Rollup merge of #88205 - danii:e0772, r=GuillaumeGomez

Add Explanation For Error E0772

I've added an error explanation for the error code E0772.

Assists with #61137
This commit is contained in:
Matthias Krüger 2022-01-29 14:46:29 +01:00 committed by GitHub
commit 11898a56c2
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4 changed files with 94 additions and 3 deletions

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@ -472,6 +472,7 @@ E0768: include_str!("./error_codes/E0768.md"),
E0769: include_str!("./error_codes/E0769.md"),
E0770: include_str!("./error_codes/E0770.md"),
E0771: include_str!("./error_codes/E0771.md"),
E0772: include_str!("./error_codes/E0772.md"),
E0773: include_str!("./error_codes/E0773.md"),
E0774: include_str!("./error_codes/E0774.md"),
E0775: include_str!("./error_codes/E0775.md"),
@ -642,5 +643,4 @@ E0787: include_str!("./error_codes/E0787.md"),
// E0723, // unstable feature in `const` context
E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
// E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.
}

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@ -0,0 +1,89 @@
A trait object has some specific lifetime `'1`, but it was used in a way that
requires it to have a `'static` lifetime.
Example of erroneous code:
```compile_fail,E0772
trait BooleanLike {}
trait Person {}
impl BooleanLike for bool {}
impl dyn Person {
fn is_cool(&self) -> bool {
// hey you, you're pretty cool
true
}
}
fn get_is_cool<'p>(person: &'p dyn Person) -> impl BooleanLike {
// error: `person` has an anonymous lifetime `'p` but calling
// `print_cool_fn` introduces an implicit `'static` lifetime
// requirement
person.is_cool()
}
```
The trait object `person` in the function `get_is_cool`, while already being
behind a reference with lifetime `'p`, also has it's own implicit lifetime,
`'2`.
Lifetime `'2` represents the data the trait object might hold inside, for
example:
```
trait MyTrait {}
struct MyStruct<'a>(&'a i32);
impl<'a> MyTrait for MyStruct<'a> {}
```
With this scenario, if a trait object of `dyn MyTrait + '2` was made from
`MyStruct<'a>`, `'a` must live as long, if not longer than `'2`. This allows the
trait object's internal data to be accessed safely from any trait methods. This
rule also goes for any lifetime any struct made into a trait object may have.
In the implementation for `dyn Person`, the `'2` lifetime representing the
internal data was ommitted, meaning that the compiler inferred the lifetime
`'static`. As a result, the implementation's `is_cool` is inferred by the
compiler to look like this:
```
# trait Person {}
#
# impl dyn Person {
fn is_cool<'a>(self: &'a (dyn Person + 'static)) -> bool {unimplemented!()}
# }
```
While the `get_is_cool` function is inferred to look like this:
```
# trait Person {}
# trait BooleanLike {}
#
fn get_is_cool<'p, R: BooleanLike>(person: &'p (dyn Person + 'p)) -> R {
unimplemented!()
}
```
Which brings us to the core of the problem; the assignment of type
`&'_ (dyn Person + '_)` to type `&'_ (dyn Person + 'static)` is impossible.
Fixing it is as simple as being generic over lifetime `'2`, as to prevent the
compiler from inferring it as `'static`:
```
# trait Person {}
#
impl<'d> dyn Person + 'd {/* ... */}
// This works too, and is more elegant:
//impl dyn Person + '_ {/* ... */}
```
See the [Rust Reference on Trait Object Lifetime Bounds][trait-objects] for
more information on trait object lifetimes.
[trait-object-lifetime-bounds]: https://doc.rust-lang.org/reference/types/trait-object.html#trait-object-lifetime-bounds

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@ -38,4 +38,5 @@ LL | impl MyTrait for Box<dyn ObjectTrait<Assoc = i32> + '_> {
error: aborting due to 4 previous errors
For more information about this error, try `rustc --explain E0515`.
Some errors have detailed explanations: E0515, E0772.
For more information about an error, try `rustc --explain E0515`.

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@ -131,4 +131,5 @@ LL | impl MyTrait for Box<dyn ObjectTrait + '_> {
error: aborting due to 6 previous errors
For more information about this error, try `rustc --explain E0759`.
Some errors have detailed explanations: E0759, E0772.
For more information about an error, try `rustc --explain E0759`.