rustbuild: Add support for a per-target default-linker option.

Author: apteryks

config.toml.example

@@ -488,9 +488,12 @@ changelog-seen = 2
 # FIXME(#75760): Some UI tests fail when this option is enabled.
 #parallel-compiler = false
 
-# The default linker that will be hard-coded into the generated compiler for
-# targets that don't specify linker explicitly in their target specifications.
-# Note that this is not the linker used to link said compiler.
+# The default linker that will be hard-coded into the generated
+# compiler for targets that don't specify a default linker explicitly
+# in their target specifications.  Note that this is not the linker
+# used to link said compiler. It can also be set per-target (via the
+# `[target.<triple>]` block), which may be useful in a cross-compilation
+# setting.
 #
 # See https://doc.rust-lang.org/rustc/codegen-options/index.html#linker for more information.
 #default-linker = <none> (path)

src/bootstrap/compile.rs

@@ -662,6 +662,8 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         .env("CFG_VERSION", builder.rust_version());
 
     let libdir_relative = builder.config.libdir_relative().unwrap_or_else(|| Path::new("lib"));
+    let target_config = builder.config.target_config.get(&target);
+
     cargo.env("CFG_LIBDIR_RELATIVE", libdir_relative);
 
     if let Some(ref ver_date) = builder.rust_info.commit_date() {
@@ -673,9 +675,15 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
     if !builder.unstable_features() {
         cargo.env("CFG_DISABLE_UNSTABLE_FEATURES", "1");
     }
-    if let Some(ref s) = builder.config.rustc_default_linker {
+
+    // Prefer the current target's own default_linker, else a globally
+    // specified one.
+    if let Some(s) = target_config.and_then(|c| c.default_linker.as_ref()) {
+        cargo.env("CFG_DEFAULT_LINKER", s);
+    } else if let Some(ref s) = builder.config.rustc_default_linker {
         cargo.env("CFG_DEFAULT_LINKER", s);
     }
+
     if builder.config.rustc_parallel {
         cargo.rustflag("--cfg=parallel_compiler");
         cargo.rustdocflag("--cfg=parallel_compiler");
@@ -700,7 +708,6 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
         }
         let llvm_config = builder.ensure(native::Llvm { target });
         cargo.env("LLVM_CONFIG", &llvm_config);
-        let target_config = builder.config.target_config.get(&target);
         if let Some(s) = target_config.and_then(|c| c.llvm_config.as_ref()) {
             cargo.env("CFG_LLVM_ROOT", s);
         }

src/bootstrap/config.rs

@@ -294,6 +294,7 @@ pub struct Target {
     pub cxx: Option<PathBuf>,
     pub ar: Option<PathBuf>,
     pub ranlib: Option<PathBuf>,
+    pub default_linker: Option<PathBuf>,
     pub linker: Option<PathBuf>,
     pub ndk: Option<PathBuf>,
     pub sanitizers: Option<bool>,
@@ -531,6 +532,7 @@ struct TomlTarget {
     cxx: Option<String>,
     ar: Option<String>,
     ranlib: Option<String>,
+    default_linker: Option<PathBuf>,
     linker: Option<String>,
     llvm_config: Option<String>,
     llvm_filecheck: Option<String>,