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Implement coherence checks for negative trait impls by spastorino · Pull Request...

 2 years ago
source link: https://github.com/rust-lang/rust/pull/90104
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spastorino commented 8 days ago

The main purpose of this PR is to be able to move Error trait to core.

This feature is necessary to handle the following from impl on box.

impl From<&str> for Box<dyn Error> { ... }

Without having negative traits affect coherence moving the error trait into core and moving that From impl to alloc will cause the from impl to no longer compiler because of a potential future incompatibility. The compiler indicates that &str could introduce an Error impl in the future, and thus prevents the From impl in alloc that would cause overlap with From<E: Error> for Box<dyn Error>. Adding impl !Error for &str {} with the negative trait coherence feature will disable this error by encoding a stability guarantee that &str will never implement Error, making the From impl compile.

We would have this in alloc:

impl From<&str> for Box<dyn Error> {} // A 
impl<E> From<E> for Box<dyn Error> where E: Error {} // B

and this in core:

trait Error {}
impl !Error for &str {}

r? @nikomatsakis

This PR was built on top of @yaahc PR #85764.

Language team proposal: to rust-lang/lang-team#96


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