[英]Rust "expected type" error prints mismatched types that are exactly the same
夜间生锈:
struct Foo<T, F: Fn(&T, &T) -> T> {
value: T,
func: F
}
fn main() {
let lambda = |&x, &y| x + y;
let foo = Foo {
value: 5 as i32,
func: lambda
};
}
错误信息:
Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
--> src/main.rs:8:15
|
8 | let foo = Foo {
| ^^^ one type is more general than the other
|
= note: expected type `std::ops::FnOnce<(&i32, &i32)>`
found type `std::ops::FnOnce<(&i32, &i32)>`
请注意,预期类型和找到的类型是相同的字符。 为什么错误消息说一种类型比另一种更通用,同时还说它们是同一类型?
夜间生锈:
这似乎只是夜间构建中的“坏”错误消息。 在 Rust 1.32(稳定版)中,错误告诉你这是一个生命周期不匹配:
error[E0631]: type mismatch in closure arguments
--> src/main.rs:8:15
|
7 | let lambda = |&x, &y| x + y;
| -------------- found signature of `fn(&_, &_) -> _`
8 | let foo = Foo {
| ^^^ expected signature of `for<'r, 's> fn(&'r i32, &'s i32) -> _`
|
note: required by `Foo`
--> src/main.rs:1:1
|
1 | struct Foo<T, F: Fn(&T, &T) -> T> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error[E0271]: type mismatch resolving `for<'r, 's> <[closure@src/main.rs:7:18: 7:32] as std::ops::FnOnce<(&'r i32, &'s i32)>>::Output == i32`
--> src/main.rs:8:15
|
8 | let foo = Foo {
| ^^^ expected bound lifetime parameter, found concrete lifetime
|
note: required by `Foo`
--> src/main.rs:1:1
|
1 | struct Foo<T, F: Fn(&T, &T) -> T> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
为什么错误消息说一种类型比另一种更通用,同时还说它们是同一类型?
类型仅在生命周期上有所不同。 每晚的消息不包括生命周期——也许是为了在生命周期不相关的情况下减少噪音。 显然,当生命周期是类型之间的唯一区别时,这根本没有帮助。
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