bootstrap: allow setting --jobs in config.toml

config.example.toml

@@ -414,6 +414,10 @@
 # Specify the location of the Android NDK. Used when targeting Android.
 #android-ndk = "/path/to/android-ndk-r26d"
 
+# Number of parallel jobs to be used for building and testing. If set to `0` or
+# omitted, it will be automatically determined.
+#jobs = 0
+
 # =============================================================================
 # General install configuration options
 # =============================================================================

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

@@ -891,6 +891,7 @@ define_config! {
         metrics: Option<bool> = "metrics",
         android_ndk: Option<PathBuf> = "android-ndk",
         optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
+        jobs: Option<u32> = "jobs",
     }
 }
 
@@ -1511,6 +1512,7 @@ impl Config {
             metrics: _,
             android_ndk,
             optimized_compiler_builtins,
+            jobs,
         } = toml.build.unwrap_or_default();
 
         if let Some(file_build) = build {
@@ -1621,6 +1623,12 @@ impl Config {
         // Verbose flag is a good default for `rust.verbose-tests`.
         config.verbose_tests = config.is_verbose();
 
+        if let Some(jobs) = jobs {
+            if jobs > 0 {
+                config.jobs = Some(jobs);
+            }
+        }
+
         if let Some(install) = toml.install {
             let Install { prefix, sysconfdir, docdir, bindir, libdir, mandir, datadir } = install;
             config.prefix = prefix.map(PathBuf::from);

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

@@ -122,6 +122,7 @@ fn override_toml() {
             "--set=rust.deny-warnings=false".to_owned(),
             "--set=build.gdb=\"bar\"".to_owned(),
             "--set=build.tools=[\"cargo\"]".to_owned(),
+            "--set=build.jobs=1".to_owned(),
             "--set=llvm.build-config={\"foo\" = \"bar\"}".to_owned(),
             "--set=target.x86_64-unknown-linux-gnu.runner=bar".to_owned(),
             "--set=target.x86_64-unknown-linux-gnu.rpath=false".to_owned(),
@@ -139,6 +140,7 @@ deny-warnings = true
 [build]
 gdb = "foo"
 tools = []
+jobs = 2
 
 [llvm]
 download-ci-llvm = false
@@ -171,6 +173,7 @@ runner = "x86_64-runner"
         Some(["cargo".to_string()].into_iter().collect()),
         "setting list value"
     );
+    assert_eq!(config.jobs, Some(1));
     assert_eq!(
         config.llvm_build_config,
         [("foo".to_string(), "bar".to_string())].into_iter().collect(),
@@ -352,3 +355,9 @@ fn parse_rust_std_features_empty() {
 fn parse_rust_std_features_invalid() {
     parse("rust.std-features = \"backtrace\"");
 }
+
+#[test]
+fn parse_jobs_zero() {
+    assert_eq!(parse("build.jobs = 0").jobs, None);
+    assert_eq!(parse("build.jobs = 1").jobs, Some(1));
+}