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)
```

Author: bors