Move self to the explicit () parameter list (proposal #7016) (#7272)

Implements proposal #7016: `self` moves from the deduced implicit list
(`fn F[self: Self]()`) to the front of the explicit list. Its type may
be written explicitly (`fn F(self: Self)`) or omitted, in which case it
defaults to `Self` (`fn F(self)`, `fn F(ref self)`); `self` in the
implicit list is rejected.

Throughout checking, `self` is modeled as the first explicit parameter.
Because a method is just a function whose first parameter is `self`, it
can also be called as an ordinary function with the receiver passed
explicitly (`Type.M(obj, ...)`), not only as `obj.M(...)`. A new
`SemIR::CallArgParamPatterns` helper chooses the parameters matched
against the explicit arguments, excluding a leading `self` only when it
is supplied as a method-call receiver; arity checking, conversion, and
generic deduction use it. The resulting SemIR and lowering are
unchanged: `self` is still `call_param0`, and witnesses, thunks, and
vtables are unaffected.

An omitted `self` type is parsed as a `SelfBindingPattern` node with no
type expression; checking synthesizes the `Self` type so it behaves
exactly like `self: Self`. However, the exact spelling used must match
between a forward declaration and a definition, following #3763's rules
around declaration matching.

Generated functions, thunks, and C++ interop import/export build `self`
as the first explicit parameter, and the `self`-type override (e.g.
Derived->Base for a virtual override) applies to the explicit `self`.
Placement is validated by new diagnostics: `SelfInImplicitParamList`,
`SelfNotFirstParam`, and `SelfOutsideParamList`. The benchmark source
generator and the documentation adopt the `(self)` shorthand; the
prelude, the examples, and the test data are migrated in the following
commits.

Assisted-by: Claude Code with Claude Opus 4.7

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

toolchain/parse/handle_binding_pattern.cpp

@@ -35,13 +35,15 @@ auto HandleBindingPattern(Context& context) -> void {
   }
 
   // The first item should be an identifier, the placeholder `_`, or `self`.
+  std::optional<Lex::TokenIndex> self_token;
   if (auto identifier = context.ConsumeIf(Lex::TokenKind::Identifier)) {
     context.AddLeafNode(NodeKind::IdentifierNameNotBeforeSignature,
                         *identifier);
   } else if (auto self =
                  context.ConsumeIf(Lex::TokenKind::SelfValueIdentifier)) {
-    // Checking will validate the `self` is only declared in the implicit
-    // parameter list of a function.
+    // Checking will validate where `self` may be declared. Its type may be
+    // omitted, in which case it defaults to `Self` (see below).
+    self_token = *self;
     context.AddLeafNode(NodeKind::SelfValueName, *self);
   } else if (auto underscore = context.ConsumeIf(Lex::TokenKind::Underscore)) {
     context.AddLeafNode(NodeKind::UnderscoreName, *underscore);
@@ -116,6 +118,17 @@ auto HandleBindingPattern(Context& context) -> void {
 
     context.PushState(state);
     context.PushStateForExpr(PrecedenceGroup::ForType());
+  } else if (self_token && !template_token) {
+    // A `self` binding may omit its type; checking supplies the implicit `Self`
+    // type. There is no type node, so this produces a `SelfBindingPattern`
+    // rather than a `LetBindingPattern`.
+    if (ref_token) {
+      context.AddNode(NodeKind::RefBindingName, *ref_token, state.has_error);
+    }
+    context.AddNode(NodeKind::SelfBindingPattern, *self_token, state.has_error);
+    if (state.has_error) {
+      context.ReturnErrorOnState();
+    }
   } else {
     on_error(/*expected_name=*/false);
     // Add a substitute for a type node.