Replace :! binding syntax with phase keywords and contextual defaults (#7479)

Implement the toolchain side of proposal #7254, removing the `:!`
binding
syntax for generic and template parameters in favor of the keywords
`generic`,
`template`, and `runtime` plus contextual defaults for phase.

For valid programs this is semantics-preserving: each binding resolves
to the
same phase, and produces the same SemIR, as it did under `:!`/`:`. The
parser
derives a binding's phase from its syntactic context plus any explicit
phase
keyword; new diagnostics and error recovery for misused keywords are
described
below.

Implementation details for each component:

- Lexer: remove the `:!` (`ColonExclaim`) token, move its virtual
parse-node
  budget onto `:`, and add the `generic` and `runtime` keywords.
- Parser: thread a `BindingContext` (`ExplicitParam`, `DeducedParam`, or
`CompileTimeEntityParam`) from declaration introducers down through
parameter
lists to each binding pattern, using a one-token lookahead to
distinguish a
name-qualifier parameter list from a declaration's own final list.
Parameters
of a compile-time entity (`class`, `interface`, `constraint`, `choice`,
`alias`, `export`, `namespace`) and deduced `[]` parameters default to
checked
generic; explicit function parameters and local bindings default to
runtime.
`HandleBindingPattern` resolves the phase from that context plus the
keyword: a
`generic` keyword needs no node of its own (the phase is carried by the
  binding's node kind), while a `runtime` keyword is preserved as a
`RuntimeBindingName` node so `check` can name it in a diagnostic. A
phase
keyword that is merely redundant with the contextual default is
diagnosed
  here, without invalidating the parse tree.
- Check: a phase keyword that is invalid for its context (for example
`runtime`
on a checked-generic parameter) is diagnosed here, and recovers by
building an
error binding that still introduces the name so that later uses of it do
not
  produce cascading errors.

The removed `:!` syntax is now rejected as an ordinary parse error.

The `form`/`:?`/`->?` ("extended types") portion of proposal #7254 is
left for a
separate change.

Assisted-by: Claude Code

toolchain/parse/handle_decl_name_and_params.cpp

@@ -8,6 +8,28 @@
 
 namespace Carbon::Parse {
 
+// Determines the binding context for an explicit `()` parameter list beginning
+// at the current position. A list that is a name qualifier (its matched `)` is
+// immediately followed by `.`) names a compile-time entity, so its parameters
+// use the compile-time-entity context; otherwise the list uses the enclosing
+// declaration's default `decl_context`.
+static auto ExplicitParamListContext(Context& context,
+                                     BindingContext decl_context)
+    -> BindingContext {
+  // A compile-time entity's parameters are compile-time whether or not this
+  // list is a name qualifier, so there is no need to look ahead for the `.`.
+  if (decl_context == BindingContext::CompileTimeEntityParam) {
+    return decl_context;
+  }
+  auto open_paren = *context.position();
+  auto close_paren = context.tokens().GetMatchedClosingToken(open_paren);
+  auto after_close = Lex::TokenIndex(close_paren.index + 1);
+  if (context.tokens().GetKind(after_close) == Lex::TokenKind::Period) {
+    return BindingContext::CompileTimeEntityParam;
+  }
+  return decl_context;
+}
+
 // Adds a leaf node for the name, and updates the state stack for parameter
 // handling.
 static auto HandleName(Context& context, Context::State state,
@@ -21,21 +43,27 @@ static auto HandleName(Context& context, Context::State state,
       context.AddNode(not_before_params_qualifier_kind,
                       context.ConsumeChecked(Lex::TokenKind::Period),
                       state.has_error);
-      context.PushState(StateKind::DeclNameAndParams);
+      // Carry the declaration's binding context across the qualified name so
+      // the final parameter list uses the declaration's default phase.
+      context.PushState(StateKind::DeclNameAndParams, *context.position(),
+                        state.binding_context);
       break;
 
     case Lex::TokenKind::OpenSquareBracket:
       context.AddLeafNode(before_params_kind, name_token);
       state.kind = StateKind::DeclNameAndParamsAfterImplicit;
       context.PushState(state);
-      context.PushState(StateKind::PatternListAsImplicit);
+      context.PushState(StateKind::PatternListAsImplicit, *context.position(),
+                        BindingContext::DeducedParam);
       break;
 
     case Lex::TokenKind::OpenParen:
       context.AddLeafNode(before_params_kind, name_token);
       state.kind = StateKind::DeclNameAndParamsAfterParams;
       context.PushState(state);
-      context.PushState(StateKind::PatternListAsExplicit);
+      context.PushState(
+          StateKind::PatternListAsExplicit, *context.position(),
+          ExplicitParamListContext(context, state.binding_context));
       break;
 
     case Lex::TokenKind::MinusGreater:
@@ -91,7 +119,8 @@ auto HandleDeclNameAndParamsAfterImplicit(Context& context) -> void {
     return;
   }
 
-  context.PushState(StateKind::PatternListAsExplicit);
+  context.PushState(StateKind::PatternListAsExplicit, *context.position(),
+                    ExplicitParamListContext(context, state.binding_context));
 }
 
 auto HandleDeclNameAndParamsAfterParams(Context& context) -> void {
@@ -102,7 +131,8 @@ auto HandleDeclNameAndParamsAfterParams(Context& context) -> void {
                  NodeKind::IdentifierNameMaybeBeforeSignature);
     context.AddNode(NodeKind::IdentifierNameQualifierWithParams, *period,
                     state.has_error);
-    context.PushState(StateKind::DeclNameAndParams);
+    context.PushState(StateKind::DeclNameAndParams, *context.position(),
+                      state.binding_context);
   }
 }