Merge pull request #278 from JohnTitor/compile-fail-subtyping

Some improvements on the "subtyping" chapter
pull/280/head
Eric Huss 3 years ago committed by GitHub
commit 484c6104db
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@ -188,6 +188,10 @@ some sense "fundamental". All the others can be understood by analogy to the oth
* `*const T` follows the logic of `&T`
* `*mut T` follows the logic of `&mut T` (or `UnsafeCell<T>`)
For more types, see the ["Variance" section][variance-table] on the reference.
[variance-table]: ../reference/subtyping.html#variance
> NOTE: the *only* source of contravariance in the language is the arguments to
> a function, which is why it really doesn't come up much in practice. Invoking
> contravariance involves higher-order programming with function pointers that
@ -265,7 +269,7 @@ enough into the place expecting something long-lived.
Here it is:
```rust,ignore
```rust,compile_fail
fn evil_feeder<T>(input: &mut T, val: T) {
*input = val;
}
@ -285,15 +289,16 @@ And what do we get when we run this?
```text
error[E0597]: `spike` does not live long enough
--> src/main.rs:9:32
--> src/main.rs:9:31
|
9 | let spike_str: &str = &spike;
| ^^^^^ borrowed value does not live long enough
10 | evil_feeder(&mut mr_snuggles, spike_str);
6 | let mut mr_snuggles: &'static str = "meow! :3"; // mr. snuggles forever!!
| ------------ type annotation requires that `spike` is borrowed for `'static`
...
9 | let spike_str: &str = &spike; // Only lives for the block
| ^^^^^^ borrowed value does not live long enough
10 | evil_feeder(&mut mr_snuggles, spike_str); // EVIL!
11 | }
| - borrowed value only lives until here
|
= note: borrowed value must be valid for the static lifetime...
| - `spike` dropped here while still borrowed
```
Good, it doesn't compile! Let's break down what's happening here in detail.

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