mk: If local-rust is the same as the current version, then force a local-rebuild

configure

@@ -600,7 +600,7 @@ 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 local-rebuild 0 "assume local-rust matches the current version, for rebuilds; implies local-rust, and is implied if local-rust already matches the current version"
 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"

mk/main.mk

@@ -64,12 +64,21 @@ CFG_FILENAME_EXTRA=$(shell printf '%s' $(CFG_RELEASE)$(CFG_EXTRA_FILENAME) | $(C
 # from users enabling unstable features on the stable compiler.
 CFG_BOOTSTRAP_KEY=$(CFG_FILENAME_EXTRA)
 
+# If local-rust is the same as the current version, then force a local-rebuild
+ifdef CFG_ENABLE_LOCAL_RUST
+ifeq ($(CFG_RELEASE),\
+      $(shell $(S)src/etc/local_stage0.sh --print-rustc-release $(CFG_LOCAL_RUST_ROOT)))
+    CFG_INFO := $(info cfg: auto-detected local-rebuild $(CFG_RELEASE))
+    CFG_ENABLE_LOCAL_REBUILD = 1
+endif
+endif
+
 # The stage0 compiler needs to use the previous key recorded in src/stage0.txt,
 # except for local-rebuild when it just uses the same current key.
 ifdef CFG_ENABLE_LOCAL_REBUILD
 CFG_BOOTSTRAP_KEY_STAGE0=$(CFG_BOOTSTRAP_KEY)
 else
-CFG_BOOTSTRAP_KEY_STAGE0=$(shell grep 'rustc_key' $(S)src/stage0.txt | sed 's/rustc_key: '//)
+CFG_BOOTSTRAP_KEY_STAGE0=$(shell sed -ne 's/^rustc_key: //p' $(S)src/stage0.txt)
 endif
 
 # The name of the package to use for creating tarballs, installers etc.

mk/stage0.mk

@@ -11,6 +11,7 @@ endif
 
 $(SNAPSHOT_RUSTC_POST_CLEANUP): \
 		$(S)src/stage0.txt \
+		$(S)src/etc/local_stage0.sh \
 		$(S)src/etc/get-stage0.py $(MKFILE_DEPS) \
 		| $(HBIN0_H_$(CFG_BUILD))/
 	@$(call E, fetch: $@)

src/etc/local_stage0.sh

@@ -49,6 +49,13 @@ if [ -z $TARG_DIR ]; then
     exit 1
 fi
 
+case "$TARG_DIR" in
+--print-rustc-release)
+  # not actually copying to TARG_DIR, just print the local rustc version and exit
+  ${PREFIX}/bin/rustc${BIN_SUF} --version --verbose | sed -ne 's/^release: //p'
+;;
+*)
+
 cp ${PREFIX}/bin/rustc${BIN_SUF} ${TARG_DIR}/stage0/bin/
 cp ${PREFIX}/${LIB_DIR}/${RUSTLIBDIR}/${TARG_DIR}/${LIB_DIR}/* ${TARG_DIR}/stage0/${LIB_DIR}/
 cp ${PREFIX}/${LIB_DIR}/${LIB_PREFIX}extra*${LIB_SUF} ${TARG_DIR}/stage0/${LIB_DIR}/
@@ -66,3 +73,5 @@ cp ${PREFIX}/${LIB_DIR}/${LIB_PREFIX}term*${LIB_SUF} ${TARG_DIR}/stage0/${LIB_DI
 
 # do not fail if one of the above fails, as all we need is a working rustc!
 exit 0
+
+esac