Handle attr parse failures

Adding nice error messages for when things go wrong.

Author: etcwilde

include/swift/AST/DiagnosticsParse.def

@@ -1432,6 +1432,8 @@ ERROR(attr_missing_label,PointsToFirstBadToken,
       "missing label '%0:' in '@%1' attribute", (StringRef, StringRef))
 ERROR(attr_expected_label,none,
       "expected label '%0:' in '@%1' attribute", (StringRef, StringRef))
+ERROR(attr_expected_colon_after_label,none,
+      "expected ':' after label '%0'", (StringRef))
 
 ERROR(alignment_must_be_positive_integer,none,
       "alignment value must be a positive integer literal", ())

lib/Parse/ParseDecl.cpp

@@ -2751,7 +2751,6 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
   }
 
   case DAK_UnavailableFromAsync: {
-
     StringRef message;
     if (consumeIf(tok::l_paren)) {
       if (!Tok.is(tok::identifier)) {
@@ -2761,27 +2760,36 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
       StringRef flag = Tok.getText();
 
       if (flag != "message") {
-        llvm_unreachable("Unknown unavailable argument");
+        diagnose(Tok.getLoc(), diag::attr_unknown_option, flag, AttrName);
+        return true;
       }
       consumeToken();
       if (!consumeIf(tok::colon)) {
-        llvm_unreachable("Expected ':' following 'message'");
+        if (!Tok.is(tok::equal)) {
+          diagnose(Tok.getLoc(), diag::attr_expected_colon_after_label, flag);
+          return false;
+        }
+        diagnose(Tok.getLoc(), diag::replace_equal_with_colon_for_value)
+          .fixItReplace(Tok.getLoc(), ": ");
+        consumeToken();
       }
       if (!Tok.is(tok::string_literal)) {
-        llvm_unreachable("Expected string literal");
+        diagnose(Tok.getLoc(), diag::attr_expected_string_literal, AttrName);
+        return false;
       }
 
       Optional<StringRef> value = getStringLiteralIfNotInterpolated(
-          Loc, "' I don't know what goes here! '");
+          Tok.getLoc(), flag);
       if (!value)
-        llvm_unreachable("Expected a message, homie");
-
+        return false;
+      Token stringTok = Tok;
       consumeToken();
-
       message = *value;
 
       if (!consumeIf(tok::r_paren))
-        llvm_unreachable("Expected a closing r-paren");
+        diagnose(stringTok.getRange().getEnd(), diag::attr_expected_rparen,
+            AttrName, /*isModifiler*/false)
+          .fixItInsertAfter(stringTok.getLoc(), ")");
     }
 
     Attributes.add(new (Context) UnavailableFromAsyncAttr(

test/Concurrency/unavailable_from_async.swift

@@ -128,3 +128,33 @@ func asyncFunc() async { // expected-error{{asynchronous global function 'asyncF
   }
 
 }
+
+// Parsing tests
+
+// expected-error@+2 {{expected declaration}}
+// expected-error@+1:24{{unknown option 'nope' for attribute '_unavailableFromAsync'}}
+@_unavailableFromAsync(nope: "almost right, but not quite")
+func blarp1() {}
+
+// expected-error@+2 {{expected declaration}}
+// expected-error@+1 {{expected ':' after label 'message'}}
+@_unavailableFromAsync(message; "almost right, but not quite")
+func blarp2() {}
+
+// expected-error@+1:31 {{'=' has been replaced with ':' in attribute arguments}}{{31-32=: }}
+@_unavailableFromAsync(message="almost right, but not quite")
+func blarp3() {}
+
+// expected-error@+2 {{expected declaration}}
+// expected-error@+1 {{expected string literal in '_unavailableFromAsync' attribute}}
+@_unavailableFromAsync(message: 32)
+func blarp4() {}
+
+// expected-error@+2 {{expected declaration}}
+// expected-error@+1 {{message cannot be an interpolated string}}
+@_unavailableFromAsync(message: "blarppy blarp \(31 + 10)")
+func blarp5() {}
+
+// expected-error@+1:48 {{expected ')' in '_unavailableFromAsync' attribute}}{{48-48=)}}
+@_unavailableFromAsync(message: "blarppy blarp"
+func blarp6() {}