Bug 1613649 - Explicitly opt-out of Rust's local ThinLTO for DEVELOPE…

…R_OPTIONS builds. r=firefox-build-system-reviewers,chmanchester

By default the Rust compiler will perform a limited kind of ThinLTO on each
crate. For local builds this additional optimization is not worth the
increase in compile time.

Differential Revision: https://phabricator.services.mozilla.com/D61821

--HG--
extra : moz-landing-system : lando

config/makefiles/rust.mk

@@ -89,6 +89,13 @@ endif
 
 rustflags_override = $(MOZ_RUST_DEFAULT_FLAGS) $(rustflags_neon)
 
+ifdef DEVELOPER_OPTIONS
+# By default the Rust compiler will perform a limited kind of ThinLTO on each
+# crate. For local builds this additional optimization is not worth the
+# increase in compile time so we opt out of it.
+rustflags_override += -Clto=off
+endif
+
 ifdef MOZ_USING_SCCACHE
 export RUSTC_WRAPPER=$(CCACHE)
 endif