[Kobzol]

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

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.

[rustbot]

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☔ The latest upstream changes (presumably #145146) made this pull request unmergeable. Please resolve the merge conflicts.

On the right track, but a few things need fixing.

There is a test called tests/ui/deriving/deriving-all-codegen.rs which tests the output of all builtin derives. It's really useful to have a test that shows the actual output, so that minor changes can be seen.

From should be added to that file. A lot of the structs don't have a single field, it would be worth adding/moving some of the ones from deriving-from.rs to it.

The derive(...) lists are in mostly-alphabetical order, so I would put From in between Default and Hash.

@rustbot author

I tried to resolve review remarks, and improve the error message(s). Before fixing tests, I'd like to know your opinion on the current errors.

Changes to the code generated for builtin derived traits.

cc @nnethercote

Good progress. The new error messages look good, thanks.

Ok: squash the final 7 commits together, undo the formatting changes to deriving-all-codegen.rs, and r=me.

@bors r=nnethercote

📌 Commit 1f3a747 has been approved by nnethercote

It is now in the queue for this repository.

still in queue; merged in #145475

@bors r-