Change:
<ins> error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:9:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = ({ LL |         1 LL ~     }) else { | error: `for...else` loops are not supported --> $DIR/bad-let-else-statement.rs:18:7 | LL |       let foo = for i in 1..2 { |                 --- `else` is attached to this loop LL |           break; LL |       } else { |  _______^ LL | | LL | |         return; LL | |     }; | |_____^ | = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:29:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = (if true { LL |         1 LL |     } else { LL |         0 LL ~     }) else { | error: `loop...else` loops are not supported --> $DIR/bad-let-else-statement.rs:38:7 | LL |       let foo = loop { |                 ---- `else` is attached to this loop LL |           break; LL |       } else { |  _______^ LL | | LL | |         return; LL | |     }; | |_____^ | = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:48:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = (match true { LL |         true => 1, LL |         false => 0 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:58:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = (X { LL |         a: 1 LL ~     }) else { | error: `while...else` loops are not supported --> $DIR/bad-let-else-statement.rs:67:7 | LL |       let foo = while false { |                 ----- `else` is attached to this loop LL |           break; LL |       } else { |  _______^ LL | | LL | |         return; LL | |     }; | |_____^ | = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:76:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = (const { LL |         1 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:85:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = &({ LL |         1 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:95:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = bar = ({ LL |         1 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:104:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = 1 + ({ LL |         1 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:113:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = 1..({ LL |         1 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:122:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = return ({ LL |         () LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:131:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = -({ LL |         1 LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:140:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = do yeet ({ LL |         () LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:149:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = become ({ LL |         () LL ~     }) else { | error: right curly brace `}` before `else` in a `let...else` statement not allowed --> $DIR/bad-let-else-statement.rs:158:5 | LL |     } else { |     ^ | help: wrap the expression in parentheses | LL ~     let foo = |x: i32| ({ LL |         x LL ~     }) else { | error: aborting due to 17 previous errors </ins>

Discussion:
I believe the intention in let-else is that this code should be considered syntactically invalid, but it compiles successfully today. If you replace with , it correctly fails to parse.
I believe here is the missing case for ExprKind::ConstBlock: This should be changed to an exhaustive match so that future new ExprKind variants are forced to touch this code. Some other unhandled cases that can end in : ExprKind::Become ExprKind::FormatArgs ExprKind::IncludedBytes ExprKind::InlineAsm ExprKind::MacCall ExprKind::OffsetOf ExprKind::Yeet