Auto merge of #60861 - Centril:let-chains-ast-intro, r=petrochenkov

[let_chains, 2/6] Introduce `Let(..)` in AST, remove IfLet + WhileLet and parse let chains

Here we remove `ast::ExprKind::{IfLet, WhileLet}` and introduce `ast::ExprKind::Let`.
Moreover, we also:
+ connect the parsing logic for let chains
+ introduce the feature gate
+ rewire HIR lowering a bit.

However, this does not connect the new syntax to semantics in HIR.
That will be the subject of a subsequent PR.

Per https://github.com/rust-lang/rust/issues/53667#issuecomment-471583239.
Next step after https://github.com/rust-lang/rust/pull/59288.

cc @Manishearth re. Clippy.

r? @oli-obk

14 files changed, 2007 insertions, 215 deletions

diff --git a/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs b/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs
index 7e819e2b34e..5716e6d45a0 100644
--- a/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs
+++ b/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs
@@ -68,7 +68,7 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
 
     let mut g = |e| f(expr(e));
 
-    for kind in 0 .. 16 {
+    for kind in 0..=19 {
         match kind {
             0 => iter_exprs(depth - 1, &mut |e| g(ExprKind::Box(e))),
             1 => iter_exprs(depth - 1, &mut |e| g(ExprKind::Call(e, vec![]))),
@@ -79,25 +79,26 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::MethodCall(
                             seg.clone(), vec![make_x(), e])));
             },
-            3 => {
-                let op = Spanned { span: DUMMY_SP, node: BinOpKind::Add };
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, e, make_x())));
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, make_x(), e)));
-            },
-            4 => {
-                let op = Spanned { span: DUMMY_SP, node: BinOpKind::Mul };
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, e, make_x())));
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, make_x(), e)));
-            },
-            5 => {
-                let op = Spanned { span: DUMMY_SP, node: BinOpKind::Shl };
+            3..=8 => {
+                let op = Spanned {
+                    span: DUMMY_SP,
+                    node: match kind {
+                        3 => BinOpKind::Add,
+                        4 => BinOpKind::Mul,
+                        5 => BinOpKind::Shl,
+                        6 => BinOpKind::And,
+                        7 => BinOpKind::Or,
+                        8 => BinOpKind::Lt,
+                        _ => unreachable!(),
+                    }
+                };
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, e, make_x())));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, make_x(), e)));
             },
-            6 => {
+            9 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Unary(UnOp::Deref, e)));
             },
-            7 => {
+            10 => {
                 let block = P(Block {
                     stmts: Vec::new(),
                     id: DUMMY_NODE_ID,
@@ -106,7 +107,7 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                 });
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::If(e, block.clone(), None)));
             },
-            8 => {
+            11 => {
                 let decl = P(FnDecl {
                     inputs: vec![],
                     output: FunctionRetTy::Default(DUMMY_SP),
@@ -120,33 +121,41 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                                           e,
                                           DUMMY_SP)));
             },
-            9 => {
+            12 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(e, make_x())));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(make_x(), e)));
             },
-            10 => {
+            13 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Field(e, Ident::from_str("f"))));
             },
-            11 => {
+            14 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Range(
                             Some(e), Some(make_x()), RangeLimits::HalfOpen)));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Range(
                             Some(make_x()), Some(e), RangeLimits::HalfOpen)));
             },
-            12 => {
+            15 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::AddrOf(Mutability::Immutable, e)));
             },
-            13 => {
+            16 => {
                 g(ExprKind::Ret(None));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Ret(Some(e))));
             },
-            14 => {
+            17 => {
                 let path = Path::from_ident(Ident::from_str("S"));
                 g(ExprKind::Struct(path, vec![], Some(make_x())));
             },
-            15 => {
+            18 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Try(e)));
             },
+            19 => {
+                let ps = vec![P(Pat {
+                    id: DUMMY_NODE_ID,
+                    node: PatKind::Wild,
+                    span: DUMMY_SP,
+                })];
+                iter_exprs(depth - 1, &mut |e| g(ExprKind::Let(ps.clone(), e)))
+            },
             _ => panic!("bad counter value in iter_exprs"),
         }
     }
diff --git a/src/test/ui/lint/lint-unnecessary-parens.rs b/src/test/ui/lint/lint-unnecessary-parens.rs
index dc74d69bd8d..c36101043a7 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.rs
+++ b/src/test/ui/lint/lint-unnecessary-parens.rs
@@ -22,8 +22,8 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
-    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `if let` head expression
-    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `while let` head expression
+    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` head expression
+    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` head expression
     let v = X { y: false };
     // struct lits needs parens, so these shouldn't warn.
     if (v == X { y: true }) {}
diff --git a/src/test/ui/lint/lint-unnecessary-parens.stderr b/src/test/ui/lint/lint-unnecessary-parens.stderr
index fe2ee38eb42..dfbefd7b03e 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.stderr
+++ b/src/test/ui/lint/lint-unnecessary-parens.stderr
@@ -40,13 +40,13 @@ error: unnecessary parentheses around `match` head expression
 LL |     match (true) {
    |           ^^^^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `if let` head expression
+error: unnecessary parentheses around `let` head expression
   --> $DIR/lint-unnecessary-parens.rs:25:16
    |
 LL |     if let 1 = (1) {}
    |                ^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `while let` head expression
+error: unnecessary parentheses around `let` head expression
   --> $DIR/lint-unnecessary-parens.rs:26:19
    |
 LL |     while let 1 = (2) {}
diff --git a/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
new file mode 100644
index 00000000000..e66d4657566
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
@@ -0,0 +1,6 @@
+// compile-pass
+// compile-flags: -Z unpretty=expanded
+
+fn main() {
+    if let 0 = 1 {}
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout
new file mode 100644
index 00000000000..a6b15f9bbf6
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout
@@ -0,0 +1,10 @@
+#![feature(prelude_import)]
+#![no_std]
+#[prelude_import]
+use ::std::prelude::v1::*;
+#[macro_use]
+extern crate std;
+// compile-pass
+// compile-flags: -Z unpretty=expanded
+
+fn main() { if let 0 = 1 { } }
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
new file mode 100644
index 00000000000..7d1e5c3d64d
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
@@ -0,0 +1,244 @@
+// Here we test that `lowering` behaves correctly wrt. `let $pats = $expr` expressions.
+//
+// We want to make sure that `let` is banned in situations other than:
+//
+// expr =
+//   | ...
+//   | "if" expr_with_let block {"else" block}?
+//   | {label ":"}? while" expr_with_let block
+//   ;
+//
+// expr_with_let =
+//   | "let" top_pats "=" expr
+//   | expr_with_let "&&" expr_with_let
+//   | "(" expr_with_let ")"
+//   | expr
+//   ;
+//
+// To that end, we check some positions which is not part of the language above.
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+#![feature(let_chains)] // Avoid inflating `.stderr` with overzealous gates in this test.
+//~^ WARN the feature `let_chains` is incomplete
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn main() {}
+
+fn nested_within_if_expr() {
+    if &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+
+    if !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR type `bool` cannot be dereferenced
+    if -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR cannot apply unary operator `-` to type `bool`
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        if let 0 = 0? {}
+        //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+        Ok(())
+    }
+    if (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result`
+
+    if true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if true && (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if true || (true && let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    if x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+
+    if true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+    if ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+    if (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+
+    // Binds as `(let ... = true)..true &&/|| false`.
+    if let Range { start: _, end: _ } = true..true && false {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    if let Range { start: _, end: _ } = true..true || false {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+
+    // Binds as `(let Range { start: F, end } = F)..(|| true)`.
+    const F: fn() -> bool = || true;
+    if let Range { start: F, end } = F..|| true {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+
+    // Binds as `(let Range { start: true, end } = t)..(&&false)`.
+    let t = &&true;
+    if let Range { start: true, end } = t..&&false {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+
+    if let true = let true = true {} //~ ERROR `let` expressions are not supported here
+}
+
+fn nested_within_while_expr() {
+    while &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+
+    while !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR type `bool` cannot be dereferenced
+    while -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR cannot apply unary operator `-` to type `bool`
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        while let 0 = 0? {}
+        //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+        Ok(())
+    }
+    while (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result`
+
+    while true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while true && (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while true || (true && let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    while x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+
+    while true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+    while ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+    while (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
+
+    // Binds as `(let ... = true)..true &&/|| false`.
+    while let Range { start: _, end: _ } = true..true && false {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    while let Range { start: _, end: _ } = true..true || false {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+
+    // Binds as `(let Range { start: F, end } = F)..(|| true)`.
+    const F: fn() -> bool = || true;
+    while let Range { start: F, end } = F..|| true {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+
+    // Binds as `(let Range { start: true, end } = t)..(&&false)`.
+    let t = &&true;
+    while let Range { start: true, end } = t..&&false {}
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+
+    while let true = let true = true {} //~ ERROR `let` expressions are not supported here
+}
+
+fn not_error_because_clarified_intent() {
+    if let Range { start: _, end: _ } = (true..true || false) { }
+
+    if let Range { start: _, end: _ } = (true..true && false) { }
+
+    while let Range { start: _, end: _ } = (true..true || false) { }
+
+    while let Range { start: _, end: _ } = (true..true && false) { }
+}
+
+fn outside_if_and_while_expr() {
+    &let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    !let 0 = 0; //~ ERROR `let` expressions are not supported here
+    *let 0 = 0; //~ ERROR `let` expressions are not supported here
+    //~^ ERROR type `bool` cannot be dereferenced
+    -let 0 = 0; //~ ERROR `let` expressions are not supported here
+    //~^ ERROR cannot apply unary operator `-` to type `bool`
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        let 0 = 0?;
+        //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+        Ok(())
+    }
+    (let 0 = 0)?; //~ ERROR `let` expressions are not supported here
+    //~^ ERROR the `?` operator can only be used in a function that returns `Result`
+    //~| ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+
+    true || let 0 = 0; //~ ERROR `let` expressions are not supported here
+    (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+    true && (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    x = let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    true..(let 0 = 0); //~ ERROR `let` expressions are not supported here
+    ..(let 0 = 0); //~ ERROR `let` expressions are not supported here
+    (let 0 = 0)..; //~ ERROR `let` expressions are not supported here
+
+    (let Range { start: _, end: _ } = true..true || false);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+
+    (let true = let true = true);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    // Check function tail position.
+    &let 0 = 0
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+}
+
+// Let's make sure that `let` inside const generic arguments are considered.
+fn inside_const_generic_arguments() {
+    struct A<const B: bool>;
+    impl<const B: bool> A<{B}> { const O: u32 = 5; }
+
+    if let A::<{
+        true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+        //~^ ERROR constant contains unimplemented expression type
+        //~| ERROR constant contains unimplemented expression type
+    }>::O = 5 {}
+
+    while let A::<{
+        true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+        //~^ ERROR constant contains unimplemented expression type
+        //~| ERROR constant contains unimplemented expression type
+    }>::O = 5 {}
+
+    if A::<{
+        true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+        //~^ ERROR constant contains unimplemented expression type
+        //~| ERROR constant contains unimplemented expression type
+    }>::O == 5 {}
+
+    // In the cases above we have `ExprKind::Block` to help us out.
+    // Below however, we would not have a block and so an implementation might go
+    // from visiting expressions to types without banning `let` expressions down the tree.
+    // This tests ensures that we are not caught by surprise should the parser
+    // admit non-IDENT expressions in const generic arguments.
+
+    if A::<
+        true && let 1 = 1 //~ ERROR expected one of `,` or `>`, found `&&`
+    >::O == 5 {}
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
new file mode 100644
index 00000000000..207d0d6d6b8
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
@@ -0,0 +1,987 @@
+error: expected one of `,` or `>`, found `&&`
+  --> $DIR/disallowed-positions.rs:242:14
+   |
+LL |         true && let 1 = 1
+   |              ^^ expected one of `,` or `>` here
+
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/disallowed-positions.rs:20:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
+warning: the feature `let_chains` is incomplete and may cause the compiler to crash
+  --> $DIR/disallowed-positions.rs:22:12
+   |
+LL | #![feature(let_chains)] // Avoid inflating `.stderr` with overzealous gates in this test.
+   |            ^^^^^^^^^^
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:32:9
+   |
+LL |     if &let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:35:9
+   |
+LL |     if !let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:36:9
+   |
+LL |     if *let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:38:9
+   |
+LL |     if -let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:46:9
+   |
+LL |     if (let 0 = 0)? {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:50:16
+   |
+LL |     if true || let 0 = 0 {}
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:51:17
+   |
+LL |     if (true || let 0 = 0) {}
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:52:25
+   |
+LL |     if true && (true || let 0 = 0) {}
+   |                         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:53:25
+   |
+LL |     if true || (true && let 0 = 0) {}
+   |                         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:56:12
+   |
+LL |     if x = let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:59:15
+   |
+LL |     if true..(let 0 = 0) {}
+   |               ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:61:11
+   |
+LL |     if ..(let 0 = 0) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:63:9
+   |
+LL |     if (let 0 = 0).. {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:67:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:71:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:78:8
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:86:8
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:92:19
+   |
+LL |     if let true = let true = true {}
+   |                   ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:96:12
+   |
+LL |     while &let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:99:12
+   |
+LL |     while !let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:100:12
+   |
+LL |     while *let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:102:12
+   |
+LL |     while -let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:110:12
+   |
+LL |     while (let 0 = 0)? {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:114:19
+   |
+LL |     while true || let 0 = 0 {}
+   |                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:115:20
+   |
+LL |     while (true || let 0 = 0) {}
+   |                    ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:116:28
+   |
+LL |     while true && (true || let 0 = 0) {}
+   |                            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:117:28
+   |
+LL |     while true || (true && let 0 = 0) {}
+   |                            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:120:15
+   |
+LL |     while x = let 0 = 0 {}
+   |               ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:123:18
+   |
+LL |     while true..(let 0 = 0) {}
+   |                  ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:125:14
+   |
+LL |     while ..(let 0 = 0) {}
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:127:12
+   |
+LL |     while (let 0 = 0).. {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:131:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:135:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:142:11
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:150:11
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:156:22
+   |
+LL |     while let true = let true = true {}
+   |                      ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:170:6
+   |
+LL |     &let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:172:6
+   |
+LL |     !let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:173:6
+   |
+LL |     *let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:175:6
+   |
+LL |     -let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:183:6
+   |
+LL |     (let 0 = 0)?;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:187:13
+   |
+LL |     true || let 0 = 0;
+   |             ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:188:14
+   |
+LL |     (true || let 0 = 0);
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:189:22
+   |
+LL |     true && (true || let 0 = 0);
+   |                      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:192:9
+   |
+LL |     x = let 0 = 0;
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:194:12
+   |
+LL |     true..(let 0 = 0);
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:195:8
+   |
+LL |     ..(let 0 = 0);
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:196:6
+   |
+LL |     (let 0 = 0)..;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:198:6
+   |
+LL |     (let Range { start: _, end: _ } = true..true || false);
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:202:6
+   |
+LL |     (let true = let true = true);
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:202:17
+   |
+LL |     (let true = let true = true);
+   |                 ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:207:6
+   |
+LL |     &let 0 = 0
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:218:17
+   |
+LL |         true && let 1 = 1
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:224:17
+   |
+LL |         true && let 1 = 1
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:230:17
+   |
+LL |         true && let 1 = 1
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:32:8
+   |
+LL |     if &let 0 = 0 {}
+   |        ^^^^^^^^^^ expected bool, found &bool
+   |
+   = note: expected type `bool`
+              found type `&bool`
+
+error[E0614]: type `bool` cannot be dereferenced
+  --> $DIR/disallowed-positions.rs:36:8
+   |
+LL |     if *let 0 = 0 {}
+   |        ^^^^^^^^^^
+
+error[E0600]: cannot apply unary operator `-` to type `bool`
+  --> $DIR/disallowed-positions.rs:38:8
+   |
+LL |     if -let 0 = 0 {}
+   |        ^^^^^^^^^^ cannot apply unary operator `-`
+   |
+   = note: an implementation of `std::ops::Neg` might be missing for `bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:46:8
+   |
+LL |     if (let 0 = 0)? {}
+   |        ^^^^^^^^^^^^ the `?` operator cannot be applied to type `bool`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `bool`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
+  --> $DIR/disallowed-positions.rs:46:8
+   |
+LL |     if (let 0 = 0)? {}
+   |        ^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:56:8
+   |
+LL |     if x = let 0 = 0 {}
+   |        ^^^^^^^^^^^^^
+   |        |
+   |        expected bool, found ()
+   |        help: try comparing for equality: `x == let 0 = 0`
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:59:8
+   |
+LL |     if true..(let 0 = 0) {}
+   |        ^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:61:8
+   |
+LL |     if ..(let 0 = 0) {}
+   |        ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeTo`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeTo<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:63:8
+   |
+LL |     if (let 0 = 0).. {}
+   |        ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeFrom`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeFrom<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:67:12
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |            |
+   |            expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:67:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:71:12
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |            |
+   |            expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:71:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:78:12
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found struct `std::ops::Range`
+   |
+   = note: expected type `fn() -> bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:78:41
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |                                         ^^^^^^^ expected bool, found closure
+   |
+   = note: expected type `bool`
+              found type `[closure@$DIR/disallowed-positions.rs:78:41: 78:48]`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:78:8
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:86:12
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^   - this match expression has type `bool`
+   |            |
+   |            expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:86:44
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |                                            ^^^^^^^ expected bool, found &&bool
+   |
+   = note: expected type `bool`
+              found type `&&bool`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:86:8
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:42:20
+   |
+LL |         if let 0 = 0? {}
+   |                    ^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `{integer}`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:96:11
+   |
+LL |     while &let 0 = 0 {}
+   |           ^^^^^^^^^^ expected bool, found &bool
+   |
+   = note: expected type `bool`
+              found type `&bool`
+
+error[E0614]: type `bool` cannot be dereferenced
+  --> $DIR/disallowed-positions.rs:100:11
+   |
+LL |     while *let 0 = 0 {}
+   |           ^^^^^^^^^^
+
+error[E0600]: cannot apply unary operator `-` to type `bool`
+  --> $DIR/disallowed-positions.rs:102:11
+   |
+LL |     while -let 0 = 0 {}
+   |           ^^^^^^^^^^ cannot apply unary operator `-`
+   |
+   = note: an implementation of `std::ops::Neg` might be missing for `bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:110:11
+   |
+LL |     while (let 0 = 0)? {}
+   |           ^^^^^^^^^^^^ the `?` operator cannot be applied to type `bool`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `bool`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
+  --> $DIR/disallowed-positions.rs:110:11
+   |
+LL |     while (let 0 = 0)? {}
+   |           ^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:120:11
+   |
+LL |     while x = let 0 = 0 {}
+   |           ^^^^^^^^^^^^^
+   |           |
+   |           expected bool, found ()
+   |           help: try comparing for equality: `x == let 0 = 0`
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:123:11
+   |
+LL |     while true..(let 0 = 0) {}
+   |           ^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:125:11
+   |
+LL |     while ..(let 0 = 0) {}
+   |           ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeTo`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeTo<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:127:11
+   |
+LL |     while (let 0 = 0).. {}
+   |           ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeFrom`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeFrom<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:131:15
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |               |
+   |               expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:131:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:135:15
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |               |
+   |               expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:135:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:142:15
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found struct `std::ops::Range`
+   |
+   = note: expected type `fn() -> bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:142:44
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |                                            ^^^^^^^ expected bool, found closure
+   |
+   = note: expected type `bool`
+              found type `[closure@$DIR/disallowed-positions.rs:142:44: 142:51]`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:142:11
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:150:15
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^   - this match expression has type `bool`
+   |               |
+   |               expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:150:47
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |                                               ^^^^^^^ expected bool, found &&bool
+   |
+   = note: expected type `bool`
+              found type `&&bool`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:150:11
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:106:23
+   |
+LL |         while let 0 = 0? {}
+   |                       ^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `{integer}`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0614]: type `bool` cannot be dereferenced
+  --> $DIR/disallowed-positions.rs:173:5
+   |
+LL |     *let 0 = 0;
+   |     ^^^^^^^^^^
+
+error[E0600]: cannot apply unary operator `-` to type `bool`
+  --> $DIR/disallowed-positions.rs:175:5
+   |
+LL |     -let 0 = 0;
+   |     ^^^^^^^^^^ cannot apply unary operator `-`
+   |
+   = note: an implementation of `std::ops::Neg` might be missing for `bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:183:5
+   |
+LL |     (let 0 = 0)?;
+   |     ^^^^^^^^^^^^ the `?` operator cannot be applied to type `bool`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `bool`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
+  --> $DIR/disallowed-positions.rs:183:5
+   |
+LL |     (let 0 = 0)?;
+   |     ^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:198:10
+   |
+LL |     (let Range { start: _, end: _ } = true..true || false);
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |          |
+   |          expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:207:5
+   |
+LL | fn outside_if_and_while_expr() {
+   |                                - help: try adding a return type: `-> &bool`
+...
+LL |     &let 0 = 0
+   |     ^^^^^^^^^^ expected (), found &bool
+   |
+   = note: expected type `()`
+              found type `&bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:179:17
+   |
+LL |         let 0 = 0?;
+   |                 ^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `{integer}`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:218:25
+   |
+LL |         true && let 1 = 1
+   |                         ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:218:21
+   |
+LL |         true && let 1 = 1
+   |                     ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:224:25
+   |
+LL |         true && let 1 = 1
+   |                         ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:224:21
+   |
+LL |         true && let 1 = 1
+   |                     ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:230:25
+   |
+LL |         true && let 1 = 1
+   |                         ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:230:21
+   |
+LL |         true && let 1 = 1
+   |                     ^
+
+error: aborting due to 109 previous errors
+
+Some errors have detailed explanations: E0019, E0277, E0308, E0600, E0614.
+For more information about an error, try `rustc --explain E0019`.
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
new file mode 100644
index 00000000000..64987663adb
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -0,0 +1,136 @@
+// gate-test-let_chains
+
+// Here we test feature gating for ´let_chains`.
+// See `disallowed-positions.rs` for the grammar
+// defining the language for gated allowed positions.
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn _if() {
+    if let 0 = 1 {} // Stable!
+
+    if (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    if (((let 0 = 1))) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    if true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    if let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    if (let 0 = 1) && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    if true && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    if (let 0 = 1) && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    if let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+}
+
+fn _while() {
+    while let 0 = 1 {} // Stable!
+
+    while (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    while (((let 0 = 1))) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    while true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    while let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    while (let 0 = 1) && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    while true && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+
+    while (let 0 = 1) && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    while let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+}
+
+fn _macros() {
+    macro_rules! noop_expr { ($e:expr) => {}; }
+
+    noop_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    macro_rules! use_expr {
+        ($e:expr) => {
+            if $e {}
+            while $e {}
+        }
+    }
+    use_expr!((let 0 = 1 && 0 == 0));
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    use_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    #[cfg(FALSE)] (let 0 = 1);
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    use_expr!(let 0 = 1);
+    //~^ ERROR no rules expected the token `let`
+    // ^--- FIXME(53667): Consider whether `Let` can be added to `ident_can_begin_expr`.
+}
+
+fn main() {}
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
new file mode 100644
index 00000000000..6167427fa9f
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -0,0 +1,570 @@
+error: no rules expected the token `let`
+  --> $DIR/feature-gate.rs:131:15
+   |
+LL |     macro_rules! use_expr {
+   |     --------------------- when calling this macro
+...
+LL |     use_expr!(let 0 = 1);
+   |               ^^^ no rules expected this token in macro call
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:14:9
+   |
+LL |     if (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:18:11
+   |
+LL |     if (((let 0 = 1))) {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:22:16
+   |
+LL |     if true && let 0 = 1 {}
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:26:8
+   |
+LL |     if let 0 = 1 && true {}
+   |        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:30:9
+   |
+LL |     if (let 0 = 1) && true {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:34:17
+   |
+LL |     if true && (let 0 = 1) {}
+   |                 ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:38:9
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:38:24
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |                        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:8
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:21
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                     ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:35
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:48
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:61
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                             ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:56:8
+   |
+LL |     if let Range { start: _, end: _ } = (true..true) && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:64:12
+   |
+LL |     while (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:68:14
+   |
+LL |     while (((let 0 = 1))) {}
+   |              ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:72:19
+   |
+LL |     while true && let 0 = 1 {}
+   |                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:76:11
+   |
+LL |     while let 0 = 1 && true {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:80:12
+   |
+LL |     while (let 0 = 1) && true {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:84:20
+   |
+LL |     while true && (let 0 = 1) {}
+   |                    ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:88:12
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:88:27
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |                           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:11
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:24
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:38
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                      ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:51
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:64
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:106:11
+   |
+LL |     while let Range { start: _, end: _ } = (true..true) && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:129:20
+   |
+LL |     #[cfg(FALSE)] (let 0 = 1);
+   |                    ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:114:17
+   |
+LL |     noop_expr!((let 0 = 1));
+   |                 ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:123:16
+   |
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:126:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:14:9
+   |
+LL |     if (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:18:11
+   |
+LL |     if (((let 0 = 1))) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:22:16
+   |
+LL |     if true && let 0 = 1 {}
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:26:8
+   |
+LL |     if let 0 = 1 && true {}
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:30:9
+   |
+LL |     if (let 0 = 1) && true {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:34:17
+   |
+LL |     if true && (let 0 = 1) {}
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:38:9
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:38:24
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |                        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:8
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:21
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                     ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:35
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:48
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:61
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                             ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:56:8
+   |
+LL |     if let Range { start: _, end: _ } = (true..true) && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:64:12
+   |
+LL |     while (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:68:14
+   |
+LL |     while (((let 0 = 1))) {}
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:72:19
+   |
+LL |     while true && let 0 = 1 {}
+   |                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:76:11
+   |
+LL |     while let 0 = 1 && true {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:80:12
+   |
+LL |     while (let 0 = 1) && true {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:84:20
+   |
+LL |     while true && (let 0 = 1) {}
+   |                    ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:88:12
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:88:27
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |                           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:11
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:24
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:38
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:51
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:64
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:106:11
+   |
+LL |     while let Range { start: _, end: _ } = (true..true) && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:123:16
+   |
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:126:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: aborting due to 63 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs
new file mode 100644
index 00000000000..1de4e5bcebe
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs
@@ -0,0 +1,18 @@
+// run-pass
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn main() {
+    let x: bool;
+    // This should associate as: `(x = (true && false));`.
+    x = true && false;
+    assert!(!x);
+
+    fn _f1() -> bool {
+        // Should associate as `(let _ = (return (true && false)))`.
+        if let _ = return true && false {};
+    }
+    assert!(!_f1());
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
deleted file mode 100644
index d79798d57e8..00000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-// edition:2015
-
-// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
-// with examples easier.
-
-#[allow(irrefutable_let_patterns)]
-fn main() {
-    use std::ops::Range;
-
-    if let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    if let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    while let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    if let true = false && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let true = (1 == 2) && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    // The following cases are not an error as parenthesis are used to
-    // clarify intent:
-
-    if let Range { start: _, end: _ } = true..(true || false) { }
-
-    if let Range { start: _, end: _ } = true..(true && false) { }
-
-    while let Range { start: _, end: _ } = true..(true || false) { }
-
-    while let Range { start: _, end: _ } = true..(true && false) { }
-}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
deleted file mode 100644
index 2cd59fe56cf..00000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
+++ /dev/null
@@ -1,56 +0,0 @@
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:10:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true && false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2015.rs:13:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true || false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:16:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true && false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2015.rs:19:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true || false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:22:19
-   |
-LL |     if let true = false && false { }
-   |                   ^^^^^^^^^^^^^^ help: consider adding parentheses: `(false && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:25:22
-   |
-LL |     while let true = (1 == 2) && false { }
-   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parentheses: `((1 == 2) && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: aborting due to 6 previous errors
-
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
deleted file mode 100644
index 687bf659416..00000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-// edition:2018
-
-// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
-// with examples easier.
-
-#[allow(irrefutable_let_patterns)]
-fn main() {
-    use std::ops::Range;
-
-    if let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    if let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    while let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    if let true = false && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let true = (1 == 2) && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    // The following cases are not an error as parenthesis are used to
-    // clarify intent:
-
-    if let Range { start: _, end: _ } = true..(true || false) { }
-
-    if let Range { start: _, end: _ } = true..(true && false) { }
-
-    while let Range { start: _, end: _ } = true..(true || false) { }
-
-    while let Range { start: _, end: _ } = true..(true && false) { }
-}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr
deleted file mode 100644
index cbba2d74733..00000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr
+++ /dev/null
@@ -1,56 +0,0 @@
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:10:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true && false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2018.rs:13:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true || false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:16:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true && false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2018.rs:19:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true || false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:22:19
-   |
-LL |     if let true = false && false { }
-   |                   ^^^^^^^^^^^^^^ help: consider adding parentheses: `(false && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:25:22
-   |
-LL |     while let true = (1 == 2) && false { }
-   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parentheses: `((1 == 2) && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: aborting due to 6 previous errors
-