Add support for source forwarding in Error derive #293
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@MegaBluejay no, let's postpone refactoring here as a minor priority task. |
| @@ -32,6 +32,11 @@ often [`From` as well](crate::From). | |||
| called `Backtrace`. Then it would return that field as the `backtrace`. | |||
| 3. One of the fields is annotated with `#[error(backtrace)]`. Then it would | |||
| return that field as the `backtrace`. | |||
| 4. The source field is annotated with `#[error(backtrace)]`. Then it would | |||
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copy paste error?
| 4. The source field is annotated with `#[error(backtrace)]`. Then it would | |
| 4. The source field is annotated with `#[error(forward)]`. Then it would |
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No that's correct, provide is forwarded only when the source is annotated with #[error(backtrace)].
That behaviour is unchanged in this PR.
The reasoning for keeping a separate attribute is to make source forwarding work on stable, since there's no good way of generating feature-gated code.
I also think this is somewhat unintuitive, but I haven't come up with a better idea.
Perhaps it could make sense to disallow #[error(backtrace)] on the source field if there's no #[error(forward)]?
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I'm having a hard time understanding the usecase for such forwarding. Could you share a concrete example for when this is useful? |
The main use case I can see is transparent error variants Something like #[derive(Debug, Display, Error)
enum Error {
...
#[error(forward)]
#[display("{_0}")]
#[debug("{_0:?}")]
Other(anyhow::Error),
}Where there's no new information added by the wrapper, so adding it to the source stack is unnecessary. |
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@MegaBluejay yes, I start thinking that maybe |
Resolves #428
Related to #110, #200
Synopsis
Adds support for
#[error(forward)]attribute, which forwards the.source()implementation to a field.Solution
Use existing forward fields in
Statefor parsing the attributes, and keep the source-inferring logic the same as without forwarding.Backtrace forwarding
The current behaviour is that if the field annotated/inferred to be the source is annotated with
#[error(backtrace)], a forwardedprovide()is generated.This was in the example usage, but not documented in the list above, so I added it.
Checklist
#[error(forward)]when no source field was specified/inferred