Bug 1623085 - Only explicitly set number of Rust codegen-units for target artifacts. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D67162

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

Cargo.toml

@@ -63,7 +63,6 @@ opt-level = 2
 rpath = false
 debug-assertions = false
 panic = "abort"
-codegen-units = 1
 
 [patch.crates-io]
 libudev-sys = { path = "dom/webauthn/libudev-sys" }

config/makefiles/rust.mk

@@ -188,6 +188,13 @@ $(target_rust_nonltoable): RUSTFLAGS:=$(rustflags_override) $(rustflags_sancov)
 
 TARGET_RECIPES := $(target_rust_ltoable) $(target_rust_nonltoable)
 
+# If this is a release build we want rustc to generate one codegen unit per
+# crate. This results in better optimization and less code duplication at the
+# cost of longer compile times.
+ifndef DEVELOPER_OPTIONS
+$(TARGET_RECIPES): RUSTFLAGS += -C codegen-units=1
+endif
+
 HOST_RECIPES := \
   $(foreach a,library program,$(foreach b,build check,force-cargo-host-$(a)-$(b)))