carbon-language/carbon-lang

Support for struct patterns

Open

#6680 opened on Feb 2, 2026

View on GitHub
 (7 comments) (0 reactions) (0 assignees)C++ (33,701 stars) (1,535 forks)batch import
good first issuetoolchain

Description

The Carbon toolchain is missing support for struct patterns, as documented here. As "first issues" go, this is probably fairly complex, but there are a couple ways of breaking it down to make it more manageable:

  • You can split it up by compilation phase, implementing parsing, then checking, then lowering in separate PRs (I don't think this will need changes to lexing).
  • You can split it up into sub-features: the core {.foo = foo: Foo, .bar = bar: Bar} syntax, and then the {foo: Foo, bar: Bar} shorthand and the trailing , _ syntax.

I'd recommend doing both: have each PR focus on a single phase of a single feature, and probably do all three phases of the core feature before starting on the shorthand or trailing , _ syntaxes.

Be sure to take a look at the toolchain docs, especially the one about pattern matching, if you're not already familiar with them. The existing support for tuple patterns should also be a useful point of reference, because the two features have a lot in common.

Contributor guide

Tech stack
cpp
Domain
backendtooling
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++ knowledgeCarbon toolchain architectureFamiliarity with pattern matching
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Implement struct patterns in the Carbon toolchain. Start with the core `{.foo = foo: Foo, .bar = bar: Bar}` syntax across parsing, checking, and lowering phases. Use the existing tuple pattern implementation (see toolchain/docs/check/pattern matching.md) as reference. Then add shorthand `{foo: Foo, bar: Bar}` and trailing comma underscore syntaxes. Refer to the design docs at trunk/docs/design/pattern matching.md for specifications.