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/check/handle_function.cpp

@@ -30,6 +30,7 @@
 #include "toolchain/sem_ir/function.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst.h"
+#include "toolchain/sem_ir/pattern.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
@@ -517,8 +518,21 @@ static auto BuildFunctionDecl(Context& context,
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::FunctionIntroducer>();
 
-  auto self_param_id =
-      FindSelfPattern(context, name.implicit_param_patterns_id);
+  auto self_param_id = FindSelfPattern(context, name.implicit_param_patterns_id,
+                                       name.param_patterns_id);
+
+  // `self` must be the first explicit parameter. (Declaring it in the implicit
+  // parameter list or outside any parameter list is diagnosed earlier.)
+  if (self_param_id.has_value()) {
+    auto explicit_params =
+        context.inst_blocks().GetOrEmpty(name.param_patterns_id);
+    if (!explicit_params.empty() && explicit_params.front() != self_param_id &&
+        llvm::is_contained(explicit_params, self_param_id)) {
+      CARBON_DIAGNOSTIC(SelfNotFirstParam, Error,
+                        "`self` must be the first explicit parameter");
+      context.emitter().Emit(SemIR::LocId(self_param_id), SelfNotFirstParam);
+    }
+  }
 
   // Process modifiers.
   auto [parent_scope_inst_id, parent_scope_inst] =
@@ -660,8 +674,8 @@ static auto DiagnoseUnusedMarkersWithoutDefinition(
   const auto& function = context.functions().Get(function_id);
   // The `unused` modifier requires a definition, so it is not valid on any
   // parameter when there is none. This applies to implicit parameters (such as
-  // `self`) too, so check the implicit parameter list as well as the explicit
-  // one.
+  // `T:! type` introduced by `[...]`) too, so check the implicit parameter list
+  // as well as the explicit one.
   for (auto param_patterns_id :
        {function.implicit_param_patterns_id, function.param_patterns_id}) {
     if (param_patterns_id.has_value()) {