impl Trait capturing lifetime of type parameter

[Plecra]

I tried this code:

fn erase<T>(_: T) -> impl core::fmt::Debug + 'static {}
fn create(v: ()) -> Box<dyn core::fmt::Debug> {
    Box::new(erase(&v))
}

I expected it to compile, but there was an error:

error[E0515]: cannot return value referencing function parameter `v`
 --> src/lib.rs:3:5
  |
3 |     Box::new(erase(&v))
  |     ^^^^^^^^^^^^^^^--^^
  |     |              |
  |     |              `v` is borrowed here
  |     returns a value referencing data owned by the current function

Meta

rustc --version --verbose:

rustc 1.48.0 (7eac88abb 2020-11-16)
binary: rustc
commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
commit-date: 2020-11-16
host: x86_64-pc-windows-msvc
release: 1.48.0
LLVM version: 11.0

rustc 1.50.0-nightly (a0d664bae 2020-11-22)       
binary: rustc
commit-hash: a0d664bae6ca79c54cc054aa2403198e105190a2
commit-date: 2020-11-22
host: x86_64-pc-windows-msvc
release: 1.50.0-nightly

The issue also affects -Zpolonius

[jonas-schievink]

I think this works as intended. impl Trait captures all type parameters, and T is not 'static, so the lifetime of the parameter is assumed to be contained in the returned value.

[Plecra]

Does that mean impl Trait can't express types that are separate from its arguments in any way? Is the 'static bound in the return position somehow ambiguous?

This behaviour seems very strange to me, and isn't mentioned in the original RFC, the reference or the Book. Why would impl Trait be defined to silently ignore its lifetime?

[estebank]

#82171 (comment):

There's no way currently to express that an impl Trait doesn't capture all the input lifetimes. Once type Ret<'a> = impl Trait + 'a; is stabilized you could express the behavior you're looking for, but for now the only option are the ones identified in the report (explicit type and Box<dyn Trait>).

[estebank]

The original code now compiles. Closing this in favor of #82171.