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/pattern_match.cpp

@@ -21,6 +21,7 @@
 #include "toolchain/check/type.h"
 #include "toolchain/diagnostics/format_providers.h"
 #include "toolchain/sem_ir/expr_info.h"
+#include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/inst_kind.h"
 #include "toolchain/sem_ir/pattern.h"
 
@@ -978,7 +979,7 @@ auto CalleePatternMatch(Context& context,
           .param_ranges = {implicit_end, explicit_end, return_end}};
 }
 
-auto ThunkPatternMatch(Context& context, SemIR::InstId self_pattern_id,
+auto ThunkPatternMatch(Context& context,
                        llvm::ArrayRef<SemIR::InstId> param_pattern_ids,
                        llvm::ArrayRef<SemIR::InstId> outer_call_args)
     -> ThunkPatternMatchResults {
@@ -986,15 +987,7 @@ auto ThunkPatternMatch(Context& context, SemIR::InstId self_pattern_id,
   MatchContext match(context);
 
   llvm::SmallVector<SemIR::InstId> inner_args;
-  inner_args.reserve(outer_call_args.size() + 1);
-
-  if (self_pattern_id.has_value()) {
-    inner_args.push_back(match.MatchWithResult(
-        &state,
-        {.pattern_id = self_pattern_id,
-         .work = MatchContext::PreWork{.scrutinee_id = SemIR::InstId::None},
-         .allow_unmarked_ref = true}));
-  }
+  inner_args.reserve(outer_call_args.size());
 
   for (SemIR::InstId inst_id : param_pattern_ids) {
     inner_args.push_back(match.MatchWithResult(
@@ -1035,18 +1028,21 @@ auto CallerPatternMatch(Context& context, SemIR::SpecificId specific_id,
   CallerState state = {.callee_specific_id = specific_id};
   MatchContext match(context);
 
-  if (self_pattern_id.has_value()) {
-    match.Match(&state,
-                {.pattern_id = self_pattern_id,
-                 .work = MatchContext::PreWork{.scrutinee_id = self_arg_id},
-                 .allow_unmarked_ref = true});
+  // When we have a separate `self_arg_id`, we concatenate that onto the front
+  // of the arg_refs to match against the first parameter.
+  llvm::ArrayRef<SemIR::InstId> self_arg_refs = {};
+  if (self_arg_id.has_value()) {
+    self_arg_refs = self_arg_id;
+    CARBON_CHECK(self_pattern_id.has_value());
   }
 
-  for (auto [arg_id, param_pattern_id] : llvm::zip_equal(
-           arg_refs, context.inst_blocks().GetOrEmpty(param_patterns_id))) {
-    match.Match(&state, {.pattern_id = param_pattern_id,
-                         .work = MatchContext::PreWork{.scrutinee_id = arg_id},
-                         .allow_unmarked_ref = is_desugared});
+  for (const auto& [arg_id, param_pattern_id] : llvm::zip_equal(
+           llvm::concat<const SemIR::InstId>(self_arg_refs, arg_refs),
+           context.inst_blocks().GetOrEmpty(param_patterns_id))) {
+    match.Match(&state,
+                {.pattern_id = param_pattern_id,
+                 .work = MatchContext::PreWork{.scrutinee_id = arg_id},
+                 .allow_unmarked_ref = arg_id == self_arg_id || is_desugared});
   }
 
   // Track the return storage, if present.