WIP Rust #29556

rust-local-rebuild.patch

diff --git a/configure b/configure
index 38f3e3b00c6d..bcb0b0fded8a 100755
--- a/configure
+++ b/configure
@@ -599,6 +599,7 @@ opt debug-assertions 0 "build with debugging assertions"
 opt fast-make 0 "use .gitmodules as timestamp for submodule deps"
 opt ccache 0 "invoke gcc/clang via ccache to reuse object files between builds"
 opt local-rust 0 "use an installed rustc rather than downloading a snapshot"
+opt local-rebuild 0 "use an installed rustc matching the current version, for rebuilds"
 opt llvm-static-stdcpp 0 "statically link to libstdc++ for LLVM"
 opt rpath 1 "build rpaths into rustc itself"
 opt stage0-landing-pads 1 "enable landing pads during bootstrap with stage0"
@@ -847,6 +848,15 @@ then
     BIN_SUF=.exe
 fi
 
+if [ -n "$CFG_ENABLE_LOCAL_REBUILD" ]
+then
+    if [ -z "$CFG_ENABLE_LOCAL_RUST" ]
+    then
+        CFG_ENABLE_LOCAL_RUST=1
+        putvar CFG_ENABLE_LOCAL_RUST
+    fi
+fi
+
 if [ -n "$CFG_ENABLE_LOCAL_RUST" ]
 then
     system_rustc=$(which rustc)
diff --git a/mk/main.mk b/mk/main.mk
index 493b61051331..15092925b15d 100644
--- a/mk/main.mk
+++ b/mk/main.mk
@@ -34,7 +34,11 @@ CFG_FILENAME_EXTRA=$(shell printf '%s' $(CFG_RELEASE)$(CFG_EXTRA_FILENAME) | $(C
 # intentionally not "secure" by any definition, this is largely just a deterrent
 # from users enabling unstable features on the stable compiler.
 CFG_BOOTSTRAP_KEY=$(CFG_FILENAME_EXTRA)
+ifdef CFG_ENABLE_LOCAL_REBUILD
+CFG_BOOTSTRAP_KEY_STAGE0=$(CFG_BOOTSTRAP_KEY)
+else
 CFG_BOOTSTRAP_KEY_STAGE0=$(shell grep 'rustc_key' $(S)src/stage0.txt | sed 's/rustc_key: '//)
+endif
 
 ifeq ($(CFG_RELEASE_CHANNEL),stable)
 # This is the normal semver version string, e.g. "0.12.0", "0.12.0-nightly"
@@ -526,6 +530,11 @@ ifneq ($(strip $(CFG_BUILD)),$(strip $(3)))
 CFGFLAG$(1)_T_$(2)_H_$(3) = stage1
 
 RPATH_VAR$(1)_T_$(2)_H_$(3) := $$(TARGET_RPATH_VAR1_T_$(2)_H_$$(CFG_BUILD))
+else
+ifdef CFG_ENABLE_LOCAL_REBUILD
+# Use the local compiler as if it were effectively stage1 too.
+CFGFLAG$(1)_T_$(2)_H_$(3) = stage1
+endif
 endif
 endif
 
diff --git a/src/bootstrap/build/config.rs b/src/bootstrap/build/config.rs
index 533c1c93d5bc..dd5e6e8e3939 100644
--- a/src/bootstrap/build/config.rs
+++ b/src/bootstrap/build/config.rs
@@ -65,6 +65,7 @@ pub struct Config {
     pub target: Vec<String>,
     pub rustc: Option<String>,
     pub cargo: Option<String>,
+    pub rebuild: bool,
 
     // libstd features
     pub debug_jemalloc: bool,
@@ -306,6 +307,7 @@ impl Config {
                 ("JEMALLOC", self.use_jemalloc),
                 ("DEBUG_JEMALLOC", self.debug_jemalloc),
                 ("RPATH", self.rust_rpath),
+                ("LOCAL_REBUILD", self.rebuild),
             }
 
             match key {
diff --git a/src/bootstrap/build/mod.rs b/src/bootstrap/build/mod.rs
index 44f161fb487f..05b74dd1095f 100644
--- a/src/bootstrap/build/mod.rs
+++ b/src/bootstrap/build/mod.rs
@@ -478,6 +478,11 @@ impl Build {
              .arg("-j").arg(self.jobs().to_string())
              .arg("--target").arg(target);
 
+        let mut stage = compiler.stage;
+        if stage == 0 && self.config.rebuild {
+            stage = 1;
+        }
+
         // 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.
@@ -486,7 +491,7 @@ 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())
@@ -712,7 +717,7 @@ 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