fix(typescript): emit declaration files for type-only source files that are not explicitly included

[ianyong]

Rollup Plugin Name: @rollup/plugin-typescript

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers:

• Fixes [@rollup/plugin-typescript] Declaration files for type-only source files that are not explicitly included are not emitted #1554.

Description

Please read the linked issue first for context.

The core issue is that TypeScript source files which contain only type definitions transpile into nothing. This means that Rollup has no equivalent JavaScript files to operate on (i.e., we cannot handle these files if they are not already included in tsconfig.json because within the Rollup build hooks and output generation hooks, these files do not exist). Specifically in the test case added, the export statement in main.ts is removed when transpiling into JavaScript because MyNumber is a type. This means that by the time Rollup operates on main.js, there is no longer any logical link between main.ts & should-be-emitted-types.ts.

However, the observation here is that we don't have to manually keep track of which files to emit because the TypeScript compiler already does it for us. Specifically, the TypeScript compiler only emits the files which are necessary (this is verified in the test case I added). This means that we can simply emit all declaration & source map files in emittedFiles to solve the linked issue.

[ianyong]

The reason for this change is that we only need to worry about emitting .d.ts.map (and equivalent .d.cts.map & .d.mts.map) files. The .js.map files are already being emitted by the TypeScript compiler (unless the noEmit option is set to true).

[shellscape]

Looks good. Will this alter any existing behavior, cause a difference in output (aside from additional files)

[ianyong]

Will this alter any existing behavior, cause a difference in output (aside from additional files)

No, the only change in behaviour is that in the generateBundle output generation hook, we output the set of files emitted by the TypeScript compiler instead of computing this set of files ourselves. Every other step in the build process is unchanged.

[shellscape]

[shellscape]

don't report bugs on closed pull requests. open a new issue and follow the issue template