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 disallows building a facet type that contains another facet type
with non-extend constraints in it. Which in turn prevents the
possibility of introducing a different `.Self` into a facet type.
Eval can still insert a facet type with non-extend constraints, as we
only prevent it for `where` being written into the facet type. There is
a TODO in handle_where.cpp for this and some tests in
toolchain/check/testdata/facet/nested_facet_types_from_eval.carbon
The none.carbon min-prelude is not just an empty prelude, it also
prevents any prelude from being imported at all. So no import machinery
runs before the test, only the `package` statement from the prelude
would run.
Use the none.carbon min-prelude in a few tests that were specifying
`--no-prelude-import` to give it a trial run.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
People seemed receptive [on
#toolchain](https://discord.com/channels/655572317891461132/655578254970716160/1379196447827689553),
so proceeding.
For the two name conflicts, I've set it up so that there's a
"foo.carbon" and "foo_with_prelude.carbon", to indicate that the
no-prelude approach is preferred (with a shorter name).
Note min_prelude will require a little more work/thought, I want to
avoid adding `--custom-core` etc.
Chandler Carruth
Dana Jansens
