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, 30 insertions, 0 deletions

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index ebc3a3ef9..524bab4e4 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -1677,6 +1677,12 @@ namespace Slang
                     expr->type.IsLeftValue = true;
                 }
             }
+            else if( auto typeOrExtensionDecl = as<AggTypeDeclBase>(containerDecl) )
+            {
+                expr->type.type = calcThisType(makeDeclRef(typeOrExtensionDecl));
+                return expr;
+            }
+#if 0
             else if (auto aggTypeDecl = as<AggTypeDecl>(containerDecl))
             {
                 ensureDecl(aggTypeDecl, DeclCheckState::CanUseAsType);
@@ -1706,6 +1712,7 @@ namespace Slang
                 expr->type.type = extensionDecl->targetType.type;
                 return expr;
             }
+#endif
 
             scope = scope->parent;
         }
@@ -1713,4 +1720,27 @@ namespace Slang
         getSink()->diagnose(expr, Diagnostics::thisExpressionOutsideOfTypeDecl);
         return CreateErrorExpr(expr);
     }
+
+    RefPtr<Expr> SemanticsExprVisitor::visitThisTypeExpr(ThisTypeExpr* expr)
+    {
+        auto scope = expr->scope;
+        while (scope)
+        {
+            auto containerDecl = scope->containerDecl;
+            if( auto typeOrExtensionDecl = as<AggTypeDeclBase>(containerDecl) )
+            {
+                auto thisType = calcThisType(makeDeclRef(typeOrExtensionDecl));
+                auto thisTypeType = getTypeType(thisType);
+
+                expr->type.type = thisTypeType;
+                return expr;
+            }
+
+            scope = scope->parent;
+        }
+
+        getSink()->diagnose(expr, Diagnostics::thisTypeOutsideOfTypeDecl);
+        return CreateErrorExpr(expr);
+    }
+
 }