bootstrap: mirror mk's local-rebuild changes

The rustc for a local-rebuild is assumed to use the current bootstrap
key and already match the current stage1 features.

Author: cuviper

src/bootstrap/build/config.rs

@@ -67,6 +67,7 @@ pub struct Config {
     pub target: Vec<String>,
     pub rustc: Option<String>,
     pub cargo: Option<String>,
+    pub rebuild: bool,
 
     // libstd features
     pub debug_jemalloc: bool,
@@ -315,6 +316,7 @@ impl Config {
                 ("RPATH", self.rust_rpath),
                 ("OPTIMIZE_TESTS", self.rust_optimize_tests),
                 ("DEBUGINFO_TESTS", self.rust_debuginfo_tests),
+                ("LOCAL_REBUILD", self.rebuild),
             }
 
             match key {

src/bootstrap/build/mod.rs

@@ -510,6 +510,14 @@ impl Build {
              .arg("-j").arg(self.jobs().to_string())
              .arg("--target").arg(target);
 
+        let stage;
+        if compiler.stage == 0 && self.config.rebuild {
+            // Assume the local-rebuild rustc already has stage1 features.
+            stage = 1;
+        } else {
+            stage = compiler.stage;
+        }
+
         // Customize the compiler we're running. Specify the compiler to cargo
         // as our shim and then pass it some various options used to configure
         // how the actual compiler itself is called.
@@ -518,7 +526,7 @@ impl Build {
         // src/bootstrap/{rustc,rustdoc.rs}
         cargo.env("RUSTC", self.out.join("bootstrap/debug/rustc"))
              .env("RUSTC_REAL", self.compiler_path(compiler))
-             .env("RUSTC_STAGE", compiler.stage.to_string())
+             .env("RUSTC_STAGE", stage.to_string())
              .env("RUSTC_DEBUGINFO", self.config.rust_debuginfo.to_string())
              .env("RUSTC_CODEGEN_UNITS",
                   self.config.rust_codegen_units.to_string())
@@ -744,7 +752,7 @@ impl Build {
         // In stage0 we're using a previously released stable compiler, so we
         // use the stage0 bootstrap key. Otherwise we use our own build's
         // bootstrap key.
-        let bootstrap_key = if compiler.is_snapshot(self) {
+        let bootstrap_key = if compiler.is_snapshot(self) && !self.config.rebuild {
             &self.bootstrap_key_stage0
         } else {
             &self.bootstrap_key