ci: Only run rustfmt when necessary

The rules we want here are pretty simple:
  - only run rustfmt when Rust code changes, to the extent that we can
    tell what has or hasn't changed (only really reliable for pre-merge)
  - make rustfmt success mandatory for pre-merge, advisory for other
    branches

Encode that, and make sure we don't run rustfmt when we don't need to.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35203>

.gitlab-ci/test-source-dep.yml

@@ -198,18 +198,25 @@
   rules:
     - !reference [.never-post-merge-rules, rules]
     - !reference [.no_scheduled_pipelines-rules, rules]
-    - changes:
+    # pre-merge: must succeed if changing Rust, skip if not
+    - if: &is-merge-attempt $GITLAB_USER_LOGIN == "marge-bot" && $CI_PIPELINE_SOURCE == "merge_request_event"
+      changes: &rustfmt_files
         - .gitlab-ci.yml
         - .gitlab-ci/**/*
-    # in merge pipeline, formatting checks are not allowed to fail
-    - if: $GITLAB_USER_LOGIN == "marge-bot" && $CI_PIPELINE_SOURCE == "merge_request_event"
-      changes: &rust_file_list
         - src/**/*.rs
       when: on_success
       allow_failure: false
-    # in other pipelines, formatting checks are allowed to fail
-    - changes: *rust_file_list
-      when: on_success
+    - if: *is-merge-attempt
+      when: never
+    - if: &is-merge-request $CI_PIPELINE_SOURCE == "merge_request_event"
+      changes:
+        *rustfmt_files
+      when: manual
+      allow_failure: false
+    - if: *is-merge-request
+      when: never
+    # non-pre-merge: run but only advisory
+    - when: manual
       allow_failure: true
 
 # Rules for .mr-label-maker.yml