fix(typescript)!: incorrect declarations directory

[georg94]

Rollup Plugin Name: 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:

• [@rollup/plugin-typescript] tsconfig declarationDir is ignored when using single file output #1230

Description

The commit 5f78173 changed the behaviour of the typescript plugin. This introduced the issue #1230: The declarationDir was now ignored for single file setups. I made some analysis and found the problem:

• In the construction of the plugin the typescript configuration is parsed
• while parsing all paths are made absolute, prepended with process.cwd() (packages/typescript/src/options/tsconfig.ts#L147)
• in order to output declaration files the baseDir is determied from the parsed config
• the id (filename generated by typescript) of the declaration file is also an absolute path, generated with the config
• as the "prefixes" match the result file misses the declarationDir

Example:

let tsConfig = {
  // [...]
  "declaration": true,
  "declarationDir": "types"
  // [...]
};
// After initial config processing
tsConfig = {
  // [...]
  "declaration": true,
  "declarationDir": "/home/user/app/types"
  // [...]
};
// declaration file generation
const id = '/home/user/app/types/index.d.ts';
const baseDir = parsedOptions.options.declarationDir; // -> '/home/user/app/types'
path.relative(baseDir, id) // gives `index.d.ts` instead of `types/index.d.ts`

There is another description of this error in #1230 (comment)

[shellscape]

Because this fix is modifying the output of the plugin, it's a breaking change and will be released as a new major.