Add support for importing JSON modules "as const"

[james-pre]

This PR implements importing JSON modules with const context, which has the same effect as an as const assertion.

This was discussed in microsoft/TypeScript#32063. The issue suggests using a const import attribute, however this PR uses a compiler option which is much simpler and should be more maintainable.

This feature is opt-in, so no existing code will be broken. It can be enabled by setting compilerOptions.importJsonAsConst to true in tsconfig.

I've already tested a patch of the compiler written in Typescript on some of my own code and it works great. I also added some new tests for completeness.

[james-pre]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

This PR adds support for importing JSON modules with const assertion semantics through a new importJsonAsConst compiler option. When enabled, JSON imports receive literal types (e.g., "string" instead of string) and readonly modifiers, equivalent to using as const in TypeScript.

Changes:

• Added importJsonAsConst boolean compiler option (opt-in, defaults to false)
• Modified type checker to apply const context to JSON file expressions when the option is enabled
• Added comprehensive test cases demonstrating both default and const-enabled behavior

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/jsonLiteralTypes.ts	Test case demonstrating JSON imports with importJsonAsConst: true producing literal types
testdata/tests/cases/compiler/jsonLiteralTypesDefault.ts	Test case verifying default behavior without the option produces wide types
testdata/baselines/reference/tsc/commandLine/help-all.js	Updated command-line help to include the new importJsonAsConst option
testdata/baselines/reference/compiler/jsonLiteralTypes.*	Baseline files showing expected literal types when option is enabled
testdata/baselines/reference/compiler/jsonLiteralTypesDefault.*	Baseline files showing expected wide types and errors in default behavior
internal/tsoptions/parsinghelpers.go	Added parsing logic for the importJsonAsConst option
internal/tsoptions/declscompiler.go	Added command-line option definition with appropriate flags
internal/diagnostics/extraDiagnosticMessages.json	Added diagnostic message for the new option
internal/diagnostics/diagnostics_generated.go	Generated diagnostic code (6933) for the option description
internal/core/compileroptions.go	Added ImportJsonAsConst field to compiler options struct
internal/checker/checker.go	Modified isConstContext to return true for JSON expressions when option is enabled, and updated type resolution to skip widening for JSON imports