Add surface syntax for "this type" (#1236)

Within the context of an aggregate type (or an `extension` of one), the programmer can use `this` to refer to the "current" instance of the surrounding type, but there is no easy way to utter the name of the type itself. This is especially relevant inside of an `interface`, where the type of `this` isn't actually the `interface` type, but rather a placeholder for the as-yet-unknown concrete type that will implement the interface.

This change adds a keyword `This` that works similarly to `this`, but names the current *type* instead of the current instance. It can be used to declare things like binary methods or factory functions in an interface:

```
interface IBasicMathType
{
    This absoluteValue();
    This sumWith(This left);
}

T doSomeMath<T:IBasicMathType>(T value)
{
    return value.sumWith(value.absoluteValue());
}
```

The `This` type is consistent with the type named `Self` in Rust and Swift (where Rust/Swift use `self` instead of `this`). Other names could be considered (e.g., `ThisType`) if we find that users don't like the name in this change.

1 files changed, 19 insertions, 0 deletions

diff --git a/source/slang/slang-parser.cpp b/source/slang/slang-parser.cpp
index e5a1ad576..288962cd8 100644
--- a/source/slang/slang-parser.cpp
+++ b/source/slang/slang-parser.cpp
@@ -1794,6 +1794,19 @@ namespace Slang
         return parseTaggedUnionType(parser);
     }
 
+        /// Parse a `This` type expression
+    static RefPtr<Expr> parseThisTypeExpr(Parser* parser)
+    {
+        RefPtr<ThisTypeExpr> expr = new ThisTypeExpr();
+        expr->scope = parser->currentScope;
+        return expr;
+    }
+
+    static RefPtr<RefObject> parseThisTypeExpr(Parser* parser, void* /*userData*/)
+    {
+        return parseThisTypeExpr(parser);
+    }
+
     static TypeSpec parseTypeSpec(Parser* parser)
     {
         TypeSpec typeSpec;
@@ -1848,6 +1861,11 @@ namespace Slang
             typeSpec.expr = parseTaggedUnionType(parser);
             return typeSpec;
         }
+        else if(AdvanceIf(parser, "This"))
+        {
+            typeSpec.expr = parseThisTypeExpr(parser);
+            return typeSpec;
+        }
 
         Token typeName = parser->ReadToken(TokenType::Identifier);
 
@@ -4995,6 +5013,7 @@ namespace Slang
         addBuiltinSyntax<Expr>(session, scope, #KEYWORD, &CALLBACK)
 
         EXPR(this,  parseThisExpr);
+        EXPR(This,  parseThisTypeExpr);
         EXPR(true,  parseTrueExpr);
         EXPR(false, parseFalseExpr);
         EXPR(__TaggedUnion, parseTaggedUnionType);