Implement #[derive(From)]

[Kobzol]

Implements the #[derive(From)] feature (tracking issue, RFC ¹).

It allows deriving the From impl on structs and tuple structs with exactly one field. Some implementation notes:

• I wasn't exactly sure which spans to use in the derive generating code, so I just used span everywhere. I don't know if it's the Right Thing To Do. In particular the errors when #[derive(From)] is used on a struct with an unsized field are weirdly duplicated.
• I had to solve an import stability problem, where if I just added the unstable macro From to core::convert, previously working code like use std::convert::From would suddenly require an unstable feature gate, because rustc would think that you're trying to import the unstable macro. @petrochenkov suggested that I add the macro the the core prelude instead. This has worked well, although it only works in edition 2021+. Not sure if I botched the prelude somehow and it should live elsewhere (?).
• I had to add Ty::AstTy, because the from function receives an argument with the type of the single field, and the existing variants of the Ty enum couldn't represent an arbitrary type.

Footnotes

1. Note that the RFC has gone through the FCP, just rfcbot hasn't realized it yet, it should be merged soon. ↩

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