E0716 without "let's call this" and spurious "requirement introduced here" #153545
Description
Edit (2026-03-07T23:28:55+01:00): massively simplified code
Code
use std::cell::RefCell;
use std::sync::Arc;
use std::collections::HashMap;
use std::collections::hash_map::Entry;
fn apply<'a>(
f: Arc<dyn Fn(Entry<'a, String, String>) + 'a>,
) -> impl Fn(RefCell<HashMap<String, String>>)
{
move |map| {
let value = map.borrow_mut().entry("foo".to_string());
let wrapped_value = value;
f(wrapped_value);
}
}Current output
error[E0597]: `map` does not live long enough
--> src/lib.rs:11:21
|
7 | f: Arc<dyn Fn(Entry<'a, String, String>) + 'a>,
| - lifetime `'1` appears in the type of `f`
...
10 | move |map| {
| --- binding `map` declared here
11 | let value = map.borrow_mut().entry("foo".to_string());
| ^^^ borrowed value does not live long enough
12 | let wrapped_value = value;
13 | f(wrapped_value);
| ---------------- argument requires that `map` is borrowed for `'1`
14 | }
| - `map` dropped here while still borrowed
|
note: requirement that the value outlives `'1` introduced here
--> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:76:34
|
76 | pub const trait Fn<Args: Tuple>: [const] FnMut<Args> {
| ^^^^^^^^^^^^^^^^^^^
error[E0716]: temporary value dropped while borrowed
--> src/lib.rs:11:21
|
7 | f: Arc<dyn Fn(Entry<'a, String, String>) + 'a>,
| - lifetime `'1` appears in the type of `f`
...
11 | let value = map.borrow_mut().entry("foo".to_string());
| ^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
| |
| creates a temporary value which is freed while still in use
12 | let wrapped_value = value;
13 | f(wrapped_value);
| ---------------- argument requires that borrow lasts for `'1`
|
note: requirement that the value outlives `'1` introduced here
--> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:76:34
|
76 | pub const trait Fn<Args: Tuple>: [const] FnMut<Args> {
| ^^^^^^^^^^^^^^^^^^^Rationale and extra context
-
The compiler says "lifetime
'1appears in the type off", but I can't figure out where and how it can appear. The trait object has an explicit lifetime 'a, and there are no elided lifetimes in Entry. -
I'm used to a line saying "let's call this lifetime '1". It's missing in this case, so I don't know where '1 even comes from.
-
I don't understand how the highlighted
[const] FnMut<Args>can introduce a lifetime. Is it related to the impl Fn for &Fn? -
It is not clear how "temporary value is freed at the end of this statement" relates to the lifetime '1. It looks like a separate error, but all the messages from the first error are duplicated.
Other cases
When replacing Arc with Box, the note "requirement ..." goes away.
Rust Version
rustc 1.93.1 (01f6ddf75 2026-02-11)
binary: rustc
commit-hash: 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf
commit-date: 2026-02-11
host: x86_64-unknown-linux-gnu
release: 1.93.1
LLVM version: 21.1.8Anything else?
No response