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

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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
This commit is contained in:
Chandler Carruth
2026-07-11 01:22:44 +00:00
committed by GitHub
parent bf106c3b4b
commit 8be274cf60
482 changed files with 16917 additions and 16226 deletions
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toolchain/check/testdata/function/generic/template_param.carbon
Vendored
+3 -3
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@@ -16,7 +16,7 @@
library "[[@TEST_NAME]]";
fn F(unused template T:! type) {
fn F(unused template T: type) {
}
fn G() {
@@ -49,9 +49,9 @@ fn G() {
// CHECK:STDOUT: %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
// CHECK:STDOUT: %T.patt.loc4_23.1: %pattern_type = symbolic_binding_pattern T, 0, template [template = %T.patt.loc4_23.2 (constants.%T.patt)]
// CHECK:STDOUT: } {
// CHECK:STDOUT: %.loc4_26.1: type = splice_block %.loc4_26.2 [concrete = type] {
// CHECK:STDOUT: %.loc4_25.1: type = splice_block %.loc4_25.2 [concrete = type] {
// CHECK:STDOUT: %.Self.frozen: %type = symbolic_binding .Self [symbolic_self = constants.%.Self.frozen]
// CHECK:STDOUT: %.loc4_26.2: type = type_literal type [concrete = type]
// CHECK:STDOUT: %.loc4_25.2: type = type_literal type [concrete = type]
// CHECK:STDOUT: }
// CHECK:STDOUT: %T.loc4_23.2: type = symbolic_binding T, 0, template [template = %T.loc4_23.1 (constants.%T)]
// CHECK:STDOUT: }