Naga should support source maps

[jimblandy]

People are generating WGSL from other sources (WESL, for example), and would like to get error messages that refer to their original sources. Naga should help them do that.

This might not really imply sourcemap support within Naga itself. It might be sufficient just to make sure that error types expose spans that a consumer could apply source maps to. But with this approach, the user would then need to use some crate like codespan_reporting themselves.

[kpreid]

IMO, it would be nice to have an API which allows simply passing multiple source strings that are each separately line-numbered (and named), for users who want to organize their shader code into multiple files. (WebGL supports this via GLSL #line directives.)

This could be expressed as a wgpu::util (or entirely external) function that constructs a source map for the concatenation. But if source map support increases code size and build time significantly, then it would be nice to have a separate feature. But-but, users concerned with that might actually prefer the option of entirely leaving span mapping and error formatting out of the binary, as you suggest as an alternative.