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Properly deal with weak alias types as self types of impls by fmease · Pull Request #120780 · rust-lang/rust

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F-lazy_type_alias

`#![feature(lazy_type_alias)]`

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Feb 8, 2024

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Feb 17, 2024

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Feb 17, 2024

bors added a commit to rust-lang-ci/rust that referenced this pull request

Feb 17, 2024 
Properly deal with weak alias types as self types of impls

Fixes rust-lang#114216.
Fixes rust-lang#116100.

Not super happy about the two ad hoc “normalization” implementations for weak alias types:

1. In `inherent_impls`: The “peeling”, normalization to [“WHNF”][whnf]: Semantically that's exactly what we want (neither proper normalization nor shallow normalization would be correct here). Basically a weak alias type is “nominal” (well...^^) if the WHNF is nominal. [rust-lang#97974](rust-lang#97974) followed the same approach.
2. In `constrained_generic_params`: Generic parameters are constrained by a weak alias type if the corresp. “normalized” type constrains them (where we only normalize *weak* alias types not arbitrary ones). Weak alias types are injective if the corresp. “normalized” type is injective.

Both have ad hoc overflow detection mechanisms.

**Coherence** is handled in rust-lang#117164.

r? `@oli-obk` or types

[whnf]: https://en.wikipedia.org/wiki/Lambda_calculus_definition#Weak_head_normal_form

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Feb 17, 2024

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Feb 17, 2024

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request

Feb 20, 2024 
Expand weak alias types before collecting constrained/referenced late bound regions + refactorings

Fixes rust-lang#114220.
Follow-up to rust-lang#120780.

r? `@oli-obk`

rust-timer added a commit to rust-lang-ci/rust that referenced this pull request

Feb 21, 2024 
Rollup merge of rust-lang#121344 - fmease:lta-constr-by-input, r=oli-obk

Expand weak alias types before collecting constrained/referenced late bound regions + refactorings

Fixes rust-lang#114220.
Follow-up to rust-lang#120780.

r? `@oli-obk`

@fmease fmease added the T-compiler

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Oct 9, 2025