Auto merge of #116447 - oli-obk:gen_fn, r=compiler-errors

Implement `gen` blocks in the 2024 edition

Coroutines tracking issue #43122
`gen` block tracking issue #117078

This PR implements `gen` blocks that implement `Iterator`. Most of the logic with `async` blocks is shared, and thus I renamed various types that were referring to `async` specifically.

An example usage of `gen` blocks is

```rust
fn foo() -> impl Iterator<Item = i32> {
    gen {
        yield 42;
        for i in 5..18 {
            if i.is_even() { continue }
            yield i * 2;
        }
    }
}
```

The limitations (to be resolved) of the implementation are listed in the tracking issue

Author: bors

clippy_lints/src/suspicious_operation_groupings.rs

@@ -578,7 +578,7 @@ fn ident_difference_expr_with_base_location(
         | (Assign(_, _, _), Assign(_, _, _))
         | (TryBlock(_), TryBlock(_))
         | (Await(_, _), Await(_, _))
-        | (Async(_, _), Async(_, _))
+        | (Gen(_, _, _), Gen(_, _, _))
         | (Block(_, _), Block(_, _))
         | (Closure(_), Closure(_))
         | (Match(_, _), Match(_, _))

clippy_utils/src/ast_utils.rs

@@ -211,7 +211,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
                 && eq_fn_decl(lf, rf)
                 && eq_expr(le, re)
         },
-        (Async(lc, lb), Async(rc, rb)) => lc == rc && eq_block(lb, rb),
+        (Gen(lc, lb, lk), Gen(rc, rb, rk)) => lc == rc && eq_block(lb, rb) && lk == rk,
         (Range(lf, lt, ll), Range(rf, rt, rl)) => ll == rl && eq_expr_opt(lf, rf) && eq_expr_opt(lt, rt),
         (AddrOf(lbk, lm, le), AddrOf(rbk, rm, re)) => lbk == rbk && lm == rm && eq_expr(le, re),
         (Path(lq, lp), Path(rq, rp)) => both(lq, rq, eq_qself) && eq_path(lp, rp),

clippy_utils/src/sugg.rs

@@ -190,7 +190,7 @@ impl<'a> Sugg<'a> {
                 (snip, false) => Sugg::MaybeParen(snip),
                 (snip, true) => Sugg::NonParen(snip),
             },
-            ast::ExprKind::Async(..)
+            ast::ExprKind::Gen(..)
             | ast::ExprKind::Block(..)
             | ast::ExprKind::Break(..)
             | ast::ExprKind::Call(..)