ci: introduce `classify-pr.sh` script which determines whether a PR should have CI run

.github/workflows/rust.yml

@@ -14,9 +14,13 @@ jobs:
       crates: ${{ steps.get_matrix.outputs.crates }}
       deps: ${{ steps.get_matrix.outputs.deps }}
       nightly_version: ${{ steps.read_toolchain.outputs.nightly_version }}
+      pr_changed_source: ${{ steps.classify_pr.outputs.pr_changed_source }}
     steps:
       - name: Checkout Crate
         uses: actions/checkout@v4
+      - name: Determine what files the PR changes.
+        id: classify_pr
+        run: contrib/classify-pr.sh ${{ github.event.pull_request.head.sha }} ${{ github.event.pull_request.base.sha }}
       - name: Read nightly version
         id: read_toolchain
         run: echo "nightly_version=$(cat .github/nightly-version)" >> $GITHUB_OUTPUT

contrib/classify-pr.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+if [ "$#" -ne 2 ]; then
+  echo "Usage: $0 <tip of master> <tip of PR>"
+  exit 1
+fi
+
+pr_tip=$1
+master_tip=$2
+
+# When called on `pull_request`, GA fails to pull down master by default.
+# When called on `push`, GA fails to pull down the PR by default, only its merge commit.
+# The simplest way to deal with this is to just pull both explicitly.
+git fetch origin "$master_tip":master_tip
+git fetch origin "$pr_tip":pr_tip
+
+pr_base=$(git merge-base master_tip pr_tip)
+
+# If something modifies any non-markdown file, it's considered a source code change.
+if git diff --name-only "$pr_base" "$pr_tip" | grep -qv "^.md$"; then
+    echo "pr_changed_source=true" >> "$GITHUB_OUTPUT"
+else
+    echo "pr_changed_source=false" >> "$GITHUB_OUTPUT"
+fi
+