Add basic support for [mutating] methods (#667)

By default, when writing a "method" (aka "member function") in Slang, the `this` parameter is implicitly an `in` parameter. So this:

```hlsl
struct Foo
{
    int state;
    int getState() { return state; }
    void setState(int s) { state = s; }
};
```

is desugared into something like this:

```hlsl
struct Foo { int state };
int Foo_getState(Foo this) { return this.state; }

// BAD:
void Foo_setState(Foo this, int s) { this.state = s; }
```

That "setter" doesn't really do what was intended. It modifies a local copy of type `Foo`, because `in` parameters in HLSL represent by-value copy-in semantics, and are mutable in the body the function. Slang was updated to give a static error on the original code to catch this kind of mistake (so that `this` parameters are unlike ordinary function parameters, and no longer mutable).

Of course, sometimes users *want* a mutable `this` parameter. Rather than make a mutable `this` the default (there are arguments both for and against this), this change adds a new attribute `[mutating]` that can be put on a method (member function) to indicate that its `this` parameter should be an `in out` parameter:

```hlsl
[mutating] void setState(int s) { state = s; }
```

The above will translate to, more or less:

```hlsl
void Foo_setState(inout Foo this, int s) { this.state = s; }
```

One added detail is that `[mutating]` can also be used on interface requirements, with the same semantics. A `[mutating]` requirement can be satisfied with a `[mutating]` or non-`[mutating]` method, while a non-`[mutating]` requirement can't be satisfied with a `[mutating]` method (the call sites would not expect mutation to happen).

The design of `[mutating]` here is heavily influenced by the equivalent `mutating` keyword in Swift.

Notes on the implementation:

* Adding the new attribute was straightforward using the existing support, but I had to change around where attributes get checked in the overall sequencing of static checks, because attributes were being checked *after* function bodies, but with this change I need to look at semantically-checked attributes to determine the mutability of `this`

* The check to restrict it so that `[mutating]` methods cannot satisfy non-`[mutating]` requirements was easy to add, but it points out the fact that there is a huge TODO comment where the actual checking of method *signatures* is supposed to happen. That is a bug waiting to bite users and needs to be fixed!

* While we had special-case logic to detect attempts to modify state accessed through an immutable `this` (e.g., `this.state = s`), that logic didn't trigger when the mutation happened through a function/operator call (e.g., `this.state += s`), so this change factors out the validation logic for that case and calls through to it from both the assignment and `out` argument cases.

* The error message for the special-case check was updated to note that the user could apply `[mutating]` to their function declaration to get rid of the error.

* The semantic checking logic for an explicit `this` expression was already walking up through the scopes (created during parsing) and looking for a scope that represents an outer type declaration that `this` might be referring to. We simply extend it to note when it passes through the scope for a function or similar declaration (`FunctionDeclBase`) and check for the `[mutating]` attribute. If the attribute is seen, it returns a mutable `this` expression, and otherwise leaves it immutable.

* The IR lowering logic then needed to be updated so that when adding an IR-level parameter to represent `this`, it gives it the appropriate "direction" based on the attributes of the function declaration being lowered. The rest of the IR logic works as-is, because it will treat `this` just like an other parameter (whether it is `in` or `inout`).

* This biggest chunk of work was the "implicit `this`" case, because ordinary name lookup may resolve an expression like `state` into `this.state`, so that the `this` expression comes out of "thin air." To handle this case, I extended the structure of the "breadcrumbs" that come along with a lookup result (the breadcrumbs are used for any case where a single identifier like `state` needs to be embellished to a more complex expression as a result of lookup), so that it can identify whether a `Breadcrumb::Kind::This` node comes from a `[mutating]` context or not. Similar to the logic for an explicit `this`, we handle this by noting when we pass through a `FunctionDeclBase` when moving up through scopes, and look for the `[mutating]` attribute on it. The rest of the work was just plumbing the additional state through.

1 files changed, 17 insertions, 2 deletions

diff --git a/source/slang/syntax.h b/source/slang/syntax.h
index 24660b7fd..019740a5e 100644
--- a/source/slang/syntax.h
+++ b/source/slang/syntax.h
@@ -864,7 +864,7 @@ namespace Slang
         class Breadcrumb : public RefObject
         {
         public:
-            enum class Kind
+            enum class Kind : uint8_t
             {
                 // The lookup process looked "through" an in-scope
                 // declaration to the fields inside of it, so that
@@ -895,6 +895,16 @@ namespace Slang
             // The kind of lookup step that was performed
             Kind kind;
 
+            // For the `Kind::This` case, is the `this` parameter
+            // mutable or not?
+            enum class ThisParameterMode : uint8_t
+            {
+                Default,
+                Mutating,
+            };
+            ThisParameterMode thisParameterMode = ThisParameterMode::Default;
+
+
             // As needed, a reference to the declaration that faciliated
             // the lookup step.
             //
@@ -910,8 +920,13 @@ namespace Slang
             // arrive at a final value.
             RefPtr<Breadcrumb> next;
 
-            Breadcrumb(Kind kind, DeclRef<Decl> declRef, RefPtr<Breadcrumb> next)
+            Breadcrumb(
+                Kind                kind,
+                DeclRef<Decl>       declRef,
+                RefPtr<Breadcrumb>  next,
+                ThisParameterMode   thisParameterMode = ThisParameterMode::Default)
                 : kind(kind)
+                , thisParameterMode(thisParameterMode)
                 , declRef(declRef)
                 , next(next)
             {}