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Support for parsing expression patterns (#6977)

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When parsing a pattern, if we encounter something that isn't pattern
syntax, try parsing as an expression instead. We only need one-token
lookahead to distinguish pattern syntax from expression syntax.

Track a precedence group through pattern parsing so that we can allow
different kinds of expressions in a top-level pattern (such as the
operand of `let`) and in a nested pattern (such as a subpattern of a
tuple pattern or within grouping parens). For example, we do not allow
`case if ...`, and for now I've chosen to also not allow logical or
relational operators at the top level of a pattern, so `case 1 + 1` is
OK, but `case 1 == 1` and `case true and false` require parentheses.
This decision should be ratified or revisited by a design proposal.

Very basic check support is also provided, only sufficient to form an
`ExprPattern` instruction and nothing beyond that. For now, all pattern
matching against an `ExprPattern` fails with a TODO error. To support
that, I've switched from calling `BeginSubpattern` in the parent handler
of a pattern and `EndSubpatternAs*` in the pattern handler itself to
calling both functions in parent handlers, with `EndSubpattern`
converting an expression into an expression pattern where needed.

Depends on #6976.

Assisted-by: Gemini via Google Antigravity
This commit is contained in:
Richard Smith
2026-03-28 00:06:06 +00:00
committed by GitHub
parent 1ef35e8299
commit 181a592b8c
49 changed files with 679 additions and 210 deletions
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toolchain/parse/testdata/function/declaration.carbon
Vendored
+5 -11
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@@ -146,12 +146,8 @@ fn F();
struct X { fn () }
fn F();
// --- fail_with_identifier_as_param.carbon
// --- with_identifier_as_param.carbon
// CHECK:STDERR: fail_with_identifier_as_param.carbon:[[@LINE+4]]:11: error: expected `:`, `:!`, or `:?` in binding pattern [ExpectedBindingPattern]
// CHECK:STDERR: fn foo(bar);
// CHECK:STDERR: ^
// CHECK:STDERR:
fn foo(bar);
// --- fail_without_name_and_many_tokens_in_params.carbon
@@ -426,17 +422,15 @@ fn ComplexReturnForm() ->? X.Y(Z);
// CHECK:STDOUT: {kind: 'FunctionDecl', text: ';', subtree_size: 5},
// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
// CHECK:STDOUT: ]
// CHECK:STDOUT: - filename: fail_with_identifier_as_param.carbon
// CHECK:STDOUT: - filename: with_identifier_as_param.carbon
// CHECK:STDOUT: parse_tree: [
// CHECK:STDOUT: {kind: 'FileStart', text: ''},
// CHECK:STDOUT: {kind: 'FunctionIntroducer', text: 'fn'},
// CHECK:STDOUT: {kind: 'IdentifierNameMaybeBeforeSignature', text: 'foo'},
// CHECK:STDOUT: {kind: 'ExplicitParamListStart', text: '('},
// CHECK:STDOUT: {kind: 'IdentifierNameNotBeforeSignature', text: 'bar'},
// CHECK:STDOUT: {kind: 'InvalidParse', text: ')', has_error: yes},
// CHECK:STDOUT: {kind: 'LetBindingPattern', text: 'bar', has_error: yes, subtree_size: 3},
// CHECK:STDOUT: {kind: 'ExplicitParamList', text: ')', has_error: yes, subtree_size: 5},
// CHECK:STDOUT: {kind: 'FunctionDecl', text: ';', subtree_size: 8},
// CHECK:STDOUT: {kind: 'IdentifierNameExpr', text: 'bar'},
// CHECK:STDOUT: {kind: 'ExplicitParamList', text: ')', subtree_size: 3},
// CHECK:STDOUT: {kind: 'FunctionDecl', text: ';', subtree_size: 6},
// CHECK:STDOUT: {kind: 'FileEnd', text: ''},
// CHECK:STDOUT: ]
// CHECK:STDOUT: - filename: fail_without_name_and_many_tokens_in_params.carbon