Proposal: support #line directive so we can use other tools and have sane line numbers for errors & outputs.

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Search Terms

#line preprocessing directive

Suggestion

the c++ preprocessor adds #line directives and some other preprocessing tools do similar as well. And it's a good way to have multiple preprecossors act on a single file in a pipeline.

This proposal is just only supporting the '#line` directive so that other tools can do the job of preprocessing. Other existing issues such as Preprocessor directives proposal propose more kinds of directives and a more complete set of directives.

Additionally this proposal would support the preprocessor output of clang/gcc for line changes in the form of

# 1 "some/path/file.ts 1

which is very similar to #line directive.

Use Cases

I want to run c preprocessor on my typescript file and feed it directly to tsc. By supporting this directive typescript will be able to give proper line numbers and generate a more sensible source mappings.

Examples

#line 33 "../../../somedir/myfile.ts"
code.logic = 7;

Now for error reporting and generation of source maps I expect it to map to respective file and line number.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[nmain]

#13944 seems like a more sensible solution; it allows the same result as #line, it doesn't require intrusive syntax changes in source files, and is already supported by many parts of the js ecosystem.

[RyanCavanaugh]

Agree with @nmain - source maps are the standard for JS tooling so #13944 would be the preferred solution in this space.