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Add: orphan rule rationale. #1755

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merged 3 commits into from
May 3, 2025
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Add: orphan rule rationale. #1755

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137860d
Add: orphan rule rationale.
TmSalviano Mar 11, 2025
009ebdd
Fix: grammar error . Fix: improved semantics
TmSalviano Mar 13, 2025
e496215
Update orphan rule introduction
ehuss May 3, 2025
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9 changes: 9 additions & 0 deletions src/items/implementations.md
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Expand Up @@ -174,6 +174,15 @@ be instantiable with the same set of types for the input type parameters. -->
r[items.impl.trait.orphan-rule]
#### Orphan rules

r[items.impl.trait.orphan-rule.rationale]
The orphan rule helps ensure that other people's code can't break your code, and vice versa.
If an external crate implements an external trait for an external type, and your crate also
implements the same trait for the same type, the compiler wouldn't know which implementation
to use.\
The orphan rule prevents this by requiring that either the trait or some type in the
implementation is local to your crate, ensuring only one crate defines the implementation and
thereby maintaining coherence.
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I would personally put a “that” here:

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implementation is local to your crate, ensuring only one crate defines the implementation and
thereby maintaining coherence.
implementation is local to your crate, ensuring that only one crate defines the implementation
and thereby maintaining coherence.

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@traviscross traviscross Mar 20, 2025 •
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Probably we'll want to move this into a note section, but it's generally going in the right direction. The one caveat I'd add is that the problem being prevented here isn't really about the compiler not knowing what to pick. The compiler knows what to do about that. It'd throw an ambiguity error. But what that means is that adding an impl for your own types or of your own traits would be breaking (in many more situations). So it's mostly about saving space for such authors, and that's the real rationale. Of course, whenever you talk about removing the rule, you do find out all the ways that we are leaning on the rule more technically, and those reasons are a bit more subtle. E.g.:


r[items.impl.trait.orphan-rule.general]
Given `impl<P1..=Pn> Trait<T1..=Tn> for T0`, an `impl` is valid only if at
least one of the following is true:
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