Bug 1551368 - Part 3: Don't remove the rust incremental cache when clobbering. r=chmanchester

Skips over the incremental cache when performing a clobber. The incremental compilation cache is located at:
  `$(objdir)/$(rust_target)/$(rust_build_kind)/incremental`

When cross compiling there can be two caches, one for the host and one for the target so we handle both.

Differential Revision: https://phabricator.services.mozilla.com/D31310

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

python/mozbuild/mozbuild/controller/clobber.py

@@ -136,12 +136,22 @@ class Clobberer(object):
         some directories (e.g. Visual Studio Project Files) will not be
         deleted.
         """
+        # Determine where cargo build artifacts are stored
+        RUST_TARGET_VARS = ('RUST_HOST_TARGET', 'RUST_TARGET')
+        rust_targets = set([self.substs[x] for x in RUST_TARGET_VARS if x in self.substs])
+        rust_build_kind = 'release'
+        if self.substs.get('MOZ_DEBUG_RUST'):
+            rust_build_kind = 'debug'
+
         # Top-level files and directories to not clobber by default.
         no_clobber = {
             '.mozbuild',
             'msvc',
         }
 
+        # Hold off on clobbering cargo build artifacts
+        no_clobber |= rust_targets
+
         if full:
             # mozfile doesn't like unicode arguments (bug 818783).
             paths = [self.topobjdir.encode('utf-8')]
@@ -150,6 +160,13 @@ class Clobberer(object):
 
         self.delete_dirs(self.topobjdir, paths)
 
+        # Now handle cargo's build artifacts and skip removing the incremental
+        # compilation cache.
+        for target in rust_targets:
+            cargo_path = os.path.join(self.topobjdir, target, rust_build_kind)
+            paths = self.collect_subdirs(cargo_path, {'incremental', })
+            self.delete_dirs(cargo_path, paths)
+
     def ensure_objdir_state(self):
         """Ensure the CLOBBER file in the objdir exists.