Support out-dir in build section of Cargo configuration file

Fixed #7555.

src/bin/cargo/commands/build.rs

@@ -63,7 +63,12 @@ pub fn exec(config: &mut Config, args: &ArgMatches<'_>) -> CliResult {
         ProfileChecking::Checked,
     )?;
 
-    compile_opts.export_dir = args.value_of_path("out-dir", config);
+    if let Some(out_dir) = args.value_of_path("out-dir", config) {
+        compile_opts.export_dir = Some(out_dir);
+    } else if let Some(out_dir) = config.build_config()?.out_dir.as_ref() {
+        let out_dir = out_dir.resolve_path(config);
+        compile_opts.export_dir = Some(out_dir);
+    }
     if compile_opts.export_dir.is_some() {
         config
             .cli_unstable()

src/cargo/util/config/mod.rs

@@ -1643,6 +1643,7 @@ pub struct CargoBuildConfig {
     pub rustc_wrapper: Option<PathBuf>,
     pub rustc: Option<PathBuf>,
     pub rustdoc: Option<PathBuf>,
+    pub out_dir: Option<ConfigRelativePath>,
 }
 
 /// A type to deserialize a list of strings from a toml file.

src/doc/src/reference/unstable.md

@@ -76,6 +76,13 @@ directory. Example:
 cargo +nightly build --out-dir=out -Z unstable-options
 ```
 
+This can also be specified in `.cargo/config` files.
+
+```toml
+[build]
+out-dir = "out"
+```
+
 ### doctest-xcompile
 * Tracking Issue: [#7040](https://github.com/rust-lang/cargo/issues/7040)
 * Tracking Rustc Issue: [#64245](https://github.com/rust-lang/rust/issues/64245)

tests/testsuite/out_dir.rs

@@ -246,6 +246,30 @@ fn avoid_build_scripts() {
     );
 }
 
+#[cargo_test]
+fn cargo_build_out_dir() {
+    let p = project()
+        .file("src/main.rs", r#"fn main() { println!("Hello, World!") }"#)
+        .file(
+            ".cargo/config",
+            r#"
+            [build]
+            out-dir = "out"
+            "#,
+        )
+        .build();
+
+    p.cargo("build -Z unstable-options")
+        .masquerade_as_nightly_cargo()
+        .run();
+    check_dir_contents(
+        &p.root().join("out"),
+        &["foo"],
+        &["foo", "foo.dSYM"],
+        &["foo.exe", "foo.pdb"],
+    );
+}
+
 fn check_dir_contents(
     out_dir: &Path,
     expected_linux: &[&str],