Bug 1437627 - part 2 - move incremental compilation logic to moz.configure; r=chmanchester

Doing checks in Python is much simpler than doing them in Makefile
logic, and we're going to be adding some more complex logic in the next patch.

build/moz.configure/toolchain.configure

@@ -1463,6 +1463,26 @@ def rust_compiler_flags(opt_level, debug_rust, debug_symbols):
 
 set_config('MOZ_RUST_DEFAULT_FLAGS', rust_compiler_flags)
 
+# Rust incremental compilation
+# ==============================================================
+
+
+@depends('MOZ_AUTOMATION')
+def cargo_incremental(automation):
+    """Return a value for the CARGO_INCREMENTAL environment variable."""
+
+    # We never want to use incremental compilation in automation.  sccache
+    # handles our automation use case much better than incremental compilation
+    # would.
+    if automation:
+        return '0'
+
+    # Don't do anything special in other situations.
+    return
+
+
+set_config('CARGO_INCREMENTAL', cargo_incremental)
+
 # Linker detection
 # ==============================================================
 

config/rules.mk

@@ -864,10 +864,8 @@ cargo_rustc_flags += -C lto
 endif
 endif
 
-# Disable incremental Rust compilation in automation builds, where
+ifdef CARGO_INCREMENTAL
-# the lack of environmental continuity makes it unhelpful.
+cargo_incremental := CARGO_INCREMENTAL=$(CARGO_INCREMENTAL)
-ifdef MOZ_AUTOMATION
-cargo_incremental := CARGO_INCREMENTAL=0
 endif
 
 rustflags_override = RUSTFLAGS='$(MOZ_RUST_DEFAULT_FLAGS) $(RUSTFLAGS)'