[Breaking] don't exclude *.d.ts files when using --project

[ajafff]

Currently when using --project CLI option without explicitly specifying files, TSLint lints all files in the project except *.d.ts files.
I guess it was implemented that way to avoid linting external dependencies from mode_modules.

Unfortunately this also excludes declaration files that are located within the project.

Starting from typescript@2.6.0 we can use an API to determine if a file comes from node_modules (#2827).
That makes excluding *.d.ts files unnecessary. I propose we change that behavior in the next major version.

[mpal9000]

program.getCompilerOptions().skipLibCheck === true && file.fileName.endsWith(".d.ts")
      || program.isSourceFileFromExternalLibrary(file)
    ? undefined
    : file.fileName

[ajafff]

@mpal9000 I don't thing skipLibCheck is a good indicator for TSLint to ignore declaration files. People mostly use it to ignore type errors in typings of external libraries (or for performance reasons).
Since many TSLint rules are about code style, users might still want to check their own declaration files with TSLint.

Declaration files of dependencies are excluded anyway. To exclude declaration files in your project, you can simply add -e '**/*.d.ts' when executing tslint from command line.

[mpal9000]

The code above is from linter.ts -> getFileNames(). So you propose to remove the endsWith() check and use the -e cli option or do the filtering manually when using the api? That would be ok.

Does a program configured with skipLibCheck provides the required type information to lint .d.ts files of a "project" (for rules that require type information)?

[ajafff]

Does a program configured with skipLibCheck provides the required type information to lint .d.ts files of a "project" (for rules that require type information)?

The linter simply ignores the flag. It's only relevant for tsc to swallow errors in declaration files.
The declarations from those files and their type information are still available.

Be aware that those declarations might have errors without you noticing. That could confuse the linter, but it will also confuse tsc. You probably already knew that from the typescript docs.

[mpal9000]

Yes, I'm using skipLibCheck for incremental building. So .d.ts files of a "project" cannot be reliably linted when the compiler option is enabled. Nevertheless I agree that it would be better to allow the user control the .d.ts exclusion separately. Waiting for this change.. Thanks

[adidahiya]

I agree with this breaking change for 6.0

[JoshuaKGoldberg]

💀 It's time! 💀

TSLint is being deprecated and no longer accepting pull requests for major new changes or features. See #4534. 😱

If you'd like to see this change implemented, you have two choices:

• Recommended: Check if this is available in ESLint + typescript-eslint ✅
• Not Recommended: Fork TSLint locally 🤷‍♂️

👋 It was a pleasure open sourcing with you!

If you believe this message was posted here in error, please comment so we can re-open the issue!