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Change the desugaring of `assert!` for better error output
In the desugaring of `assert!`, we now expand to a `match` expression instead of `if !cond {..}`.
The span of incorrect conditions will point only at the expression, and not the whole `assert!` invocation.
```
error[E0308]: mismatched types
--> $DIR/issue-14091.rs:2:13
|
LL | assert!(1,1);
| ^ expected `bool`, found integer
```
We no longer mention the expression needing to implement the `Not` trait.
```
error[E0308]: mismatched types
--> $DIR/issue-14091-2.rs:15:13
|
LL | assert!(x, x);
| ^ expected `bool`, found `BytePos`
```
Now `assert!(val)` desugars to:
```rust
match val {
true => {},
_ => $crate::panic::panic_2021!(),
}
```
Fix #122159.