[test] Add test that verify correct usage of REQUIRES for Swift features

The test will look for other tests using `RUN:` lines that use
experimental or upcoming features and will check that the tests also are
checking with the right `REQUIRES:` lines for the used features. This
should avoid new tests being introduced without the right `REQUIRES` and
should avoid breaking the toolchain in less tested configurations.

Author: drodriguez

test/Misc/verify-swift-feature-testing.test-sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# RUN: bash %s '%swift_src_root'
+# UNSUPPORTED: OS=windows-msvc
+
+if [[ ${DEBUG-} =~ ^1|yes|true$ ]]; then
+  set -o xtrace
+fi
+
+set -o errexit
+set -o nounset
+set -o pipefail
+set -o errtrace
+
+swift_src_root=$1
+# Tests that check for the behaviour of  experimental/upcoming features, so
+# they cannot automatically be checked.
+exceptional_files=(
+  test/Frontend/experimental-features-no-asserts.swift
+  test/Frontend/upcoming_feature.swift
+)
+egrep="grep --extended-regexp"
+exit_result=0
+
+# Look for every test file in the test directories with `RUN` lines that
+# mention `-enable-experimental-feature` or `-enable-upcoming-feature`.
+# Be careful of not using REQUIRES or RUN with a colon after them or Lit will
+# pick them up.
+for f in $(${egrep} --recursive -e 'RUN[:].*-enable-(experimental|upcoming)-feature' --files-with-matches  "${swift_src_root}/test" "${swift_src_root}/validation-test"); do
+  # First lets check this is not one of the exceptional files
+  relative_f=${f#"${swift_src_root}"/}
+  for ef in "${exceptional_files[@]}"; do
+    if [ "${ef}" == "${relative_f}" ]; then
+      continue 2
+    fi
+  done
+
+  features=$(${egrep} --no-filename -e 'RUN[:]' "${f}" | ${egrep} --only-matching -e '-enable-(experimental|upcoming)-feature (-Xfrontend )?[A-Za-z0-9]*' | rev | cut -d' ' -f1 | rev | sort -u)
+  for feature in ${features}; do
+    # No warning if the necessary `REQUIRES` is already there
+    if ${egrep} --quiet -e "REQUIRES[:] swift_feature_${feature}" "${f}"; then
+      continue
+    fi
+
+    # Some tests check for the errors themselves, so we can skip them as well
+    if rg --quiet -e "requires '-enable-(experimental|upcoming)-feature ${feature}'" "${f}"; then
+      continue
+    fi
+
+    # For everything else, print a warning and for an invalid exit code
+    printf "error: %s: Missing '%s: swift_feature_%s'\n" "${f}" "REQUIRES" "${feature}"
+    exit_result=1
+  done
+done
+
+exit $exit_result