rust-lang / rust-lang/rust

A lifetime-dependent bound is not merged into a higher-ranked lifetime bound

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A-lifetimes A-trait-system C-bug fixed-by-next-solver T-compiler T-types
Dominant language
Rust
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Description

MRE:

trait Lifetime<'a> {}
trait AnyLifetime: for<'a> Lifetime<'a> {}
struct A<T: AnyLifetime>(core::marker::PhantomData<T>);
trait MyTrait<'a> {}
impl<'a, T: AnyLifetime + Lifetime<'a>> MyTrait<'a> for A<T> {}

The last line produces an error

error[E0283]: type annotations needed: cannot satisfy `T: Lifetime<'a>`
  --> src/main.rs:10:27
   |
10 | impl<'a, T: AnyLifetime + Lifetime<'a>> MyTrait<'a> for A<T> {}
   |                           ^^^^^^^^^^^^
   |
note: multiple `impl`s or `where` clauses satisfying `T: Lifetime<'a>` found
  --> src/main.rs:10:13
   |
10 | impl<'a, T: AnyLifetime + Lifetime<'a>> MyTrait<'a> for A<T> {}

Clearly, if T: for<'a> Lifetime<'a> then T: Lifetime<'a>, so the second bound should have just been ignored.

This issue arises in a generated code (in my case, in serde-generated impls), so the additional bound cannot be removed manually.

Meta
rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: aarch64-apple-darwin
release: 1.88.0
LLVM version: 20.1.5

(same behavior on nightly)

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the MRE in src/main.rs with rustc 1.88.0 and nightly, then trace the compiler path handling lifetime bounds and higher-ranked lifetime bounds. Done means the impl is accepted without requiring the redundant Lifetime<'a> bound to be removed manually, with regression coverage for the reported case.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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