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Rollup merge of rust-lang#107416 - czzrr:issue-80618, r=GuillaumeGomez
Error code E0794 for late-bound lifetime parameter error. This PR addresses [rust-lang#80618](rust-lang#80618).
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,64 @@ | ||
| A lifetime parameter of a function definition is called *late-bound* if it both: | ||
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| 1. appears in an argument type | ||
| 2. does not appear in a generic type constraint | ||
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| You cannot specify lifetime arguments for late-bound lifetime parameters. | ||
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| Erroneous code example: | ||
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| ```compile_fail,E0794 | ||
| fn foo<'a>(x: &'a str) -> &'a str { x } | ||
| let _ = foo::<'static>; | ||
| ``` | ||
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| The type of a concrete instance of a generic function is universally quantified | ||
| over late-bound lifetime parameters. This is because we want the function to | ||
| work for any lifetime substituted for the late-bound lifetime parameter, no | ||
| matter where the function is called. Consequently, it doesn't make sense to | ||
| specify arguments for late-bound lifetime parameters, since they are not | ||
| resolved until the function's call site(s). | ||
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| To fix the issue, remove the specified lifetime: | ||
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| ``` | ||
| fn foo<'a>(x: &'a str) -> &'a str { x } | ||
| let _ = foo; | ||
| ``` | ||
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| ### Additional information | ||
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| Lifetime parameters that are not late-bound are called *early-bound*. | ||
| Confusion may arise from the fact that late-bound and early-bound | ||
| lifetime parameters are declared the same way in function definitions. | ||
| When referring to a function pointer type, universal quantification over | ||
| late-bound lifetime parameters can be made explicit: | ||
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| ``` | ||
| trait BarTrait<'a> {} | ||
| struct Bar<'a> { | ||
| s: &'a str | ||
| } | ||
| impl<'a> BarTrait<'a> for Bar<'a> {} | ||
| fn bar<'a, 'b, T>(x: &'a str, _t: T) -> &'a str | ||
| where T: BarTrait<'b> | ||
| { | ||
| x | ||
| } | ||
| let bar_fn: for<'a> fn(&'a str, Bar<'static>) -> &'a str = bar; // OK | ||
| let bar_fn2 = bar::<'static, Bar>; // Not allowed | ||
| let bar_fn3 = bar::<Bar>; // OK | ||
| ``` | ||
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| In the definition of `bar`, the lifetime parameter `'a` is late-bound, while | ||
| `'b` is early-bound. This is reflected in the type annotation for `bar_fn`, | ||
| where `'a` is universally quantified and `'b` is substituted by a specific | ||
| lifetime. It is not allowed to explicitly specify early-bound lifetime | ||
| arguments when late-bound lifetime parameters are present (as for `bar_fn2`, | ||
| see issue #42868: https://github.com/rust-lang/rust/issues/42868), although the | ||
| types that are constrained by early-bound parameters can be specified (as for | ||
| `bar_fn3`). |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,8 @@ | ||
| fn foo<'a>(x: &'a str) -> &'a str { | ||
| x | ||
| } | ||
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| fn main() { | ||
| let _ = foo::<'static>; | ||
| //~^ ERROR cannot specify lifetime arguments explicitly if late bound lifetime parameters are present [E0794] | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,15 @@ | ||
| error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present | ||
| --> $DIR/issue-80618.rs:6:19 | ||
| | | ||
| LL | let _ = foo::<'static>; | ||
| | ^^^^^^^ | ||
| | | ||
| note: the late bound lifetime parameter is introduced here | ||
| --> $DIR/issue-80618.rs:1:8 | ||
| | | ||
| LL | fn foo<'a>(x: &'a str) -> &'a str { | ||
| | ^^ | ||
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| error: aborting due to previous error | ||
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| For more information about this error, try `rustc --explain E0794`. |
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