Closed
Description
openedon Sep 12, 2021
The compiler behaves strangely. For a given lifetime, if it bounds itself ('a: 'a), it will compile, otherwise it will not. I think it is a bug that this subtle difference can change whether it can be compiled or not.
I tried this code:
fn main() {
let wrapped_vec = WrappedVec {
vec: vec!["1"],
};
let first = first_str_len(wrapped_vec);
println!("first: {}", first);
}
struct WrappedVec<'a> {
vec: Vec<&'a str>,
}
impl<'a> WrappedVec<'a> {
fn iter(&'a self) -> impl Iterator<Item=&'a &'a str> {
self.vec.iter()
}
}
fn first_str_len<'a>(arg: WrappedVec<'a>) -> usize {
let first = arg.iter().next().unwrap();
let return_arg = |str: &'a str| -> &'a str {
str
};
return_arg(first).len()
}This code does not compile.
error[E0597]: `arg` does not live long enough
--> src/main.rs:21:17
|
20 | fn first_str_len<'a>(arg: WrappedVec<'a>) -> usize {
| -- lifetime `'a` defined here
21 | let first = arg.iter().next().unwrap();
| ^^^-------
| |
| borrowed value does not live long enough
| argument requires that `arg` is borrowed for `'a`
...
28 | }
| - `arg` dropped here while still borrowed
For more information about this error, try `rustc --explain E0597`.
You can compile it by rewriting the part shown below.
fn first_str_len<'a>(arg: WrappedVec<'a>) -> usize {
// ↓ <'a> -> <'a: 'a>
fn first_str_len<'a: 'a>(arg: WrappedVec<'a>) -> usize {The code is unnatural because it was rebuilt by extracting only the parts that seemed necessary for replication from the code where the problem was found.
Meta
rustc --version --verbose:
rustc 1.55.0 (c8dfcfe04 2021-09-06)
binary: rustc
commit-hash: c8dfcfe046a7680554bf4eb612bad840e7631c4b
commit-date: 2021-09-06
host: x86_64-pc-windows-msvc
release: 1.55.0
LLVM version: 12.0.1
I tried it in the Rust playground, and the same thing happened in beta and nightly.
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=05a374855198719d56d41ac02be0d59e
Backtrace
There was no backtrace
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