[Wesley Wigham (weswigham)]

Fixes #4525

As a side-effect, this makes exported binding patterns (potentially) isolatedDeclarations-emittable, since we can lookup their local type node/assertion/inference result the same as any other variable declaration.

Honestly, we've supported binding patterns in declaration files for... as long as I can remember, at least (and we've emitted them in parameter positions for many many major releases at this point), so I don't think there's really any harm in this - unless you'd get mad that when you write export const [,...rest] = [first(), ...others()] we now expose the type of first() in the declaration file even though it's not actually an accessible part of the exported shape. Still - that's probably worthwhile in exchange for making binding patterns isolatedDeclarations-emittable.

[Wesley Wigham (weswigham)]

The "interesting" part implementation-wise is that variable declarations hosting binding patterns do have an overall type (ofc they do), but don't have a symbol bound to them (only the individual names in the binding pattern do). Somehow, this is the first time we've needed to serialize such symbol-less declarations, so I had to adjust the pipeline to actually handle nil symbols.

(Compare with parameters where even the nameless binding pattern parameters get a bound symbol that refers to them, since that's how the argument type is looked up externally.)

[Copilot]

Pull request overview

Preserves variable binding patterns in declaration emit and updates type lookup to support symbol-less declarations.

Changes:

• Emits destructuring patterns rather than flattened declarations.
• Adds regression coverage and updates affected baselines.
• Handles symbol-less declaration type resolution safely.

Reviewed changes

Copilot reviewed 50 out of 50 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/declarationEmitVariableBindingPatternsRetained.ts	Adds regression coverage.
testdata/submoduleAccepted.txt	Registers accepted baseline differences.
testdata/baselines/reference/submoduleAccepted/conformance/stringLiteralTypesAndTuples01.js.diff	Accepts tuple-pattern emit.
testdata/baselines/reference/submoduleAccepted/conformance/dependentDestructuredVariables.js.diff	Accepts object-pattern emit.
testdata/baselines/reference/submoduleAccepted/compiler/isolatedDeclarationErrorsExpressions.errors.txt.diff	Accepts isolated-declaration diagnostics.
testdata/baselines/reference/submoduleAccepted/compiler/destructuredDeclarationEmit.js.diff	Accepts nested-pattern emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitRetainsJsdocyComments.js.diff	Updates comment-preserving emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks3.js.diff	Accepts mapped-type pattern emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks2.js.diff	Accepts mapped-type pattern emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks1.js.diff	Accepts mapped-type pattern emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitNonExportedBindingPattern.js.diff	Accepts non-exported patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitExpressionInExtends6.js.diff	Updates destructured class dependency emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringPrivacyError.js.diff	Updates privacy-case emit.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringObjectLiteralPattern2.js.diff	Accepts nested object patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringObjectLiteralPattern1.js.diff	Accepts object patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringObjectLiteralPattern.js.diff	Accepts comprehensive object patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern5.js.diff	Accepts omitted array elements.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern4.js.diff	Accepts array rest patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern3.js.diff	Accepts namespace array patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern2.js.diff	Accepts nested array patterns.
testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern1.js.diff	Accepts basic array patterns.
testdata/baselines/reference/submodule/conformance/stringLiteralTypesAndTuples01.js	Updates tuple declaration output.
testdata/baselines/reference/submodule/conformance/dependentDestructuredVariables.js	Updates object declaration output.
testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsExpressions.errors.txt	Removes binding-pattern diagnostics.
testdata/baselines/reference/submodule/compiler/destructuredDeclarationEmit.js	Records nested destructuring output.
testdata/baselines/reference/submodule/compiler/declarationEmitRetainsJsdocyComments.js	Records comments inside patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks3.js	Records mapped-type pattern output.
testdata/baselines/reference/submodule/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks2.js	Records mapped-type pattern output.
testdata/baselines/reference/submodule/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks1.js	Records mapped-type pattern output.
testdata/baselines/reference/submodule/compiler/declarationEmitNonExportedBindingPattern.js	Records non-exported patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitExpressionInExtends6.js	Records namespace destructuring.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringPrivacyError.js	Records privacy-case pattern.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringObjectLiteralPattern2.js	Records nested object patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringObjectLiteralPattern1.js	Records object patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringObjectLiteralPattern.js	Records comprehensive object patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern5.js	Records omitted array elements.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern4.js	Records array rest patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern3.js	Records namespace array patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern2.js	Records nested array patterns.
testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern1.js	Records basic array patterns.
testdata/baselines/reference/conformance/dependentDestructuredVariablesNoCrash3.js	Updates malformed-pattern emit baseline.
testdata/baselines/reference/compiler/subpathImportDeclarationEmit.js	Preserves patterns across subpath imports.
testdata/baselines/reference/compiler/declarationEmitVariableBindingPatternsRetained.types	Records inferred types.
testdata/baselines/reference/compiler/declarationEmitVariableBindingPatternsRetained.symbols	Records symbol bindings.
testdata/baselines/reference/compiler/declarationEmitVariableBindingPatternsRetained.js	Records JavaScript and declaration emit.
testdata/baselines/reference/compiler/declarationEmitSubpathImportsReexport.js	Updates re-export declaration output.
internal/transformers/declarations/transform.go	Retains variable binding patterns.
internal/pseudochecker/lookup.go	Handles symbol-less variables.
internal/checker/nodebuilderimpl.go	Resolves symbol-less declaration types.
internal/checker/emitresolver.go	Avoids nil-symbol literal checks.

[Jake Bailey (jakebailey)]

Does this need a refreshed message that mentions the initializer? Sort of curious how this differs from export const foo = 1, though

[Wesley Wigham (weswigham)]

Sort of curious how this differs from export const foo = 1, though

The = 1 effectively removes the | undefined present in the type annotation for that member of the binding element - meaning that the type annotation alone won't give you the actual type of b. If we had a syntax like export const [,,b!]: [number, number, number | undefined], that'd work for the emit and we'd be able to always use the type annotation (simply replacing the initializer expression with an assertion), but... we do not have a "definitely assigned assertion" for binding elements (and even then, that's actually a questionable transform, since maybe the initializer still has undefined in its' type - there's really just no way around doing expression checking without an actual per-element type annotation!).

Does this need a refreshed message that mentions the initializer?

I can update it, if you'd like, since the cases when the error is emitted has been narrowed to only initialized binding elements.

[Milan Patel (thempatel)]

hey team! i think this PR caused this regression. most of this is over my head but git bisect seems to point to this change