Try adding Github Problem Matchers to libc++ workflow.

The problem matchers are simple regex's that will be applied
to the output of particular steps, and when a match is found,
an annotation is applied to the relevent github workflow.

The idea is to more easily surface relevent failure information,
which may otherwise be hidden among cancelled tasks.

The Problem Matcher specification/format is rather limited in its
descriptive abliity, meaning we lose most context about the error.
Further, because many of the source files are re-arranged before use,
it prevents us from actually pointing to the file:line in the github UI;
Boo :-(.

A more complete solution would likely hook into LIT, remap source file
paths as needed, and use the "echo ::error::" toolkit command to create
the annotations.

Author: EricWF

.github/libcxx-problem-matchers.json

@@ -0,0 +1,27 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "clang",
+            "pattern": [
+                {
+                    "regexp": "^(.+):(\\d+):(\\d+):\\s+(error|warning):\\s+(.+)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "severity": 4,
+                    "message": 5
+                }
+            ]
+        },
+        {
+              "owner": "lit",
+              "severity": "error",
+              "pattern": [
+                  {
+                      "regexp": "^\\s*(FAIL|XPASS):\\s+([^\\s]+)\\s+::\\s+(.+?)\\s+\\(\\d+\\s+of\\s+\\d+\\)$",
+                      "message": 2
+                  }
+              ]
+          }
+    ]
+}

.github/workflows/libcxx-build-and-test.yaml

@@ -57,10 +57,17 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: ${{ matrix.config }}.${{ matrix.cxx }}
-        run: libcxx/utils/ci/run-buildbot ${{ matrix.config }}
+        run: |
+          echo "::add-matcher::.github/libcxx-problem-matchers.json"
+          libcxx/utils/ci/run-buildbot ${{ matrix.config }}
         env:
           CC: ${{ matrix.cc }}
           CXX: ${{ matrix.cxx }}
+      - if: always()
+        name: Disable  Problem Matchers
+        run: |
+          echo "::remove-matcher owner=lit::"
+          echo "::remove-matcher owner=clang::"
       - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 # v4.3.0
         if: always()
         with:
@@ -103,10 +110,17 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: ${{ matrix.config }}
-        run: libcxx/utils/ci/run-buildbot ${{ matrix.config }}
+        run: |
+          echo "::add-matcher::.github/libcxx-problem-matchers.json"
+          libcxx/utils/ci/run-buildbot ${{ matrix.config }}
         env:
           CC: ${{ matrix.cc }}
           CXX: ${{ matrix.cxx }}
+      - if: always()
+        name: Disable Problem Matchers
+        run: |
+          echo "::remove-matcher owner=lit::"
+          echo "::remove-matcher owner=clang::"
       - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 # v4.3.0
         if: always()  # Upload artifacts even if the build or test suite fails
         with:
@@ -167,10 +181,17 @@ jobs:
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: ${{ matrix.config }}
-        run: libcxx/utils/ci/run-buildbot ${{ matrix.config }}
+        run: |
+          echo "::add-matcher::.github/libcxx-problem-matchers.json"
+          libcxx/utils/ci/run-buildbot ${{ matrix.config }}
         env:
           CC: clang-21
           CXX: clang++-21
+      - if: always()
+        name: Disable Problem Matchers
+        run: |
+          echo "::remove-matcher owner=lit::"
+          echo "::remove-matcher owner=clang::"
       - uses: actions/upload-artifact@26f96dfa697d77e81fd5907df203aa23a56210a8 # v4.3.0
         if: always()
         with:
@@ -221,6 +242,7 @@ jobs:
       - uses: seanmiddleditch/gha-setup-ninja@3b1f8f94a2f8254bd26914c4ab9474d4f0015f67 # v6
       - name: Build and test
         run: |
+          echo "::add-matcher::.github/libcxx-problem-matchers.json"
           python3 -m venv .venv
           source .venv/bin/activate
           python -m pip install psutil