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/builder.rs

@@ -1298,8 +1298,12 @@ 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 = self
+                .config
+                .initial_cargo_clippy
+                .clone()
+                .unwrap_or_else(|| 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

@@ -344,6 +344,7 @@ pub struct Config {
     // These are either the stage0 downloaded binaries or the locally installed ones.
     pub initial_cargo: PathBuf,
     pub initial_rustc: PathBuf,
+    pub initial_cargo_clippy: Option<PathBuf>,
 
     #[cfg(not(test))]
     initial_rustfmt: RefCell<RustfmtState>,
@@ -834,6 +835,7 @@ define_config! {
         cargo: Option<PathBuf> = "cargo",
         rustc: Option<PathBuf> = "rustc",
         rustfmt: Option<PathBuf> = "rustfmt",
+        cargo_clippy: Option<PathBuf> = "cargo-clippy",
         docs: Option<bool> = "docs",
         compiler_docs: Option<bool> = "compiler-docs",
         library_docs_private_items: Option<bool> = "library-docs-private-items",
@@ -1439,6 +1441,7 @@ impl Config {
             cargo,
             rustc,
             rustfmt,
+            cargo_clippy,
             docs,
             compiler_docs,
             library_docs_private_items,
@@ -1491,6 +1494,14 @@ impl Config {
             config.out = absolute(&config.out).expect("can't make empty path absolute");
         }
 
+        if cargo_clippy.is_some() && rustc.is_none() {
+            println!(
+                "WARNING: Using `build.cargo-clippy` without `build.rustc` usually fails due to toolchain conflict."
+            );
+        }
+
+        config.initial_cargo_clippy = cargo_clippy;
+
         config.initial_rustc = if let Some(rustc) = rustc {
             if !flags.skip_stage0_validation {
                 config.check_stage0_version(&rustc, "rustc");