feat: add target specific rustflags to 'bootstrap.toml`

Author: karolzwolak

src/bootstrap/src/core/builder/cargo.rs

@@ -604,6 +604,15 @@ impl Builder<'_> {
             rustflags.arg(arg);
         }
 
+        // target specific rustflags take precedence over general rustflags
+        if let Some(target_rustflags) =
+            self.config.target_config.get(&target).map(|t| &t.rustflags[..])
+        {
+            for arg in target_rustflags {
+                rustflags.arg(&arg);
+            }
+        }
+
         rustflags.propagate_cargo_env("RUSTFLAGS");
 
         if build_compiler_stage != 0 {

src/bootstrap/src/core/config/config.rs

@@ -856,6 +856,7 @@ impl Config {
                     sanitizers: target_sanitizers,
                     profiler: target_profiler,
                     rpath: target_rpath,
+                    rustflags: target_rustflags,
                     crt_static: target_crt_static,
                     musl_root: target_musl_root,
                     musl_libdir: target_musl_libdir,
@@ -939,6 +940,7 @@ impl Config {
                 target.sanitizers = target_sanitizers;
                 target.profiler = target_profiler;
                 target.rpath = target_rpath;
+                target.rustflags = target_rustflags.unwrap_or_default();
                 target.optimized_compiler_builtins = target_optimized_compiler_builtins;
                 target.jemalloc = target_jemalloc;
                 if let Some(backends) = target_codegen_backends {

src/bootstrap/src/core/config/toml/target.rs

@@ -37,6 +37,7 @@ define_config! {
         sanitizers: Option<bool> = "sanitizers",
         profiler: Option<StringOrBool> = "profiler",
         rpath: Option<bool> = "rpath",
+        rustflags: Option<Vec<String>> = "rustflags",
         crt_static: Option<bool> = "crt-static",
         musl_root: Option<String> = "musl-root",
         musl_libdir: Option<String> = "musl-libdir",
@@ -70,6 +71,7 @@ pub struct Target {
     pub sanitizers: Option<bool>,
     pub profiler: Option<StringOrBool>,
     pub rpath: Option<bool>,
+    pub rustflags: Vec<String>,
     pub crt_static: Option<bool>,
     pub musl_root: Option<PathBuf>,
     pub musl_libdir: Option<PathBuf>,