[Breaking change] Remove all use of the language service. Use only the Program.

[ghost]

PR checklist

• Addresses an existing issue: sourceFile duplication for TypedRule #1969
  • Should fix Exception thrown when no-use-before-declare is enabled. #2181 (that error was in document highlights, which we no longer call)
  • Fixes Wrong no-use-before-declare error #2210
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

What changes did you make?

Tslint now only provides the Program, not the LanguageService. LanguageService is for editors; tslint should use the Program and TypeChecker API.
This would be break any custom typed rules, so should be in a major version change. So long as the custom typed rules do not use the language service, conversion is simple. See await-promise for an example.

Only no-use-before-declare and no-unused-variable used the language service.
Rewrote no-use-before-declare. It now gets the symbol at each use and checks that it comes after the declaration.
I think no-unused-variable was going to be deprecated anyway, so removed it. It would be possible to rewrite that too if you want to keep it.

As a side-effect, fixes #1969 since we don't have wrapProgram, which creates a new language service, which then has its own program. This probably helps performance too, but I haven't tested.

Also changed tslint's own linting to use the program. Using typed rules in this repo could be done in another PR.

Will rebase after #2234 is in.

[ajafff]

Also rename the parameter

[nchen63]

actually, #1481 is no longer being deprecated due to community response. Can you put it back?

[nchen63]

The parameter originally used program, and was changed to use language service in #1608

Need to verify that the original issue is doesn't break

[ghost]

Rewrote no-unused-variable to not need the language service. Done by creating a duplicate program with the unused-checking compiler options set. Added workaround for microsoft/TypeScript#9944 (which #1608 fixed) that doesn't require checkEdit.
This rule would presumably be unnecessary if we fixed that bug and implemented microsoft/TypeScript#11051.

[nchen63]

changes look good

[ghost]

Note that this makes no-unused-variable and no-use-before-declare typed rules, while before they worked without a type checker.

[nchen63]

That might be a problem for people who use plug-ins like the VSCode-TSLint since typed rules are not supported. People seem to use this rule to see the warnings instead of using the compiler flag so it doesn't block compilation; see #1481 (comment).

[ajafff]

I'm already working on a solution to use both rules without type checking.
no-use-before-declare is no problem.
no-unused-variable would only find unused private class members with type checking enabled. The problem with implicit required imports may also still be there - haven't looked at the code yet - so we would exclude them from the check without type checking. This way we would at least get most of the failures and find the rest with type checking.

[ghost]

It may be possible to create a program consisting of a single file and run the rest as usual. no-unused-variable has a lot of corner cases so I wouldn't want to duplicate the work done in the TS compiler.