Label pull requests by binary compatibility

.mergify.yml

@@ -32,3 +32,21 @@ pull_request_rules:
       - label!=do-not-merge
     actions:
       queue:
+
+  - name: flag non breaking changes
+    conditions:
+      - check-success~=^test / binary-compatibility
+      - -check-pending~=^test / binary-compatibility
+      - -check-failure~=^test / binary-compatibility
+    actions:
+      label:
+        toggle:
+          - non-breaking-change
+
+  - name: flag binary incompatible changes
+    conditions:
+      - check-failure~=^test / binary-compatibility
+    actions:
+      label:
+        toggle:
+          - release-bump-needed

README.md

@@ -176,10 +176,18 @@ jobs:
 ## Merge queue (Mergify)
 
 Rebases an approved pull request on the default branch, reruns its CI, then merges it. `scala-steward`
-updates are queued without an approval.
+updates are queued without an approval. Also labels a pull request `non-breaking-change` or
+`release-bump-needed` from the outcome of the binary compatibility checks.
 
 ### Setup
 
+Create the two labels if the project doesn't have them:
+
+```sh
+gh label create non-breaking-change -d "binary compatible with the current release" -c 0e8a16
+gh label create release-bump-needed -d "breaks binary compatibility, needs a major release" -c d93f0b
+```
+
 Install the Mergify app on the project and on this repository, then create `.mergify.yml`:
 
 ```yaml