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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.