Rollup merge of #125263 - lqd:lld-fallback, r=petrochenkov

rust-lld: fallback to rustc's sysroot if there's no path to the linker in the target sysroot

As seen in #125246, some sysroots don't expect to contain `rust-lld` and want to keep it that way, so we fallback to the default rustc sysroot if there is no path to the linker in any of the sysroot tools search paths. This is how we locate codegen-backends' dylibs already.

People also have requested an error if none of these search paths contain the self-contained linker directory, so there's also an error in that case.

r? `@petrochenkov` cc `@ehuss` `@RalfJung`

I'm not sure where we check for `rust-lld`'s existence on the targets where we use it by default, and if we just ignore it when missing or emit a warning (as I assume we don't emit an error), so I just checked for the existence of `gcc-ld`, where `cc` will look for the lld-wrapper binaries.

<sub>*Feel free to point out better ways to do this, it's the middle of the night here.*</sub>

Fixes #125246

compiler/rustc_codegen_ssa/messages.ftl

@@ -212,6 +212,8 @@ codegen_ssa_rlib_only_rmeta_found = could not find rlib for: `{$crate_name}`, fo
 
 codegen_ssa_select_cpp_build_tool_workload = in the Visual Studio installer, ensure the "C++ build tools" workload is selected
 
+codegen_ssa_self_contained_linker_missing = the self-contained linker was requested, but it wasn't found in the target's sysroot, or in rustc's sysroot
+
 codegen_ssa_shuffle_indices_evaluation = could not evaluate shuffle_indices at compile time
 
 codegen_ssa_specify_libraries_to_link = use the `-l` flag to specify native libraries to link

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -3140,13 +3140,21 @@ fn add_lld_args(
 
     let self_contained_linker = self_contained_cli || self_contained_target;
     if self_contained_linker && !sess.opts.cg.link_self_contained.is_linker_disabled() {
+        let mut linker_path_exists = false;
         for path in sess.get_tools_search_paths(false) {
+            let linker_path = path.join("gcc-ld");
+            linker_path_exists |= linker_path.exists();
             cmd.arg({
                 let mut arg = OsString::from("-B");
-                arg.push(path.join("gcc-ld"));
+                arg.push(linker_path);
                 arg
             });
         }
+        if !linker_path_exists {
+            // As a sanity check, we emit an error if none of these paths exist: we want
+            // self-contained linking and have no linker.
+            sess.dcx().emit_fatal(errors::SelfContainedLinkerMissing);
+        }
     }
 
     // 2. Implement the "linker flavor" part of this feature by asking `cc` to use some kind of

compiler/rustc_codegen_ssa/src/errors.rs

@@ -413,6 +413,10 @@ pub struct UnableToExeLinker {
 #[diag(codegen_ssa_msvc_missing_linker)]
 pub struct MsvcMissingLinker;
 
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_self_contained_linker_missing)]
+pub struct SelfContainedLinkerMissing;
+
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_check_installed_visual_studio)]
 pub struct CheckInstalledVisualStudio;