rust-lang / rust-lang/rust

Confusing error message when closures return references based on their arguments

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A-diagnostics T-compiler
Dominant language
Rust
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Description

Code
struct Val {
    inner: u32
}

fn main() {
    let get_str = |val: &Val| &val.inner;
    
    println!("{}", get_str(&Val { inner: 1 }));
}
Current output
error: lifetime may not live long enough
 --> src/main.rs:6:31
  |
6 |     let get_str = |val: &Val| &val.inner;
  |                         -   - ^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
  |                         |   |
  |                         |   return type of closure is &'2 u32
  |                         let's call the lifetime of this reference `'1`
Desired output

Rationale and extra context

I'm not sure what this specific limitation would be called, but I ran into it recently and thought that the error could give some advice on how to solve it. I'm not sure what the best way to do that is, so I haven't filled out the desired output section.

I was a bit surprised to learn that

struct Val {
    inner: u32
}

fn main() {
    let get_str = |val: &Val| -> &u32 { &val.inner };
    
    println!("{}", get_str(&Val { inner: 1 }));
}

still has the same error (namely, &Val and &u32 are assigned two separate lifetimes), whereas

struct Val {
    inner: u32
}

fn main() {
    let get_str: fn(&Val) -> &u32 = |val: &Val| &val.inner;
    
    println!("{}", get_str(&Val { inner: 1 }));
}

fixes the issue. I'm guessing there's some subtle differences in the way a type is created for a closure (even when the return type explicitly written out) versus specifying the type manually, but I'm honestly not sure. If there's some documentation regarding this, it might be a good idea to mention that in the error message.

Other cases

Rust Version
rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: x86_64-unknown-linux-gnu
release: 1.84.0
LLVM version: 19.1.5
Anything else?

No response

Contributor guide

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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

Start by reproducing the closure example with rustc 1.84.0, then compare its diagnostic with the explicitly typed function-pointer version described in the issue. Investigate the closure lifetime error diagnostic; done should provide clear, actionable guidance or relevant documentation for this limitation.

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
Needs clarification
Newbie friendliness
30/100

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