Bug 1670538 - Use an allow-list in RUSTC_BOOTSTRAP for rustc >= 1.50.0. r=firefox-build-system-reviewers,andi,sheehan,mhentges

While we could change qcms, encoding_rs and packed_simd to not emit
RUSTC_BOOTSTRAP on newer versions of rust, like we do for gkrust-shared,
it's not worth the effort (especially for those that are vendored).

Differential Revision: https://phabricator.services.mozilla.com/D105423

config/makefiles/rust.mk

@@ -208,6 +208,16 @@ export COREAUDIO_SDK_PATH=$(MACOS_SDK_DIR)
 endif
 endif
 
+ifndef RUSTC_BOOTSTRAP
+ifeq (,$(filter 1.47.% 1.48.% 1.49.%,$(RUSTC_VERSION)))
+RUSTC_BOOTSTRAP := gkrust_shared,qcms
+ifdef MOZ_RUST_SIMD
+RUSTC_BOOTSTRAP := $(RUSTC_BOOTSTRAP),encoding_rs,packed_simd
+endif
+export RUSTC_BOOTSTRAP
+endif
+endif
+
 target_rust_ltoable := force-cargo-library-build
 target_rust_nonltoable := force-cargo-test-run force-cargo-library-check $(foreach b,build check,force-cargo-program-$(b))
 

toolkit/library/rust/shared/build.rs

@@ -22,7 +22,7 @@ fn main() {
     // This is a rather awful thing to do, but we're only doing it on
     // versions of rustc that are not going to change the unstable APIs
     // we use from under us, all being already released or beta.
-    if bootstrap {
+    if bootstrap && ver < Version::parse("1.50.0").unwrap() {
         println!("cargo:rustc-env=RUSTC_BOOTSTRAP=1");
     }
 }