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As per issue #136827 submitting updates to E0495 documentation #137042

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As per issue #136827 submitting updates to E0495 documentation #137042

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50 changes: 23 additions & 27 deletions compiler/rustc_error_codes/src/error_codes/E0495.md
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Original file line number Diff line number Diff line change
@@ -1,40 +1,36 @@
#### Note: this error code is no longer emitted by the compiler.
A trait implementation returns a reference without an explicit lifetime linking it to `self`.

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@Noratrieb Noratrieb Feb 16, 2025 •
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I don't think this is what this error stands for, I think this is a much more generic lifetime error from searching for the code in ./tests.

I would probably just keep the old documentation, it's not amazing but I don't think it's possible to write documentation with great examples for this error code, it just seems like this is a very generic error code emitted in all sorts of cases.

@compiler-errors maybe you know more about this error code and whether what I said is correct. (emitted here

rust/compiler/rustc_trait_selection/src/error_reporting/infer/region.rs

Line 994 in c3fe9e7

fn report_inference_failure(&self, var_origin: RegionVariableOrigin) -> Diag<'_> {
)


A lifetime cannot be determined in the given situation.
```compile_fail,E0495
trait Tr<X> {
fn f(&self) -> X;
}

Erroneous code example:
struct Wr<'b> {
ri: &'b i32,
f: f32,
}

```compile_fail
fn transmute_lifetime<'a, 'b, T>(t: &'a (T,)) -> &'b T {
match (&t,) { // error!
((u,),) => u,
impl<'b> Tr<&f32> for Wr<'b> {
fn f(&self) -> &f32 { // error: missing lifetime linking &f32 to self
&self.f
}
}

let y = Box::new((42,));
let x = transmute_lifetime(&y);
```
Specify an explicit lifetime in the trait to ensure that the reference returned is valid for at least as long as `self`:

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In this code, you have two ways to solve this issue:
1. Enforce that `'a` lives at least as long as `'b`.
2. Use the same lifetime requirement for both input and output values.

So for the first solution, you can do it by replacing `'a` with `'a: 'b`:

```
fn transmute_lifetime<'a: 'b, 'b, T>(t: &'a (T,)) -> &'b T {
match (&t,) { // ok!
((u,),) => u,
}
```rust
trait Tr<'a, X> {
fn f(&'a self) -> X;
}
```

In the second you can do it by simply removing `'b` so they both use `'a`:
struct Wr<'b> {
ri: &'b i32,
f: f32,
}

```
fn transmute_lifetime<'a, T>(t: &'a (T,)) -> &'a T {
match (&t,) { // ok!
((u,),) => u,
impl<'a, 'b: 'a> Tr<'a, &'a f32> for Wr<'b> {
fn f(&'a self) -> &'a f32 { // ok!
&self.f
}
}
```
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