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Auto merge of rust-lang#102388 - JohnTitor:rollup-mbyw6fl, r=JohnTitor
Rollup of 8 pull requests Successful merges: - rust-lang#100747 (Add long description and test for E0311) - rust-lang#102232 (Stabilize bench_black_box) - rust-lang#102288 (Suggest unwrapping `???<T>` if a method cannot be found on it but is present on `T`.) - rust-lang#102338 (Deny associated type bindings within associated type bindings) - rust-lang#102347 (Unescaping cleanups) - rust-lang#102348 (Tweak `FulfillProcessor`.) - rust-lang#102378 (Use already resolved `self_ty` in `confirm_fn_pointer_candidate`) - rust-lang#102380 (rustdoc: remove redundant mobile `.source > .sidebar` CSS) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Original file line number | Diff line number | Diff line change |
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This error occurs when there is an unsatisfied outlives bound involving an | ||
elided region and a generic type parameter or associated type. | ||
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Erroneous code example: | ||
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```compile_fail,E0311 | ||
fn no_restriction<T>(x: &()) -> &() { | ||
with_restriction::<T>(x) | ||
} | ||
fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () { | ||
x | ||
} | ||
``` | ||
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Why doesn't this code compile? It helps to look at the lifetime bounds that are | ||
automatically added by the compiler. For more details see the documentation for | ||
[lifetime elision]( https://doc.rust-lang.org/reference/lifetime-elision.html). | ||
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The compiler elides the lifetime of `x` and the return type to some arbitrary | ||
lifetime `'anon` in `no_restriction()`. The only information available to the | ||
compiler is that `'anon` is valid for the duration of the function. When | ||
calling `with_restriction()`, the compiler requires the completely unrelated | ||
type parameter `T` to outlive `'anon` because of the `T: 'a` bound in | ||
`with_restriction()`. This causes an error because `T` is not required to | ||
outlive `'anon` in `no_restriction()`. | ||
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If `no_restriction()` were to use `&T` instead of `&()` as an argument, the | ||
compiler would have added an implied bound, causing this to compile. | ||
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This error can be resolved by explicitly naming the elided lifetime for `x` and | ||
then explicily requiring that the generic parameter `T` outlives that lifetime: | ||
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``` | ||
fn no_restriction<'a, T: 'a>(x: &'a ()) -> &'a () { | ||
with_restriction::<T>(x) | ||
} | ||
fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () { | ||
x | ||
} | ||
``` |
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