[Serialized diagnostics] Avoid generating filenames that break the reader

The serialized diagnostics reader has one very specific limitation it
places on filenames: they must not end in `/`, because that makes them
look like a directory. This is not documented, and the diagnostics
reader will unceremoniously crash when trying to read such a file.

While the reader should be fixed to at least fail gracefully in such
cases, Swift also shouldn't generate such filenames. Right now, they
can be generated when referencing an entity named `/` that is
synthesized or comes from a module. When we encounter such file names,
append `_operator` to avoid the problem.

Fixes rdar://118217560.

Author: DougGregor

lib/Frontend/SerializedDiagnosticConsumer.cpp

@@ -225,6 +225,17 @@ namespace serialized_diagnostics {
 } // namespace serialized_diagnostics
 } // namespace swift
 
+/// Sanitize a filename for the purposes of the serialized diagnostics reader.
+static StringRef sanitizeFilename(
+    StringRef filename, SmallString<32> &scratch) {
+  if (!filename.endswith("/") && !filename.endswith("\\"))
+    return filename;
+
+  scratch = filename;
+  scratch += "_operator";
+  return scratch;
+}
+
 unsigned SerializedDiagnosticConsumer::getEmitFile(
     SourceManager &SM, StringRef Filename, unsigned bufferID
 ) {
@@ -248,9 +259,14 @@ unsigned SerializedDiagnosticConsumer::getEmitFile(
   Record.push_back(entry);
   Record.push_back(0); // For legacy.
   Record.push_back(0); // For legacy.
-  Record.push_back(Filename.size());
+  
+  // Sanitize the filename enough that the serialized diagnostics reader won't
+  // reject it.
+  SmallString<32> filenameScratch;
+  auto sanitizedFilename = sanitizeFilename(Filename, filenameScratch);
+  Record.push_back(sanitizedFilename.size());
   State->Stream.EmitRecordWithBlob(State->Abbrevs.get(RECORD_FILENAME),
-                                   Record, Filename.data());
+                                   Record, sanitizedFilename.data());
 
   // If the buffer contains code that was synthesized by the compiler,
   // emit the contents of the buffer.

test/diagnostics/Inputs/slash.swift

@@ -0,0 +1,2 @@
+public func /(lhs: String, b: Int) -> Bool { false }
+

test/diagnostics/bad-generated-filenames.swift

@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t/SlashA.swiftmodule %S/Inputs/slash.swift
+// RUN: %target-swift-frontend -emit-module -o %t/SlashB.swiftmodule %S/Inputs/slash.swift
+// RUN: not %target-swift-frontend -typecheck -I %t -serialize-diagnostics-path %t/serialized.dia %s
+// RUN: c-index-test -read-diagnostics %t/serialized.dia > %t/serialized.txt 2>&1
+// RUN: %FileCheck %s -check-prefix CHECK-DIA < %t/serialized.txt
+
+import SlashA
+import SlashB
+
+func test(a: String, b: Int) -> Bool {
+  // CHECK-DIA: [[@LINE+3]]:5: error: ambiguous use of operator '/'
+  // CHECK-DIA: SlashA./_operator
+  // CHECK-DIA: SlashB./_operator
+  a / b
+}