Change expr_trailing_brace to an exhaustive match to force new expression kinds to specify whether they contain a brace

Add inline const and other possible curly brace expressions to expr_trailing_brace

Add tests for `}` before `else` in `let...else` error

Change to explicit cases for expressions with optional values when being checked for trailing braces

Add tests for more complex cases of `}` before `else` in `let..else` statement

Move other possible `}` cases into separate arm and add FIXME for future reference

compiler/rustc_ast/src/util/classify.rs

@@ -40,15 +40,44 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
             | Range(_, Some(e), _)
             | Ret(Some(e))
             | Unary(_, e)
-            | Yield(Some(e)) => {
+            | Yield(Some(e))
+            | Yeet(Some(e))
+            | Become(e) => {
                 expr = e;
             }
             Closure(closure) => {
                 expr = &closure.body;
             }
             Gen(..) | Block(..) | ForLoop(..) | If(..) | Loop(..) | Match(..) | Struct(..)
-            | TryBlock(..) | While(..) => break Some(expr),
+            | TryBlock(..) | While(..) | ConstBlock(_) => break Some(expr),
-            _ => break None,
+
+            // FIXME: These can end in `}`, but changing these would break stable code.
+            InlineAsm(_) | OffsetOf(_, _) | MacCall(_) | IncludedBytes(_) | FormatArgs(_) => {
+                break None;
+            }
+
+            Break(_, None)
+            | Range(_, None, _)
+            | Ret(None)
+            | Yield(None)
+            | Array(_)
+            | Call(_, _)
+            | MethodCall(_)
+            | Tup(_)
+            | Lit(_)
+            | Cast(_, _)
+            | Type(_, _)
+            | Await(_, _)
+            | Field(_, _)
+            | Index(_, _, _)
+            | Underscore
+            | Path(_, _)
+            | Continue(_)
+            | Repeat(_, _)
+            | Paren(_)
+            | Try(_)
+            | Yeet(None)
+            | Err => break None,
         }
     }
 }

tests/ui/parser/bad-let-else-statement.rs

@@ -0,0 +1,164 @@
+#![feature(inline_const)]
+#![feature(yeet_expr)]
+#![allow(incomplete_features)] // Necessary for now, while explicit_tail_calls is incomplete
+#![feature(explicit_tail_calls)]
+
+fn a() {
+    let foo = {
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn b() {
+    let foo = for i in 1..2 {
+        break;
+    } else {
+        //~^ ERROR `for...else` loops are not supported
+        return;
+    };
+}
+
+fn c() {
+    let foo = if true {
+        1
+    } else {
+        0
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn d() {
+    let foo = loop {
+        break;
+    } else {
+        //~^ ERROR loop...else` loops are not supported
+        return;
+    };
+}
+
+fn e() {
+    let foo = match true {
+        true => 1,
+        false => 0
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+struct X {a: i32}
+fn f() {
+    let foo = X {
+        a: 1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn g() {
+    let foo = while false {
+        break;
+    } else {
+        //~^ ERROR `while...else` loops are not supported
+        return;
+    };
+}
+
+fn h() {
+    let foo = const {
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn i() {
+    let foo = &{
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn j() {
+    let bar = 0;
+    let foo = bar = {
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn k() {
+    let foo = 1 + {
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn l() {
+    let foo = 1..{
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn m() {
+    let foo = return {
+        ()
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn n() {
+    let foo = -{
+        1
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn o() -> Result<(), ()> {
+    let foo = do yeet {
+        ()
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return Ok(());
+    };
+}
+
+fn p() {
+    let foo = become {
+        ()
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn q() {
+    let foo = |x: i32| {
+        x
+    } else {
+        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
+        return;
+    };
+}
+
+fn main() {}

tests/ui/parser/bad-let-else-statement.stderr

@@ -0,0 +1,232 @@
+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
+