Auto merge of #111928 - c410-f3r:dqewdas, r=eholk
[RFC-2011] Expand more expressions
cc #44838
Expands `if`, `let`, `match` and also makes `generic_assert_internals` an allowed feature when using `assert!`. `#![feature(generic_assert)]` is still needed to activate everything.
```rust
#![feature(generic_assert)]
fn fun(a: Option<i32>, b: Option<i32>, c: Option<i32>) {
assert!(
if a.is_some() { 1 } else { 2 } == 3
&& if let Some(elem) = b { elem == 4 } else { false }
&& match c { Some(_) => true, None => false }
);
}
fn main() {
fun(Some(1), None, Some(2));
}
// Assertion failed: assert!(
// if a.is_some() { 1 } else { 2 } == 3
// && if let Some(elem) = b { elem == 4 } else { false }
// && match c { Some(_) => true, None => false }
// );
//
// With captures:
// a = Some(1)
// b = None
// c = Some(2)
```
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