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fix E0371 description #156752

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fix E0371 description #156752

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16 changes: 12 additions & 4 deletions compiler/rustc_error_codes/src/error_codes/E0371.md
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Original file line number Diff line number Diff line change
Expand Up @@ -7,10 +7,18 @@ trait Foo { fn foo(&self) { } }
trait Bar: Foo { }
trait Baz: Bar { }

impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
impl Baz for Bar { } // Note: This is OK
impl Bar for dyn Baz { } // error, `Baz` implements `Bar` by definition
impl Foo for dyn Baz { } // error, `Baz` implements `Bar` which implements `Foo`
impl Baz for dyn Baz { } // error, `Baz` (trivially) implements `Baz`
```

This is okay, because `Bar` does not implement `Baz` by definition:

```
# trait Foo { fn foo(&self) { } }
# trait Bar: Foo { }
# trait Baz: Bar { }
impl Baz for dyn Bar { } // Note: This is OK
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@oli-obk oli-obk May 20, 2026 •
edited by rustbot
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If you pull this last one out into a separate block that is not compile_error, we make sure it actually doesn't error.

You can render it nicely by putting # in front of the duplicated definition lines (you'll need to duplicate the trait defs into the new block)

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@42triangles 42triangles May 20, 2026

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(Mostly for future reference,) that should catch the (in this case only warnings IIRC) that were emitted as well in the success case I assume?

```

When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
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