[Diagnostics] Support "swift" style diagnostics at EOF

Adjust the valid position checking and special handle EOF position.
If the requested location is at EOF, use the last token, but still
emit the diagnostics at the specificied location.

rdar://138426038
(cherry picked from commit 40c8d81)

Author: rintaro

lib/ASTGen/Sources/ASTGen/DiagnosticsBridge.swift

@@ -262,15 +262,15 @@ public func addQueuedDiagnostic(
   text: UnsafePointer<UInt8>,
   textLength: Int,
   severity: BridgedDiagnosticSeverity,
-  position: BridgedSourceLoc,
+  cLoc: BridgedSourceLoc,
   highlightRangesPtr: UnsafePointer<BridgedSourceLoc>?,
   numHighlightRanges: Int
 ) {
   let queuedDiagnostics = queuedDiagnosticsPtr.assumingMemoryBound(
     to: QueuedDiagnostics.self
   )
 
-  guard let rawPosition = position.getOpaquePointerValue() else {
+  guard let rawPosition = cLoc.getOpaquePointerValue() else {
     return
   }
 
@@ -280,18 +280,33 @@ public func addQueuedDiagnostic(
       return false
     }
 
-    return rawPosition >= baseAddress && rawPosition < baseAddress + sf.buffer.count
+    return rawPosition >= baseAddress && rawPosition <= baseAddress + sf.buffer.count
   }
   guard let sourceFile = sourceFile else {
     // FIXME: Hard to report an error here...
     return
   }
 
-  // Find the token at that offset.
   let sourceFileBaseAddress = UnsafeRawPointer(sourceFile.buffer.baseAddress!)
   let sourceFileEndAddress = sourceFileBaseAddress + sourceFile.buffer.count
   let offset = rawPosition - sourceFileBaseAddress
-  guard let token = sourceFile.syntax.token(at: AbsolutePosition(utf8Offset: offset)) else {
+  let position = AbsolutePosition(utf8Offset: offset)
+
+  // Find the token at that offset.
+  let node: Syntax
+  if let token = sourceFile.syntax.token(at: position) {
+    node = Syntax(token)
+  } else if position == sourceFile.syntax.endPosition {
+    // FIXME: EOF token is not included in '.token(at: position)'
+    // We might want to include it, but want to avoid special handling.
+    // Also 'sourceFile.syntax' is not guaranteed to be 'SourceFileSyntax'.
+    if let token = sourceFile.syntax.lastToken(viewMode: .all) {
+      node = Syntax(token)
+    } else {
+      node = sourceFile.syntax
+    }
+  } else {
+    // position out of range.
     return
   }
 
@@ -346,7 +361,8 @@ public func addQueuedDiagnostic(
 
   let textBuffer = UnsafeBufferPointer(start: text, count: textLength)
   let diagnostic = Diagnostic(
-    node: Syntax(token),
+    node: node,
+    position: position,
     message: SimpleDiagnostic(
       message: String(decoding: textBuffer, as: UTF8.self),
       severity: severity.asSeverity

test/NameLookup/nonempty-brace-in-brace.swift

@@ -1,7 +1,9 @@
 // RUN: not %target-swift-frontend -typecheck %s  2>&1 |  %FileCheck %s --check-prefix=CHECK-NO-ASSERTION
 
+// CHECK-NO-ASSERTION-NOT: Assertion
 // Used to trip an assertion
 
+
 public struct Foo {
     func bar() {
         var copySelf = self
@@ -14,5 +16,4 @@ private extension String {}
 
 
 
-
-// CHECK-NO-ASSERTION-NOT: Assertion
+// EOF

test/diagnostics/pretty-printed-diagnostics-eof.swift

@@ -0,0 +1,13 @@
+// RUN: not %target-swift-frontend -diagnostic-style=swift -typecheck %/s 2>&1 | %FileCheck %s
+
+// REQUIRES: swift_swift_parser
+
+// CHECK:      {{SOURCE_DIR[/\]test[/\]diagnostics[/\]pretty-printed-diagnostics-eof\.swift}}:[[#LINE:]]:1: error: expected '}' in struct
+// CHECK:      [[#LINE-2]] | struct MyStruct {
+// CHECK-NEXT:             |                 `- note: to match this opening '{'
+// CHECK-NEXT: [[#LINE-1]] |   func foo() {}
+// CHECK-NEXT: [[#LINE]]   |
+// CHECK-NEXT:             | `- error: expected '}' in struct
+
+struct MyStruct {
+  func foo() {}