bootstrap: allow setting --jobs in config.toml

config.example.toml

@@ -414,6 +414,11 @@
 # 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. This is the `-j`/`--jobs` flag
+# passed to cargo invocations.
+#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",
     }
 }
 
@@ -1289,7 +1290,8 @@ impl Config {
         config.rustc_error_format = flags.rustc_error_format;
         config.json_output = flags.json_output;
         config.on_fail = flags.on_fail;
-        config.jobs = Some(threads_from_config(flags.jobs as u32));
+        config.jobs = flags.jobs.filter(|j| *j > 0);
+
         config.cmd = flags.cmd;
         config.incremental = flags.incremental;
         config.dry_run = if flags.dry_run { DryRun::UserSelected } else { DryRun::Disabled };
@@ -1511,8 +1513,19 @@ impl Config {
             metrics: _,
             android_ndk,
             optimized_compiler_builtins,
+            jobs,
         } = toml.build.unwrap_or_default();
 
+        if config.jobs.is_none() {
+            config.jobs = match jobs {
+                Some(jobs) if jobs > 0 => Some(jobs),
+                _ => Some(
+                    std::thread::available_parallelism().map_or(1, std::num::NonZeroUsize::get)
+                        as u32,
+                ),
+            };
+        }
+
         if let Some(file_build) = build {
             config.build = TargetSelection::from_user(&file_build);
         };

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

@@ -110,11 +110,10 @@ pub struct Flags {
         short,
         long,
         value_hint = clap::ValueHint::Other,
-        default_value_t = std::thread::available_parallelism().map_or(1, std::num::NonZeroUsize::get),
         value_name = "JOBS"
     )]
     /// number of jobs to run in parallel
-    pub jobs: usize,
+    pub jobs: Option<u32>,
     // This overrides the deny-warnings configuration option,
     // which passes -Dwarnings to the compiler invocations.
     #[arg(global = true, long)]

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

@@ -171,6 +171,7 @@ 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 +353,61 @@ fn parse_rust_std_features_empty() {
 fn parse_rust_std_features_invalid() {
     parse("rust.std-features = \"backtrace\"");
 }
+
+#[test]
+fn parse_jobs() {
+    assert_eq!(parse("build.jobs = 1").jobs, Some(1));
+}
+
+#[test]
+fn jobs_precedence() {
+    // `--jobs` should take precedence over using `--set build.jobs`.
+
+    let config = Config::parse_inner(
+        Flags::parse(&[
+            "check".to_owned(),
+            "--config=/does/not/exist".to_owned(),
+            "--jobs=67890".to_owned(),
+            "--set=build.jobs=12345".to_owned(),
+        ]),
+        |&_| toml::from_str(""),
+    );
+    assert_eq!(config.jobs, Some(67890));
+
+    // `--set build.jobs` should take precedence over `config.toml`.
+    let config = Config::parse_inner(
+        Flags::parse(&[
+            "check".to_owned(),
+            "--config=/does/not/exist".to_owned(),
+            "--set=build.jobs=12345".to_owned(),
+        ]),
+        |&_| {
+            toml::from_str(
+                r#"
+            [build]
+            jobs = 67890
+        "#,
+            )
+        },
+    );
+    assert_eq!(config.jobs, Some(67890));
+
+    // `--jobs` > `--set build.jobs` > `config.toml`
+    let config = Config::parse_inner(
+        Flags::parse(&[
+            "check".to_owned(),
+            "--jobs=123".to_owned(),
+            "--config=/does/not/exist".to_owned(),
+            "--set=build.jobs=456".to_owned(),
+        ]),
+        |&_| {
+            toml::from_str(
+                r#"
+            [build]
+            jobs = 789
+        "#,
+            )
+        },
+    );
+    assert_eq!(config.jobs, Some(123));
+}

src/bootstrap/src/utils/change_tracker.rs

@@ -275,4 +275,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "New option `./x setup editor` added, replacing `./x setup vscode` and adding support for vim, emacs and helix.",
     },
+    ChangeInfo {
+        change_id: 131838,
+        severity: ChangeSeverity::Info,
+        summary: "Allow setting `--jobs` in config.toml with `build.jobs`.",
+    },
 ];