[Support][Error] Add a 'cantFail' utility function for known-safe calls to

fallible functions.

Some fallible functions (those returning Error or Expected<T>) may only fail
for a subset of their inputs. For example, a "safe" square root function will
succeed for all finite positive inputs:

  Expected<double> safeSqrt(double d) {
    if (d < 0 && !isnan(d) && !isinf(d))
      return make_error<...>("Cannot sqrt -ve values, nans or infs");
    return sqrt(d);
  }

At a safe callsite for such a function, checking the error return value is
redundant:

  if (auto ValOrErr = safeSqrt(42.0)) {
    // use *ValOrErr.
  } else
    llvm_unreachable("safeSqrt should always succeed for +ve values");

The cantFail function wraps this check and extracts the contained value,
simplifying control flow:

  double Result = cantFail(safeSqrt(42.0));

This function should be used with care: it is a programmatic error to wrap a
call with cantFail if it can in fact fail. For debug builds this will
result in llvm_unreachable being called. For release builds the behavior is
undefined.

Use of this function is likely to be rare in library code, but more common
for tool and unit-test code where inputs and mock functions may be known to be
safe.

llvm-svn: 296384

1 files changed, 28 insertions, 0 deletions

diff --git a/llvm/unittests/Support/ErrorTest.cpp b/llvm/unittests/Support/ErrorTest.cpp
index 29a173a..382346c 100644
--- a/llvm/unittests/Support/ErrorTest.cpp
+++ b/llvm/unittests/Support/ErrorTest.cpp
@@ -469,6 +469,34 @@ TEST(Error, ExitOnError) {
       << "exitOnError returned an unexpected error result";
 }
 
+// Test that the ExitOnError utility works as expected.
+TEST(Error, CantFailSuccess) {
+  cantFail(Error::success());
+
+  int X = cantFail(Expected<int>(42));
+  EXPECT_EQ(X, 42) << "Expected value modified by cantFail";
+}
+
+// Test that cantFail results in a crash if you pass it a failure value.
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+TEST(Error, CantFailDeath) {
+  EXPECT_DEATH(
+      cantFail(make_error<StringError>("foo", inconvertibleErrorCode())),
+      "Failure value returned from cantFail wrapped call")
+    << "cantFail(Error) did not cause an abort for failure value";
+
+  EXPECT_DEATH(
+      {
+        auto IEC = inconvertibleErrorCode();
+        int X = cantFail(Expected<int>(make_error<StringError>("foo", IEC)));
+        (void)X;
+      },
+      "Failure value returned from cantFail wrapped call")
+    << "cantFail(Expected<int>) did not cause an abort for failure value";
+}
+#endif
+
+
 // Test Checked Expected<T> in success mode.
 TEST(Error, CheckedExpectedInSuccessMode) {
   Expected<int> A = 7;