LLD
- It's not possible to use a system installed LLD that's named `lld`

With this commit:

- `-C linker=rust-lld -Z linker-flavor=ld.lld` uses rustc's LLD
- `-C linker=lld -Z linker-flavor=ld.lld` uses the system installed LLD

we don't offer guarantees about the availability of LLD in the rustc sysroot so we can rename the tool as long as we don't break the wasm32-unknown-unknown target which depends on it.

r? @alexcrichton we discussed this before

Author: bors

src/bootstrap/compile.rs

@@ -821,8 +821,11 @@ fn copy_lld_to_sysroot(builder: &Builder,
         .join("bin");
     t!(fs::create_dir_all(&dst));
 
-    let exe = exe("lld", &target);
-    builder.copy(&lld_install_root.join("bin").join(&exe), &dst.join(&exe));
+    let src_exe = exe("lld", &target);
+    let dst_exe = exe("rust-lld", &target);
+    // we prepend this bin directory to the user PATH when linking Rust binaries. To
+    // avoid shadowing the system LLD we rename the LLD we provide to `rust-lld`.
+    builder.copy(&lld_install_root.join("bin").join(&src_exe), &dst.join(&dst_exe));
 }
 
 /// Cargo's output path for the standard library in a given stage, compiled

src/bootstrap/dist.rs

@@ -493,12 +493,13 @@ impl Step for Rustc {
 
             // Copy over lld if it's there
             if builder.config.lld_enabled {
-                let exe = exe("lld", &compiler.host);
+                let exe = exe("rust-lld", &compiler.host);
                 let src = builder.sysroot_libdir(compiler, host)
                     .parent()
                     .unwrap()
                     .join("bin")
                     .join(&exe);
+                // for the rationale about this rename check `compile::copy_lld_to_sysroot`
                 let dst = image.join("lib/rustlib")
                     .join(&*host)
                     .join("bin")

src/librustc_target/spec/wasm32_unknown_unknown.rs

@@ -51,6 +51,9 @@ pub fn target() -> Result<Target, String> {
         // no dynamic linking, no need for default visibility!
         default_hidden_visibility: true,
 
+        // we use the LLD shipped with the Rust toolchain by default
+        linker: Some("rust-lld".to_owned()),
+
         .. Default::default()
     };
     Ok(Target {