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15b7792a65
Allow specifying multiple bounds for same associated item, except in trait objects Supersedes https://github.com/rust-lang/rust/pull/143146, fixes https://github.com/rust-lang/rust/issues/143143. This PR proposes to stop enforcing E0719 in all contexts other than trait object types. E0719 forbids constraining the same associated item twice within the same angle-bracket delimited associated item bound list (the `…` inside `T: Trait<…>`). For example, the following are forbidden: | Forbidden | Working alternative | |--------------------------------------------|--------------------------------------------------------------------| | `T: Trait<Gat<u32> = u32, Gat<u64> = u64>` | `T: Trait<Gat<u32> = u32> + Trait<Gat<u64> = u64>` | | `T: Iterator<Item = u32, Item = i32>` | `T: Iterator<Item = u32> + Iterator<Item = i32>` (trivially false) | | `T: Iterator<Item = u32, Item = u32>` | `T: Iterator<Item = u32>` | | `T: Iterator<Item: Send, Item: Sync>` | `T: Iterator<Item: Send + Sync>` | | `T: Trait<ASSOC = 3, ASSOC = 4>` | `T: Trait<ASSOC = 3> + Trait<ASSOC = 4>` (trivially false) | | `T: Trait<ASSOC = 3, ASSOC = 3>` | `T: Trait<ASSOC = 3>` | With this PR, all those previously forbidden examples would start working, as well as their APIT and RPIT equivalents. Types like `dyn Iterator<Item = u32, Item = u32>` will continue to be rejected, however. See https://github.com/rust-lang/rust/pull/143146#issuecomment-3274421752 for the reason why. ```@rustbot``` label T-lang T-types needs-fcp
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.