Mutable reference on owner in builder

[gwenn]

Would it be possible to have &'a mut $Owner:

fn try_new<Err>(
    owner: $Owner,
    dependent_builder: impl for<'a> ::core::ops::FnOnce(&'a mut $Owner) -> Result<$Dependent<'a>, Err>
) -> Result<Self, Err>

instead of

fn try_new<Err>(
    owner: $Owner,
    dependent_builder: impl for<'a> ::core::ops::FnOnce(&'a $Owner) -> Result<$Dependent<'a>, Err>
) -> Result<Self, Err>

(use case: https://github.com/rusqlite/rusqlite/pull/1462/files#diff-c595ea0bc5c6bee5e8dfc649ad92279225e2e9a354b8f4d355fbaf4012ebe1e8R19)

[Voultapher]

Hi, this has been discussed here a couple of times, for example here #36. It's one of the main differences to ouroboros API wise, allowing it easily leads to UB, the available API would need to be more variable than it is now. Although now that I'm revisiting the topic I have a new idea how it could be tackled without making the API variable.

[Voultapher]

@gwenn could you please try the main branch and let me know if the added MutBorrow works for your use-cases. This example should explain how to use it.

[gwenn]

Tested successfully:
rusqlite/rusqlite@93787c6
Thanks.

[Voultapher]

Cool!

I plan on doing another feature, after which I plan on releasing a new version that contains MutBorrow. Will close this issue once the new version is released.

[hniksic]

Thanks for implementing this feature - please consider uploading a release that contains it. It'd be nice to replace uses of ouroboros with self-cell, as self-cell generates much less code. But the switch is blocked by cases where the builder requires a mutable owner (and it doesn't make sense to use two self-referential crates in the same workspace).

[Voultapher]

Thanks for the reminder, I have to admit to myself that the idea will just not be ready soon so while it would have been nice to have a bigger 1.1 release, let's just release this.

[Voultapher]

Done https://github.com/Voultapher/self_cell/releases/tag/v1.1.0.

[Voultapher]

@gwenn I just released v1.1.0 with that version you should be able to port the rusqlite use cases.

[gwenn]

@Voultapher
Wonderful !