fix(build): read the symbol graph from SwiftPM and say why a build fails

scripts/check-validation-coverage.sh

@@ -13,17 +13,28 @@ set -euo pipefail
 cd "$(dirname "$0")/.."
 
 MANIFEST="scripts/validation-coverage.tsv"
-GRAPH_DIR=".build/symbolgraph"
+GRAPH_DIR=".build/out/symbolgraph"
 
 if [[ ! -f "$MANIFEST" ]]; then
     echo "check-validation-coverage: missing manifest $MANIFEST" >&2
     exit 1
 fi
 
-mkdir -p "$GRAPH_DIR"
-swift build --target PureYAML \
-    -Xswiftc -emit-symbol-graph \
-    -Xswiftc -emit-symbol-graph-dir -Xswiftc "$GRAPH_DIR" >/dev/null
+# SwiftPM's own command writes the graph for every toolchain. Passing the compiler flags through
+# -Xswiftc wrote nothing on Swift 6.4 and the gate then failed with no message. Its output is kept
+# so a failed build says why, and a missing graph is named.
+log=$(mktemp)
+trap 'rm -f "$log"' EXIT
+if ! swift package dump-symbol-graph >"$log" 2>&1; then
+    cat "$log" >&2
+    echo "check-validation-coverage: swift package dump-symbol-graph failed" >&2
+    exit 1
+fi
+if [[ ! -f "$GRAPH_DIR/PureYAML.symbols.json" ]]; then
+    cat "$log" >&2
+    echo "check-validation-coverage: no symbol graph at $GRAPH_DIR/PureYAML.symbols.json" >&2
+    exit 1
+fi
 
 python3 - "$GRAPH_DIR/PureYAML.symbols.json" "$MANIFEST" <<'PY'
 import json