feat: add rust.rustflags and per target rustflags options to bootstrap.toml

This makes easy to persistently pass any flag to the compiler when
building rustc.
For example you can use a different linker by putting the following in
`bootstrap.toml`:
```toml
[rust]
rustflags = ["-Clinker=clang", "-Clink-arg=--ld-path=wild"]
```

Author: karolzwolak

bootstrap.example.toml

@@ -708,6 +708,12 @@
 # desired in distributions, for example.
 #rust.rpath = true
 
+# Additional flags to pass to `rustc`.
+# Takes precedence over bootstrap's own flags but not over per target rustflags nor env. vars. like RUSTFLAGS.
+# Applies to all stages and targets.
+#
+#rust.rustflags = []
+
 # Indicates whether symbols should be stripped using `-Cstrip=symbols`.
 #rust.strip = false
 
@@ -1013,6 +1019,12 @@
 # and will override the same option under [rust] section. It only works on Unix platforms
 #rpath = rust.rpath (bool)
 
+# Additional flags to pass to `rustc`.
+# Takes precedence over bootstrap's own flags and `rust.rustflags` but not over env. vars. like RUSTFLAGS.
+# Applies to all stages.
+#
+#rustflags = rust.rustflags
+
 # Force static or dynamic linkage of the standard library for this target. If
 # this target is a host for rustc, this will also affect the linkage of the
 # compiler itself. This is useful for building rustc on targets that normally

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

@@ -105,6 +105,7 @@ pub struct Cargo {
     allow_features: String,
     release_build: bool,
     build_compiler_stage: u32,
+    extra_rustflags: Vec<String>,
 }
 
 impl Cargo {
@@ -403,6 +404,11 @@ impl From<Cargo> for BootstrapCommand {
             cargo.args.insert(0, "--release".into());
         }
 
+        for arg in &cargo.extra_rustflags {
+            cargo.rustflags.arg(arg);
+            cargo.rustdocflags.arg(arg);
+        }
+
         // Propagate the envs here at the very end to make sure they override any previously set flags.
         cargo.rustflags.propagate_rustflag_envs(cargo.build_compiler_stage);
         cargo.rustdocflags.propagate_rustflag_envs(cargo.build_compiler_stage);
@@ -1379,6 +1385,15 @@ impl Builder<'_> {
             rustflags.arg("-Zmir_strip_debuginfo=locals-in-tiny-functions");
         }
 
+        // take target-specific extra rustflags if any otherwise take `rust.rustflags`
+        let extra_rustflags = self
+            .config
+            .target_config
+            .get(&target)
+            .map(|t| &t.rustflags)
+            .unwrap_or(&self.config.rust_rustflags)
+            .clone();
+
         let release_build = self.config.rust_optimize.is_release() &&
             // cargo bench/install do not accept `--release` and miri doesn't want it
             !matches!(cmd_kind, Kind::Bench | Kind::Install | Kind::Miri | Kind::MiriSetup | Kind::MiriTest);
@@ -1394,6 +1409,7 @@ impl Builder<'_> {
             allow_features,
             release_build,
             build_compiler_stage,
+            extra_rustflags,
         }
     }
 }

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

@@ -225,6 +225,7 @@ pub struct Config {
     pub rust_std_features: BTreeSet<String>,
     pub rust_break_on_ice: bool,
     pub rust_parallel_frontend_threads: Option<u32>,
+    pub rust_rustflags: Vec<String>,
 
     pub llvm_profile_use: Option<String>,
     pub llvm_profile_generate: bool,
@@ -575,6 +576,7 @@ impl Config {
             bootstrap_override_lld_legacy: rust_bootstrap_override_lld_legacy,
             std_features: rust_std_features,
             break_on_ice: rust_break_on_ice,
+            rustflags: rust_rustflags,
         } = toml.rust.unwrap_or_default();
 
         let Llvm {
@@ -864,6 +866,7 @@ 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,
@@ -947,6 +950,7 @@ 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 {
@@ -1441,6 +1445,7 @@ impl Config {
             rust_randomize_layout: rust_randomize_layout.unwrap_or(false),
             rust_remap_debuginfo: rust_remap_debuginfo.unwrap_or(false),
             rust_rpath: rust_rpath.unwrap_or(true),
+            rust_rustflags: rust_rustflags.unwrap_or_default(),
             rust_stack_protector,
             rust_std_features: rust_std_features
                 .unwrap_or(BTreeSet::from([String::from("panic-unwind")])),

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

@@ -33,6 +33,7 @@ define_config! {
         channel: Option<String> = "channel",
         musl_root: Option<String> = "musl-root",
         rpath: Option<bool> = "rpath",
+        rustflags: Option<Vec<String>> = "rustflags",
         strip: Option<bool> = "strip",
         frame_pointers: Option<bool> = "frame-pointers",
         stack_protector: Option<String> = "stack-protector",
@@ -375,6 +376,7 @@ pub fn check_incompatible_options_for_ci_rustc(
         parallel_frontend_threads: _,
         bootstrap_override_lld: _,
         bootstrap_override_lld_legacy: _,
+        rustflags: _,
     } = ci_rust_config;
 
     // There are two kinds of checks for CI rustc incompatible options:

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>,

src/bootstrap/src/utils/change_tracker.rs

@@ -596,4 +596,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "The `-Zannotate-moves` option is now always enabled when building rustc, sysroot and tools.",
     },
+    ChangeInfo {
+        change_id: 148795,
+        severity: ChangeSeverity::Info,
+        summary: "New options `rust.rustflags` for all targets and `rustflags` par target that will pass specified flags to rustc for all stages. Target specific flags override global `rust.rustflags` ones.",
+    },
 ];