support custom clippy

Similar to cargo, rustc, and rustfmt, this adds the support of using custom clippy on bootstrap.
It’s designed for those who want to test their own clippy builds or avoid downloading the stage0 clippy.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/build_steps/tool.rs

@@ -1148,3 +1148,32 @@ impl<'a> Builder<'a> {
         cmd
     }
 }
+
+pub(crate) fn install_external_stage0_clippy(builder: &Builder<'_>) -> PathBuf {
+    if builder.config.dry_run() {
+        return PathBuf::default();
+    }
+
+    builder.info("Installing external clippy to stage0 sysroot.");
+
+    let (Some(external_cargo_clippy), Some(external_clippy_driver)) =
+        (&builder.config.external_cargo_clippy, &builder.config.external_clippy_driver)
+    else {
+        panic!("`build.cargo-clippy` and `build.clippy-driver` must be configured together.");
+    };
+
+    let host = builder.config.build;
+    let bindir = builder.out.join(host.triple).join("stage0/bin");
+    t!(fs::create_dir_all(&bindir));
+
+    let cargo_clippy_destination = bindir.join(exe("cargo-clippy", host));
+    let _ = fs::remove_file(&cargo_clippy_destination);
+    builder.copy_link(external_cargo_clippy, &cargo_clippy_destination);
+
+    let clippy_driver_destination = bindir.join(exe("clippy-driver", host));
+    let _ = fs::remove_file(&clippy_driver_destination);
+    builder.copy_link(external_clippy_driver, &clippy_driver_destination);
+
+    assert!(cargo_clippy_destination.exists());
+    cargo_clippy_destination
+}

src/bootstrap/src/core/builder.rs

@@ -1298,8 +1298,13 @@ impl<'a> Builder<'a> {
 
     pub fn cargo_clippy_cmd(&self, run_compiler: Compiler) -> BootstrapCommand {
         if run_compiler.stage == 0 {
-            // `ensure(Clippy { stage: 0 })` *builds* clippy with stage0, it doesn't use the beta clippy.
-            let cargo_clippy = self.build.config.download_clippy();
+            let cargo_clippy = if self.config.external_cargo_clippy.is_some() {
+                tool::install_external_stage0_clippy(self)
+            } else {
+                // `ensure(Clippy { stage: 0 })` *builds* clippy with stage0, it doesn't use the beta clippy.
+                self.build.config.download_clippy()
+            };
+
             let mut cmd = command(cargo_clippy);
             cmd.env("CARGO", &self.initial_cargo);
             return cmd;

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

@@ -337,6 +337,9 @@ pub struct Config {
     pub initial_cargo: PathBuf,
     pub initial_rustc: PathBuf,
 
+    pub external_cargo_clippy: Option<PathBuf>,
+    pub external_clippy_driver: Option<PathBuf>,
+
     #[cfg(not(test))]
     initial_rustfmt: RefCell<RustfmtState>,
     #[cfg(test)]
@@ -826,6 +829,8 @@ define_config! {
         cargo: Option<PathBuf> = "cargo",
         rustc: Option<PathBuf> = "rustc",
         rustfmt: Option<PathBuf> = "rustfmt",
+        cargo_clippy: Option<PathBuf> = "cargo-clippy",
+        clippy_driver: Option<PathBuf> = "clippy-driver",
         docs: Option<bool> = "docs",
         compiler_docs: Option<bool> = "compiler-docs",
         library_docs_private_items: Option<bool> = "library-docs-private-items",
@@ -1420,6 +1425,8 @@ impl Config {
             cargo,
             rustc,
             rustfmt,
+            cargo_clippy,
+            clippy_driver,
             docs,
             compiler_docs,
             library_docs_private_items,
@@ -1502,6 +1509,14 @@ impl Config {
                 .join(exe("cargo", config.build))
         };
 
+        assert_eq!(
+            cargo_clippy.is_some(),
+            clippy_driver.is_some(),
+            "`build.cargo-clippy` and `build.clippy-driver` must be configured together."
+        );
+        config.external_cargo_clippy = cargo_clippy;
+        config.external_clippy_driver = clippy_driver;
+
         // NOTE: it's important this comes *after* we set `initial_rustc` just above.
         if config.dry_run() {
             let dir = config.out.join("tmp-dry-run");