implement leading bar and ampersand in types (#1286)

Implements the [Leading `|` and `&` in types](https://rfcs.luau-lang.org/syntax-leading-bar-and-ampersand.html) RFC.

The changes to the parser are exactly as described in the RFC.

---------

Co-authored-by: Alexander McCord <11488393+alexmccord@users.noreply.github.com>

Ast/src/Parser.cpp

@@ -17,6 +17,7 @@ LUAU_FASTINTVARIABLE(LuauParseErrorLimit, 100)
 // flag so that we don't break production games by reverting syntax changes.
 // See docs/SyntaxChanges.md for an explanation.
 LUAU_FASTFLAGVARIABLE(DebugLuauDeferredConstraintResolution, false)
+LUAU_FASTFLAGVARIABLE(LuauLeadingBarAndAmpersand, false)
 
 namespace Luau
 {
@@ -1523,7 +1524,11 @@ AstType* Parser::parseFunctionTypeTail(const Lexeme& begin, AstArray<AstGenericT
 AstType* Parser::parseTypeSuffix(AstType* type, const Location& begin)
 {
     TempVector<AstType*> parts(scratchType);
+
+    if (!FFlag::LuauLeadingBarAndAmpersand || type != nullptr)
+    {
         parts.push_back(type);
+    }
 
     incrementRecursionCounter("type annotation");
 
@@ -1623,15 +1628,34 @@ AstTypeOrPack Parser::parseTypeOrPack()
 AstType* Parser::parseType(bool inDeclarationContext)
 {
     unsigned int oldRecursionCount = recursionCounter;
-    // recursion counter is incremented in parseSimpleType
+    // recursion counter is incremented in parseSimpleType and/or parseTypeSuffix
 
     Location begin = lexer.current().location;
 
+    if (FFlag::LuauLeadingBarAndAmpersand)
+    {
+        AstType* type = nullptr;
+
+        Lexeme::Type c = lexer.current().type;
+        if (c != '|' && c != '&')
+        {
+            type = parseSimpleType(/* allowPack= */ false, /* in declaration context */ inDeclarationContext).type;
+            recursionCounter = oldRecursionCount;
+        }
+
+        AstType* typeWithSuffix = parseTypeSuffix(type, begin);
+        recursionCounter = oldRecursionCount;
+
+        return typeWithSuffix;
+    }
+    else
+    {
         AstType* type = parseSimpleType(/* allowPack= */ false, /* in declaration context */ inDeclarationContext).type;
 
         recursionCounter = oldRecursionCount;
 
         return parseTypeSuffix(type, begin);
+    }
 }
 
 // Type ::= nil | Name[`.' Name] [ `<' Type [`,' ...] `>' ] | `typeof' `(' expr `)' | `{' [PropList] `}'

tests/Parser.test.cpp

@@ -16,6 +16,7 @@ LUAU_FASTINT(LuauRecursionLimit);
 LUAU_FASTINT(LuauTypeLengthLimit);
 LUAU_FASTINT(LuauParseErrorLimit);
 LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution);
+LUAU_FASTFLAG(LuauLeadingBarAndAmpersand);
 
 namespace
 {
@@ -3167,4 +3168,26 @@ TEST_CASE_FIXTURE(Fixture, "read_write_table_properties")
     LUAU_ASSERT(pr.errors.size() == 0);
 }
 
+TEST_CASE_FIXTURE(Fixture, "can_parse_leading_bar_unions_successfully")
+{
+    ScopedFastFlag sff{FFlag::LuauLeadingBarAndAmpersand, true};
+
+    parse(R"(type A = | "Hello" | "World")");
+}
+
+TEST_CASE_FIXTURE(Fixture, "can_parse_leading_ampersand_intersections_successfully")
+{
+    ScopedFastFlag sff{FFlag::LuauLeadingBarAndAmpersand, true};
+
+    parse(R"(type A = & { string } & { number })");
+}
+
+TEST_CASE_FIXTURE(Fixture, "mixed_leading_intersection_and_union_not_allowed")
+{
+    ScopedFastFlag sff{FFlag::LuauLeadingBarAndAmpersand, true};
+
+    matchParseError("type A = & number | string | boolean", "Mixing union and intersection types is not allowed; consider wrapping in parentheses.");
+    matchParseError("type A = | number & string & boolean", "Mixing union and intersection types is not allowed; consider wrapping in parentheses.");
+}
+
 TEST_SUITE_END();