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Rollup merge of rust-lang#89922 - JohnTitor:update-e0637, r=jackh726
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Update E0637 description to mention `&` w/o an explicit lifetime name

Deal with rust-lang#89824 (comment). Another solution would be splitting the error code into two as (I think) it's a bit unclear to users why they have the same error code.
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JohnTitor authored Oct 21, 2021
2 parents 1625f14 + 4333091 commit bb8c62d
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36 changes: 26 additions & 10 deletions compiler/rustc_error_codes/src/error_codes/E0637.md
Original file line number Diff line number Diff line change
@@ -1,35 +1,51 @@
An underscore `_` character has been used as the identifier for a lifetime.
`'_` lifetime name or `&T` without an explicit lifetime name has been used
on illegal place.

Erroneous code example:

```compile_fail,E0106,E0637
fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
//^^ `'_` is a reserved lifetime name
fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
//^^ `'_` is a reserved lifetime name
if str1.len() > str2.len() {
str1
} else {
str2
}
}
fn and_without_explicit_lifetime<T>()
where
T: Into<&u32>,
//^ `&` without an explicit lifetime name
{
}
```

`'_`, cannot be used as a lifetime identifier because it is a reserved for the
anonymous lifetime. To fix this, use a lowercase letter such as 'a, or a series
of lowercase letters such as `'foo`. For more information, see [the
book][bk-no]. For more information on using the anonymous lifetime in rust
nightly, see [the nightly book][bk-al].
First, `'_` cannot be used as a lifetime identifier in some places
because it is a reserved for the anonymous lifetime. Second, `&T`
without an explicit lifetime name cannot also be used in some places.
To fix them, use a lowercase letter such as `'a`, or a series
of lowercase letters such as `'foo`. For more information about lifetime
identifier, see [the book][bk-no]. For more information on using
the anonymous lifetime in Rust 2018, see [the Rust 2018 blog post][blog-al].

Corrected example:

```
fn longest<'a>(str1: &'a str, str2: &'a str) -> &'a str {
fn underscore_lifetime<'a>(str1: &'a str, str2: &'a str) -> &'a str {
if str1.len() > str2.len() {
str1
} else {
str2
}
}
fn and_without_explicit_lifetime<'foo, T>()
where
T: Into<&'foo u32>,
{
}
```

[bk-no]: https://doc.rust-lang.org/book/appendix-02-operators.html#non-operator-symbols
[bk-al]: https://doc.rust-lang.org/nightly/edition-guide/rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html
[blog-al]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#more-lifetime-elision-rules
18 changes: 13 additions & 5 deletions src/test/ui/error-codes/E0637.rs
Original file line number Diff line number Diff line change
@@ -1,9 +1,17 @@
struct Foo<'a: '_>(&'a u8); //~ ERROR cannot be used here
fn foo<'a: '_>(_: &'a u8) {} //~ ERROR cannot be used here
fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
//~^ ERROR: `'_` cannot be used here [E0637]
//~| ERROR: missing lifetime specifier
if str1.len() > str2.len() {
str1
} else {
str2
}
}

struct Bar<'a>(&'a u8);
impl<'a: '_> Bar<'a> { //~ ERROR cannot be used here
fn bar() {}
fn and_without_explicit_lifetime<T>()
where
T: Into<&u32>, //~ ERROR: `&` without an explicit lifetime name cannot be used here [E0637]
{
}

fn main() {}
31 changes: 19 additions & 12 deletions src/test/ui/error-codes/E0637.stderr
Original file line number Diff line number Diff line change
@@ -1,21 +1,28 @@
error[E0637]: `'_` cannot be used here
--> $DIR/E0637.rs:1:16
--> $DIR/E0637.rs:1:24
|
LL | struct Foo<'a: '_>(&'a u8);
| ^^ `'_` is a reserved lifetime name
LL | fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
| ^^ `'_` is a reserved lifetime name

error[E0637]: `'_` cannot be used here
--> $DIR/E0637.rs:2:12
error[E0637]: `&` without an explicit lifetime name cannot be used here
--> $DIR/E0637.rs:13:13
|
LL | fn foo<'a: '_>(_: &'a u8) {}
| ^^ `'_` is a reserved lifetime name
LL | T: Into<&u32>,
| ^ explicit lifetime name needed here

error[E0637]: `'_` cannot be used here
--> $DIR/E0637.rs:5:10
error[E0106]: missing lifetime specifier
--> $DIR/E0637.rs:1:62
|
LL | fn underscore_lifetime<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
| ------- ------- ^^ expected named lifetime parameter
|
= help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `str1` or `str2`
help: consider introducing a named lifetime parameter
|
LL | impl<'a: '_> Bar<'a> {
| ^^ `'_` is a reserved lifetime name
LL | fn underscore_lifetime<'a, '_>(str1: &'a str, str2: &'a str) -> &'a str {
| +++ ~~ ~~ ~~

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0637`.
Some errors have detailed explanations: E0106, E0637.
For more information about an error, try `rustc --explain E0106`.

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