ci: Check formatting of build! macros

.github/workflows/build.yml

@@ -1,6 +1,6 @@
 name: Build and Test
 
-on: 
+on:
   pull_request:
   push:
     branches:
@@ -12,7 +12,7 @@ env:
 jobs:
   test:
     runs-on: ${{ matrix.os }}
-    strategy: 
+    strategy:
       matrix:
         include:
         - os: windows-latest
@@ -50,3 +50,14 @@ jobs:
     - name: fmt
       run: cargo fmt --all -- --check
       if: matrix.os == 'windows-latest' && matrix.rust == 'stable'
+
+  format-macro:
+    runs-on: windows-latest
+    name: Format `build!` macros
+    steps:
+    - uses: actions/checkout@v2
+    - name: Invoke `windows_fmt`
+      run: cargo r -p windows_fmt
+    - name: Diff formatting result
+      shell: bash
+      run: git diff --exit-code || (echo '::error::Some `build!` macros were improperly formatted. Please run `cargo run -p windows_fmt` and push again'; exit 1)