Use ccache for stage0 tool and rustc builds

Author: Kobzol

src/bootstrap/src/core/build_steps/compile.rs

@@ -1278,6 +1278,16 @@ pub fn rustc_cargo(
         ));
     }
 
+    // The stage0 compiler changes infrequently and does not directly depend on code
+    // in the current working directory. Therefore, caching it with sccache should be
+    // useful.
+    // This is only performed for non-incremental builds, as ccache cannot deal with these.
+    if let Some(ref ccache) = builder.config.ccache {
+        if compiler.stage == 0 && !builder.config.incremental {
+            cargo.env("RUSTC_WRAPPER", ccache);
+        }
+    }
+
     rustc_cargo_env(builder, cargo, target, compiler.stage);
 }
 

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

@@ -148,6 +148,16 @@ impl Step for ToolBuild {
             &self.extra_features,
         );
 
+        // The stage0 compiler changes infrequently and does not directly depend on code
+        // in the current working directory. Therefore, caching it with sccache should be
+        // useful.
+        // This is only performed for non-incremental builds, as ccache cannot deal with these.
+        if let Some(ref ccache) = builder.config.ccache {
+            if matches!(self.mode, Mode::ToolBootstrap) && !builder.config.incremental {
+                cargo.env("RUSTC_WRAPPER", ccache);
+            }
+        }
+
         // Rustc tools (miri, clippy, cargo, rustfmt, rust-analyzer)
         // could use the additional optimizations.
         if self.mode == Mode::ToolRustc && is_lto_stage(&self.compiler) {