Rust error stratergy in the hg codebase

[Raphaël Gomès]

Update,

I've posted the same thing over on Reddit¹ and have gotten advice 
consistent with my discussions with other people (thanks Yuya!). So 
unless someone objects to any of this, I'll have a somewhat difficult 
refactor ahead of my, but at least with a pretty clear plan.

Raphaël

[1] 
https://old.reddit.com/r/rust/comments/wuomjt/help_needed_for_error_stratergy_inside_mercurial/

On 8/22/22 11:06, Raphaël Gomès wrote:
> Hi all,
>
> I've been having an ergonomic issue with error handling lately. Error 
> handling strategy used to be kind of a rough edge in Rust because of 
> the different constraints of both the compiler and the standard 
> library. Lately it seems that most projects are happy with using a mix 
> of `thiserror`/`anyhow`/`eyere`/`snafu`, however when investigating 
> using (a potential combination) of them, they don't appear to allow 
> for all the constraints that we have.
>
> Rust errors in the hg codebase:
>
> - need to be matchable at the library layers (basically that 
> everything inside `hg-core` exposes enums)
> - need to carry enough information for end-users to make sense of the 
> issue and/or devs to be able to help end-users with it
> - need to match the Python implementation's user-facing behavior 
> basically exactly (we can have some leeway for OS errors, I guess, but 
> the point is to leave the formatting to us and not a library)
> - shouldn't do any formatting inside `hg-core`, that should be left to 
> consumers of `hg-core` (though having a default formatting isn't a bad 
> idea for debugging)
> - shouldn't assume that data is somehow UTF8 (ties in to the last 
> point), as we have non-UTF8 data we need to format using 
> `format_bytes` or equivalent (no popular crate seems to care since 
> it's a pretty niche need)
> - need to compose somehow, since not all code can be perfectly 
> hierarchical: the `dirstate` module needs to call functions that may 
> return a `RevsetError` and the `revset` module needs to call functions 
> that may return a `DirstateError`, creating cycles in error enums 
> (this is the big one for me that started this refactor and questioning).
>
> So I'm calling to the Rust experts, or at least people that have some 
> experience with working in larger Rust codebases about what the 
> optimal strategy would be.
>
> Thanks,
> Raphaël
>
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