Added support for negative --jobs parameter, counting backwards from max CPUs.

src/cargo/core/compiler/build_config.rs

@@ -50,14 +50,14 @@ impl BuildConfig {
     /// * `target.$target.libfoo.metadata`
     pub fn new(
         config: &Config,
-        jobs: Option<u32>,
+        jobs: Option<i32>,
         requested_targets: &[String],
         mode: CompileMode,
     ) -> CargoResult<BuildConfig> {
         let cfg = config.build_config()?;
         let requested_kinds = CompileKind::from_requested_targets(config, requested_targets)?;
         if jobs == Some(0) {
-            anyhow::bail!("jobs must be at least 1")
+            anyhow::bail!("jobs must not be zero")
         }
         if jobs.is_some() && config.jobserver_from_env().is_some() {
             config.shell().warn(
@@ -66,7 +66,11 @@ impl BuildConfig {
                  its environment, ignoring the `-j` parameter",
             )?;
         }
-        let jobs = jobs.or(cfg.jobs).unwrap_or(::num_cpus::get() as u32);
+        let jobs = match jobs.or(cfg.jobs) {
+            None => ::num_cpus::get() as u32,
+            Some(j) if j < 0 => (::num_cpus::get() as i32 + j).max(1) as u32,
+            Some(j) => j as u32,
+        };
 
         Ok(BuildConfig {
             requested_kinds,

src/cargo/core/compiler/context/mod.rs

@@ -95,8 +95,8 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
         let jobserver = match bcx.config.jobserver_from_env() {
             Some(c) => c.clone(),
             None => {
-                let client = Client::new(bcx.build_config.jobs as usize)
-                    .chain_err(|| "failed to create jobserver")?;
+                let client =
+                    Client::new(bcx.jobs() as usize).chain_err(|| "failed to create jobserver")?;
                 client.acquire_raw()?;
                 client
             }
@@ -558,7 +558,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
     }
 
     pub fn new_jobserver(&mut self) -> CargoResult<Client> {
-        let tokens = self.bcx.build_config.jobs as usize;
+        let tokens = self.bcx.jobs() as usize;
         let client = Client::new(tokens).chain_err(|| "failed to create jobserver")?;
 
         // Drain the client fully

src/cargo/core/compiler/timings.rs

@@ -461,7 +461,7 @@ impl<'cfg> Timings<'cfg> {
             self.total_dirty,
             self.total_fresh + self.total_dirty,
             max_concurrency,
-            bcx.build_config.jobs,
+            bcx.jobs(),
             num_cpus::get(),
             self.start_str,
             total_time,

src/cargo/ops/cargo_package.rs

@@ -27,7 +27,7 @@ pub struct PackageOpts<'cfg> {
     pub check_metadata: bool,
     pub allow_dirty: bool,
     pub verify: bool,
-    pub jobs: Option<u32>,
+    pub jobs: Option<i32>,
     pub targets: Vec<String>,
     pub features: Vec<String>,
     pub all_features: bool,

src/cargo/ops/registry.rs

@@ -47,7 +47,7 @@ pub struct PublishOpts<'cfg> {
     pub index: Option<String>,
     pub verify: bool,
     pub allow_dirty: bool,
-    pub jobs: Option<u32>,
+    pub jobs: Option<i32>,
     pub targets: Vec<String>,
     pub dry_run: bool,
     pub registry: Option<String>,

src/cargo/util/command_prelude.rs

@@ -67,7 +67,8 @@ pub trait AppExt: Sized {
         self._arg(
             opt("jobs", "Number of parallel jobs, defaults to # of CPUs")
                 .short("j")
-                .value_name("N"),
+                .value_name("N")
+                .allow_hyphen_values(true),
         )
     }
 
@@ -287,6 +288,16 @@ pub trait ArgMatchesExt {
         Ok(arg)
     }
 
+    fn value_of_i32(&self, name: &str) -> CargoResult<Option<i32>> {
+        let arg = match self._value_of(name) {
+            None => None,
+            Some(arg) => Some(arg.parse::<i32>().map_err(|_| {
+                clap::Error::value_validation_auto(format!("could not parse `{}` as a number", arg))
+            })?),
+        };
+        Ok(arg)
+    }
+
     /// Returns value of the `name` command-line argument as an absolute path
     fn value_of_path(&self, name: &str, config: &Config) -> Option<PathBuf> {
         self._value_of(name).map(|path| config.cwd().join(path))
@@ -320,8 +331,8 @@ pub trait ArgMatchesExt {
         Ok(ws)
     }
 
-    fn jobs(&self) -> CargoResult<Option<u32>> {
-        self.value_of_u32("jobs")
+    fn jobs(&self) -> CargoResult<Option<i32>> {
+        self.value_of_i32("jobs")
     }
 
     fn targets(&self) -> Vec<String> {

src/cargo/util/config/mod.rs

@@ -1874,7 +1874,7 @@ pub struct CargoBuildConfig {
     pub target_dir: Option<ConfigRelativePath>,
     pub incremental: Option<bool>,
     pub target: Option<ConfigRelativePath>,
-    pub jobs: Option<u32>,
+    pub jobs: Option<i32>,
     pub rustflags: Option<StringList>,
     pub rustdocflags: Option<StringList>,
     pub rustc_wrapper: Option<PathBuf>,

tests/testsuite/bad_config.rs

@@ -138,32 +138,6 @@ Caused by:
         .run();
 }
 
-#[cargo_test]
-fn bad_cargo_config_jobs() {
-    let p = project()
-        .file("src/lib.rs", "")
-        .file(
-            ".cargo/config",
-            r#"
-                [build]
-                jobs = -1
-            "#,
-        )
-        .build();
-    p.cargo("build -v")
-        .with_status(101)
-        .with_stderr(
-            "\
-[ERROR] error in [..].cargo/config: \
-could not load config key `build.jobs`
-
-Caused by:
-  invalid value: integer `-1`, expected u32
-",
-        )
-        .run();
-}
-
 #[cargo_test]
 fn invalid_global_config() {
     let p = project()

tests/testsuite/build.rs

@@ -4719,18 +4719,28 @@ fn good_cargo_config_jobs() {
     p.cargo("build -v").run();
 }
 
+#[cargo_test]
+fn good_jobs() {
+    let p = project()
+        .file("Cargo.toml", &basic_bin_manifest("foo"))
+        .file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
+        .build();
+
+    p.cargo("build --jobs 1").run();
+
+    p.cargo("build --jobs -1").run();
+}
+
 #[cargo_test]
 fn invalid_jobs() {
     let p = project()
         .file("Cargo.toml", &basic_bin_manifest("foo"))
         .file("src/foo.rs", &main_file(r#""i am foo""#, &[]))
         .build();
 
-    p.cargo("build --jobs -1")
-        .with_status(1)
-        .with_stderr_contains(
-            "error: Found argument '-1' which wasn't expected, or isn't valid in this context",
-        )
+    p.cargo("build --jobs 0")
+        .with_status(101)
+        .with_stderr_contains("error: jobs must not be zero")
         .run();
 
     p.cargo("build --jobs over9000")