diff options
| -rw-r--r-- | compiler/rustc_error_codes/src/error_codes.rs | 2 | ||||
| -rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0482.md | 73 | ||||
| -rw-r--r-- | src/tools/tidy/src/error_codes_check.rs | 2 |
3 files changed, 75 insertions, 2 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs index 45d91c2047d..1b4b58314b3 100644 --- a/compiler/rustc_error_codes/src/error_codes.rs +++ b/compiler/rustc_error_codes/src/error_codes.rs @@ -242,6 +242,7 @@ E0468: include_str!("./error_codes/E0468.md"), E0469: include_str!("./error_codes/E0469.md"), E0477: include_str!("./error_codes/E0477.md"), E0478: include_str!("./error_codes/E0478.md"), +E0482: include_str!("./error_codes/E0482.md"), E0491: include_str!("./error_codes/E0491.md"), E0492: include_str!("./error_codes/E0492.md"), E0493: include_str!("./error_codes/E0493.md"), @@ -599,7 +600,6 @@ E0785: include_str!("./error_codes/E0785.md"), // E0479, // the type `..` (provided as the value of a type parameter) is... // E0480, // lifetime of method receiver does not outlive the method call // E0481, // lifetime of function argument does not outlive the function call - E0482, // lifetime of return value does not outlive the function call // E0483, // lifetime of operand does not outlive the operation // E0484, // reference is not valid at the time of borrow // E0485, // automatically reference is not valid at the time of borrow diff --git a/compiler/rustc_error_codes/src/error_codes/E0482.md b/compiler/rustc_error_codes/src/error_codes/E0482.md new file mode 100644 index 00000000000..58ebf43cc98 --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0482.md @@ -0,0 +1,73 @@ +A lifetime of a returned value does not outlive the function call. + +Erroneous code example: + +```compile_fail,E0482 +fn prefix<'a>( + words: impl Iterator<Item = &'a str> +) -> impl Iterator<Item = String> { // error! + words.map(|v| format!("foo-{}", v)) +} +``` + +To fix this error, make the lifetime of the returned value explicit: + +``` +fn prefix<'a>( + words: impl Iterator<Item = &'a str> + 'a +) -> impl Iterator<Item = String> + 'a { // ok! + words.map(|v| format!("foo-{}", v)) +} +``` + +The [`impl Trait`] feature in this example uses an implicit `'static` lifetime +restriction in the returned type. However the type implementing the `Iterator` +passed to the function lives just as long as `'a`, which is not long enough. + +The solution involves adding lifetime bound to both function argument and +the return value to make sure that the values inside the iterator +are not dropped when the function goes out of the scope. + +An alternative solution would be to guarantee that the `Item` references +in the iterator are alive for the whole lifetime of the program. + +``` +fn prefix( + words: impl Iterator<Item = &'static str> +) -> impl Iterator<Item = String> { // ok! + words.map(|v| format!("foo-{}", v)) +} +``` + +A similar lifetime problem might arise when returning closures: + +```compile_fail,E0482 +fn foo( + x: &mut Vec<i32> +) -> impl FnMut(&mut Vec<i32>) -> &[i32] { // error! + |y| { + y.append(x); + y + } +} +``` + +Analogically, a solution here is to use explicit return lifetime +and move the ownership of the variable to the closure. + +``` +fn foo<'a>( + x: &'a mut Vec<i32> +) -> impl FnMut(&mut Vec<i32>) -> &[i32] + 'a { // ok! + move |y| { + y.append(x); + y + } +} +``` + +To better understand the lifetime treatment in the [`impl Trait`], +please see the [RFC 1951]. + +[`impl Trait`]: https://doc.rust-lang.org/reference/types/impl-trait.html +[RFC 1951]: https://rust-lang.github.io/rfcs/1951-expand-impl-trait.html diff --git a/src/tools/tidy/src/error_codes_check.rs b/src/tools/tidy/src/error_codes_check.rs index 53c75a46339..ce169867b7b 100644 --- a/src/tools/tidy/src/error_codes_check.rs +++ b/src/tools/tidy/src/error_codes_check.rs @@ -11,7 +11,7 @@ use regex::Regex; // A few of those error codes can't be tested but all the others can and *should* be tested! const EXEMPTED_FROM_TEST: &[&str] = &[ "E0227", "E0279", "E0280", "E0313", "E0377", "E0461", "E0462", "E0464", "E0465", "E0476", - "E0482", "E0514", "E0519", "E0523", "E0554", "E0640", "E0717", "E0729", + "E0514", "E0519", "E0523", "E0554", "E0640", "E0717", "E0729", ]; // Some error codes don't have any tests apparently... |