Revert let_chains stabilization

This reverts commit 3266460749.

This is the revert against master, the beta revert was already done in #100538.

compiler/rustc_ast/src/lib.rs

@@ -14,6 +14,7 @@
 #![feature(const_trait_impl)]
 #![feature(if_let_guard)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
+#![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(slice_internals)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -31,6 +31,7 @@
 //! in the HIR, especially for multiple identifiers.
 
 #![feature(box_patterns)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -119,7 +119,33 @@ impl<'a> AstValidator<'a> {
 
     /// Emits an error banning the `let` expression provided in the given location.
     fn ban_let_expr(&self, expr: &'a Expr, forbidden_let_reason: ForbiddenLetReason) {
-        self.session.emit_err(ForbiddenLet { span: expr.span, reason: forbidden_let_reason });
+        let sess = &self.session;
+        if sess.opts.unstable_features.is_nightly_build() {
+            let err = "`let` expressions are not supported here";
+            let mut diag = sess.struct_span_err(expr.span, err);
+            diag.note("only supported directly in conditions of `if` and `while` expressions");
+            match forbidden_let_reason {
+                ForbiddenLetReason::GenericForbidden => {}
+                ForbiddenLetReason::NotSupportedOr(span) => {
+                    diag.span_note(
+                        span,
+                        "`||` operators are not supported in let chain expressions",
+                    );
+                }
+                ForbiddenLetReason::NotSupportedParentheses(span) => {
+                    diag.span_note(
+                        span,
+                        "`let`s wrapped in parentheses are not supported in a context with let \
+                        chains",
+                    );
+                }
+            }
+            diag.emit();
+        } else {
+            sess.struct_span_err(expr.span, "expected expression, found statement (`let`)")
+                .note("variable declaration using `let` is a statement")
+                .emit();
+        }
     }
 
     fn check_gat_where(

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -777,6 +777,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
         "`if let` guards are experimental",
         "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
     );
+    gate_all!(let_chains, "`let` expressions in this position are unstable");
     gate_all!(
         async_closure,
         "async closures are unstable",

compiler/rustc_ast_passes/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(iter_is_partitioned)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![recursion_limit = "256"]
 

compiler/rustc_attr/src/lib.rs

@@ -4,6 +4,7 @@
 //! The goal is to move the definition of `MetaItem` and things that don't need to be in `syntax`
 //! to this crate.
 
+#![feature(let_chains)]
 #![feature(let_else)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]

compiler/rustc_borrowck/src/lib.rs

@@ -2,6 +2,7 @@
 
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

compiler/rustc_builtin_macros/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(is_sorted)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]

compiler/rustc_codegen_llvm/src/lib.rs

@@ -6,6 +6,7 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(hash_raw_entry)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(extern_types)]
 #![feature(once_cell)]

compiler/rustc_const_eval/src/lib.rs

@@ -9,6 +9,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(control_flow_enum)]
 #![feature(decl_macro)]
 #![feature(exact_size_is_empty)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

compiler/rustc_error_messages/src/lib.rs

@@ -1,3 +1,4 @@
+#![feature(let_chains)]
 #![feature(once_cell)]
 #![feature(rustc_attrs)]
 #![feature(type_alias_impl_trait)]

compiler/rustc_errors/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
 #![feature(adt_const_params)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![feature(result_option_inspect)]

compiler/rustc_expand/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(associated_type_bounds)]
 #![feature(associated_type_defaults)]
 #![feature(if_let_guard)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(macro_metavar_expr)]
 #![feature(proc_macro_diagnostic)]

compiler/rustc_feature/src/accepted.rs

@@ -188,8 +188,6 @@ declare_features! (
     (accepted, item_like_imports, "1.15.0", Some(35120), None),
     /// Allows `'a: { break 'a; }`.
     (accepted, label_break_value, "CURRENT_RUSTC_VERSION", Some(48594), None),
-    /// Allows `if/while p && let q = r && ...` chains.
-    (accepted, let_chains, "1.64.0", Some(53667), None),
     /// Allows `break {expr}` with a value inside `loop`s.
     (accepted, loop_break_value, "1.19.0", Some(37339), None),
     /// Allows use of `?` as the Kleene "at most one" operator in macros.

compiler/rustc_feature/src/active.rs

@@ -422,6 +422,8 @@ declare_features! (
     (active, isa_attribute, "1.48.0", Some(74727), None),
     // Allows setting the threshold for the `large_assignments` lint.
     (active, large_assignments, "1.52.0", Some(83518), None),
+    /// Allows `if/while p && let q = r && ...` chains.
+    (active, let_chains, "1.37.0", Some(53667), None),
     /// Allows `let...else` statements.
     (active, let_else, "1.56.0", Some(87335), None),
     /// Allows `#[link(..., cfg(..))]`.

compiler/rustc_infer/src/lib.rs

@@ -18,6 +18,7 @@
 #![feature(control_flow_enum)]
 #![feature(extend_one)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

compiler/rustc_lint/src/lib.rs

@@ -33,6 +33,7 @@
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(iter_order_by)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]

compiler/rustc_metadata/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(generators)]
 #![feature(generic_associated_types)]
 #![feature(iter_from_generator)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(once_cell)]
 #![feature(proc_macro_internals)]

compiler/rustc_middle/src/lib.rs

@@ -39,6 +39,7 @@
 #![feature(extern_types)]
 #![feature(new_uninit)]
 #![feature(once_cell)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(trusted_len)]

compiler/rustc_mir_build/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(if_let_guard)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(once_cell)]

compiler/rustc_mir_transform/src/lib.rs

@@ -1,5 +1,6 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

compiler/rustc_parse/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

compiler/rustc_parse/src/parser/expr.rs

@@ -2251,7 +2251,15 @@ impl<'a> Parser<'a> {
 
     /// Parses the condition of a `if` or `while` expression.
     fn parse_cond_expr(&mut self) -> PResult<'a, P<Expr>> {
-        self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL | Restrictions::ALLOW_LET, None)
+        let cond =
+            self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL | Restrictions::ALLOW_LET, None)?;
+
+        if let ExprKind::Let(..) = cond.kind {
+            // Remove the last feature gating of a `let` expression since it's stable.
+            self.sess.gated_spans.ungate_last(sym::let_chains, cond.span);
+        }
+
+        Ok(cond)
     }
 
     /// Parses a `let $pat = $expr` pseudo-expression.
@@ -2280,6 +2288,7 @@ impl<'a> Parser<'a> {
             this.parse_assoc_expr_with(1 + prec_let_scrutinee_needs_par(), None.into())
         })?;
         let span = lo.to(expr.span);
+        self.sess.gated_spans.gate(sym::let_chains, span);
         Ok(self.mk_expr(span, ExprKind::Let(pat, expr, span)))
     }
 
@@ -2571,13 +2580,15 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
         // Used to check the `let_chains` and `if_let_guard` features mostly by scaning
         // `&&` tokens.
-        fn check_let_expr(expr: &Expr) -> bool {
+        fn check_let_expr(expr: &Expr) -> (bool, bool) {
             match expr.kind {
                 ExprKind::Binary(BinOp { node: BinOpKind::And, .. }, ref lhs, ref rhs) => {
-                    check_let_expr(lhs) || check_let_expr(rhs)
+                    let lhs_rslt = check_let_expr(lhs);
+                    let rhs_rslt = check_let_expr(rhs);
+                    (lhs_rslt.0 || rhs_rslt.0, false)
                 }
-                ExprKind::Let(..) => true,
+                ExprKind::Let(..) => (true, true),
-                _ => false,
+                _ => (false, true),
             }
         }
         let attrs = self.parse_outer_attributes()?;
@@ -2592,7 +2603,12 @@ impl<'a> Parser<'a> {
             let guard = if this.eat_keyword(kw::If) {
                 let if_span = this.prev_token.span;
                 let cond = this.parse_expr_res(Restrictions::ALLOW_LET, None)?;
-                if check_let_expr(&cond) {
+                let (has_let_expr, does_not_have_bin_op) = check_let_expr(&cond);
+                if has_let_expr {
+                    if does_not_have_bin_op {
+                        // Remove the last feature gating of a `let` expression since it's stable.
+                        this.sess.gated_spans.ungate_last(sym::let_chains, cond.span);
+                    }
                     let span = if_span.to(cond.span);
                     this.sess.gated_spans.gate(sym::if_let_guard, span);
                 }

compiler/rustc_passes/src/lib.rs

@@ -7,6 +7,7 @@
 #![allow(rustc::potential_query_instability)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(iter_intersperse)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

compiler/rustc_resolve/src/lib.rs

@@ -11,6 +11,7 @@
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]

compiler/rustc_session/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(if_let_guard)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

compiler/rustc_trait_selection/src/lib.rs

@@ -17,6 +17,7 @@
 #![feature(drain_filter)]
 #![feature(hash_drain_filter)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(if_let_guard)]
 #![feature(never_type)]

compiler/rustc_typeck/src/lib.rs

@@ -65,6 +65,7 @@ This API is completely unstable and subject to change.
 #![feature(is_sorted)]
 #![feature(iter_intersperse)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

library/std/src/lib.rs

@@ -254,6 +254,7 @@
 #![feature(intra_doc_pointers)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(lang_items)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(linkage)]
 #![feature(link_cfg)]

src/librustdoc/lib.rs

@@ -8,6 +8,7 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(test)]
 #![feature(never_type)]

src/test/ui/expr/if/attrs/let-chains-attr.rs

@@ -1,5 +1,7 @@
 // check-pass
 
+#![feature(let_chains)]
+
 #[cfg(FALSE)]
 fn foo() {
     #[attr]

src/test/ui/expr/if/bad-if-let-suggestion.rs

@@ -4,6 +4,7 @@
 fn a() {
     if let x = 1 && i = 2 {}
     //~^ ERROR cannot find value `i` in this scope
+    //~| ERROR `let` expressions in this position are unstable
     //~| ERROR mismatched types
     //~| ERROR `let` expressions are not supported here
 }

src/test/ui/expr/if/bad-if-let-suggestion.stderr

@@ -13,7 +13,7 @@ LL |     if let x = 1 && i = 2 {}
    |                     ^ not found in this scope
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:12:9
+  --> $DIR/bad-if-let-suggestion.rs:13:9
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -22,7 +22,7 @@ LL |     if (i + j) = i {}
    |         ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `j` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:12:13
+  --> $DIR/bad-if-let-suggestion.rs:13:13
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -31,7 +31,7 @@ LL |     if (i + j) = i {}
    |             ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `i` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:12:18
+  --> $DIR/bad-if-let-suggestion.rs:13:18
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -40,7 +40,7 @@ LL |     if (i + j) = i {}
    |                  ^ help: a function with a similar name exists: `a`
 
 error[E0425]: cannot find value `x` in this scope
-  --> $DIR/bad-if-let-suggestion.rs:19:8
+  --> $DIR/bad-if-let-suggestion.rs:20:8
    |
 LL | fn a() {
    | ------ similarly named function `a` defined here
@@ -48,13 +48,22 @@ LL | fn a() {
 LL |     if x[0] = 1 {}
    |        ^ help: a function with a similar name exists: `a`
 
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/bad-if-let-suggestion.rs:5:8
+   |
+LL |     if let x = 1 && i = 2 {}
+   |        ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
 error[E0308]: mismatched types
   --> $DIR/bad-if-let-suggestion.rs:5:8
    |
 LL |     if let x = 1 && i = 2 {}
    |        ^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
 
-error: aborting due to 7 previous errors
+error: aborting due to 8 previous errors
 
-Some errors have detailed explanations: E0308, E0425.
+Some errors have detailed explanations: E0308, E0425, E0658.
 For more information about an error, try `rustc --explain E0308`.

src/test/ui/mir/issue-92893.rs

@@ -1,5 +1,6 @@
 struct Bug<A = [(); (let a = (), 1).1]> {
     //~^ `let` expressions are not supported here
+    //~| `let` expressions in this position are unstable [E0658]
     //~| expected expression, found `let` statement
     a: A
 }

src/test/ui/mir/issue-92893.stderr

@@ -12,5 +12,15 @@ LL | struct Bug<A = [(); (let a = (), 1).1]> {
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: aborting due to 2 previous errors
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/issue-92893.rs:1:22
+   |
+LL | struct Bug<A = [(); (let a = (), 1).1]> {
+   |                      ^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
 
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/mir/mir_let_chains_drop_order.rs

@@ -4,6 +4,7 @@
 
 // See `mir_drop_order.rs` for more information
 
+#![feature(let_chains)]
 #![allow(irrefutable_let_patterns)]
 
 use std::cell::RefCell;

src/test/ui/rfc-2294-if-let-guard/feature-gate.rs

@@ -8,35 +8,49 @@ fn _if_let_guard() {
         //~^ ERROR `if let` guards are experimental
 
         () if (let 0 = 1) => {}
-        //~^ ERROR expected expression, found `let` statement
+        //~^ ERROR `let` expressions in this position are unstable
+        //~| ERROR expected expression, found `let` statement
 
         () if (((let 0 = 1))) => {}
-        //~^ ERROR expected expression, found `let` statement
+        //~^ ERROR `let` expressions in this position are unstable
+        //~| ERROR expected expression, found `let` statement
 
         () if true && let 0 = 1 => {}
         //~^ ERROR `if let` guards are experimental
+        //~| ERROR `let` expressions in this position are unstable
 
         () if let 0 = 1 && true => {}
         //~^ ERROR `if let` guards are experimental
+        //~| ERROR `let` expressions in this position are unstable
 
         () if (let 0 = 1) && true => {}
-        //~^ ERROR expected expression, found `let` statement
+        //~^ ERROR `let` expressions in this position are unstable
+        //~| ERROR expected expression, found `let` statement
 
         () if true && (let 0 = 1) => {}
-        //~^ ERROR expected expression, found `let` statement
+        //~^ ERROR `let` expressions in this position are unstable
+        //~| ERROR expected expression, found `let` statement
 
         () if (let 0 = 1) && (let 0 = 1) => {}
-        //~^ ERROR expected expression, found `let` statement
+        //~^ ERROR `let` expressions in this position are unstable
+        //~| ERROR `let` expressions in this position are unstable
+        //~| ERROR expected expression, found `let` statement
         //~| ERROR expected expression, found `let` statement
 
         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
         //~^ ERROR `if let` guards are experimental
+        //~| ERROR `let` expressions in this position are unstable
+        //~| ERROR `let` expressions in this position are unstable
+        //~| ERROR `let` expressions in this position are unstable
+        //~| ERROR `let` expressions in this position are unstable
+        //~| ERROR `let` expressions in this position are unstable
         //~| ERROR expected expression, found `let` statement
         //~| ERROR expected expression, found `let` statement
         //~| ERROR expected expression, found `let` statement
 
         () if let Range { start: _, end: _ } = (true..true) && false => {}
         //~^ ERROR `if let` guards are experimental
+        //~| ERROR `let` expressions in this position are unstable
 
         _ => {}
     }
@@ -52,9 +66,11 @@ fn _macros() {
         }
     }
     use_expr!((let 0 = 1 && 0 == 0));
-    //~^ ERROR expected expression, found `let` statement
+    //~^ ERROR `let` expressions in this position are unstable
+    //~| ERROR expected expression, found `let` statement
     use_expr!((let 0 = 1));
-    //~^ ERROR expected expression, found `let` statement
+    //~^ ERROR `let` expressions in this position are unstable
+    //~| ERROR expected expression, found `let` statement
     match () {
         #[cfg(FALSE)]
         () if let 0 = 1 => {}

src/test/ui/rfc-2294-if-let-guard/feature-gate.stderr

@@ -5,67 +5,67 @@ LL |         () if (let 0 = 1) => {}
    |                ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:13:18
+  --> $DIR/feature-gate.rs:14:18
    |
 LL |         () if (((let 0 = 1))) => {}
    |                  ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:22:16
+  --> $DIR/feature-gate.rs:26:16
    |
 LL |         () if (let 0 = 1) && true => {}
    |                ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:25:24
+  --> $DIR/feature-gate.rs:30:24
    |
 LL |         () if true && (let 0 = 1) => {}
    |                        ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:28:16
+  --> $DIR/feature-gate.rs:34:16
    |
 LL |         () if (let 0 = 1) && (let 0 = 1) => {}
    |                ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:28:31
+  --> $DIR/feature-gate.rs:34:31
    |
 LL |         () if (let 0 = 1) && (let 0 = 1) => {}
    |                               ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:32:42
+  --> $DIR/feature-gate.rs:40:42
    |
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |                                          ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:32:55
+  --> $DIR/feature-gate.rs:40:55
    |
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |                                                       ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:32:68
+  --> $DIR/feature-gate.rs:40:68
    |
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |                                                                    ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:54:16
+  --> $DIR/feature-gate.rs:68:16
    |
 LL |     use_expr!((let 0 = 1 && 0 == 0));
    |                ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/feature-gate.rs:56:16
+  --> $DIR/feature-gate.rs:71:16
    |
 LL |     use_expr!((let 0 = 1));
    |                ^^^
 
 error: no rules expected the token `let`
-  --> $DIR/feature-gate.rs:64:15
+  --> $DIR/feature-gate.rs:80:15
    |
 LL |     macro_rules! use_expr {
    |     --------------------- when calling this macro
@@ -84,7 +84,7 @@ LL |         () if let 0 = 1 => {}
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
 error[E0658]: `if let` guards are experimental
-  --> $DIR/feature-gate.rs:16:12
+  --> $DIR/feature-gate.rs:18:12
    |
 LL |         () if true && let 0 = 1 => {}
    |            ^^^^^^^^^^^^^^^^^^^^
@@ -94,7 +94,7 @@ LL |         () if true && let 0 = 1 => {}
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
 error[E0658]: `if let` guards are experimental
-  --> $DIR/feature-gate.rs:19:12
+  --> $DIR/feature-gate.rs:22:12
    |
 LL |         () if let 0 = 1 && true => {}
    |            ^^^^^^^^^^^^^^^^^^^^
@@ -104,7 +104,7 @@ LL |         () if let 0 = 1 && true => {}
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
 error[E0658]: `if let` guards are experimental
-  --> $DIR/feature-gate.rs:32:12
+  --> $DIR/feature-gate.rs:40:12
    |
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -114,7 +114,7 @@ LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 =
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
 error[E0658]: `if let` guards are experimental
-  --> $DIR/feature-gate.rs:38:12
+  --> $DIR/feature-gate.rs:51:12
    |
 LL |         () if let Range { start: _, end: _ } = (true..true) && false => {}
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -124,7 +124,7 @@ LL |         () if let Range { start: _, end: _ } = (true..true) && false => {}
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
 error[E0658]: `if let` guards are experimental
-  --> $DIR/feature-gate.rs:60:12
+  --> $DIR/feature-gate.rs:76:12
    |
 LL |         () if let 0 = 1 => {}
    |            ^^^^^^^^^^^^
@@ -133,6 +133,150 @@ LL |         () if let 0 = 1 => {}
    = help: add `#![feature(if_let_guard)]` to the crate attributes to enable
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
-error: aborting due to 18 previous errors
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:10:16
+   |
+LL |         () if (let 0 = 1) => {}
+   |                ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:14:18
+   |
+LL |         () if (((let 0 = 1))) => {}
+   |                  ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:18:23
+   |
+LL |         () if true && let 0 = 1 => {}
+   |                       ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:22:15
+   |
+LL |         () if let 0 = 1 && true => {}
+   |               ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:26:16
+   |
+LL |         () if (let 0 = 1) && true => {}
+   |                ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:30:24
+   |
+LL |         () if true && (let 0 = 1) => {}
+   |                        ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:34:16
+   |
+LL |         () if (let 0 = 1) && (let 0 = 1) => {}
+   |                ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:34:31
+   |
+LL |         () if (let 0 = 1) && (let 0 = 1) => {}
+   |                               ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:40:15
+   |
+LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
+   |               ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:40:28
+   |
+LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
+   |                            ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:40:42
+   |
+LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
+   |                                          ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:40:55
+   |
+LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
+   |                                                       ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:40:68
+   |
+LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
+   |                                                                    ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:51:15
+   |
+LL |         () if let Range { start: _, end: _ } = (true..true) && false => {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:68:16
+   |
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:71:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error: aborting due to 34 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2497-if-let-chains/allowed-syntax.rs

@@ -1,30 +0,0 @@
-// check-pass
-
-#![allow(irrefutable_let_patterns)]
-
-use std::ops::Range;
-
-fn _if() {
-    if let 0 = 1 {}
-
-    if true && let 0 = 1 {}
-
-    if let 0 = 1 && true {}
-
-    if let Range { start: _, end: _ } = (true..true) && false {}
-
-    if let 1 = 1 && let true = { true } && false {
-    }
-}
-
-fn _while() {
-    while let 0 = 1 {}
-
-    while true && let 0 = 1 {}
-
-    while let 0 = 1 && true {}
-
-    while let Range { start: _, end: _ } = (true..true) && false {}
-}
-
-fn main() {}

src/test/ui/rfc-2497-if-let-chains/ast-lowering-does-not-wrap-let-chains.rs

@@ -1,5 +1,6 @@
 // run-pass
 
+#![feature(let_chains)]
 #![allow(irrefutable_let_patterns)]
 
 fn main() {

src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs

@@ -17,6 +17,8 @@
 //
 // To that end, we check some positions which is not part of the language above.
 
+#![feature(let_chains)] // Avoid inflating `.stderr` with overzealous gates in this test.
+
 #![allow(irrefutable_let_patterns)]
 
 use std::ops::Range;
@@ -102,12 +104,6 @@ fn _macros() {
     //~^ ERROR `let` expressions are not supported here
     //~| ERROR `let` expressions are not supported here
     //~| ERROR expected expression, found `let` statement
-    use_expr!(true && let 0 = 1);
-    //~^ ERROR expected expression, found `let` statement
-
-    macro_rules! noop_expr { ($e:expr) => {}; }
-    noop_expr!((let 0 = 1));
-    //~^ ERROR expected expression, found `let` statement
 }
 
 fn nested_within_if_expr() {
@@ -481,7 +477,4 @@ fn with_parenthesis() {
         ([1, 2, 3][let _ = ()])
         //~^ ERROR expected expression, found `let` statement
     }
-
-    #[cfg(FALSE)] (let 0 = 1);
-    //~^ ERROR expected expression, found `let` statement
 }

src/test/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.rs

@@ -1,4 +1,4 @@
-#![feature(let_else)]
+#![feature(let_chains, let_else)]
 
 fn main() {
     let opt = Some(1i32);

src/test/ui/rfc-2497-if-let-chains/feature-gate.rs

@@ -0,0 +1,62 @@
+// 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 true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+
+    if let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+
+    if let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+
+    if let 1 = 1 && let true = { true } && false {
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+    //~| ERROR `let` expressions in this position are unstable [E0658]
+    }
+}
+
+fn _while() {
+    while let 0 = 1 {} // Stable!
+
+    while true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+
+    while let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+
+    while let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+}
+
+fn _macros() {
+    macro_rules! noop_expr { ($e:expr) => {}; }
+
+    noop_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+    //~| ERROR expected expression, found `let` statement
+
+    macro_rules! use_expr {
+        ($e:expr) => {
+            if $e {}
+            while $e {}
+        }
+    }
+    #[cfg(FALSE)] (let 0 = 1);
+    //~^ ERROR `let` expressions in this position are unstable [E0658]
+    //~| ERROR expected expression, found `let` statement
+    use_expr!(let 0 = 1);
+    //~^ ERROR no rules expected the token `let`
+}
+
+fn main() {}

src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr

@@ -0,0 +1,114 @@
+error: expected expression, found `let` statement
+  --> $DIR/feature-gate.rs:55:20
+   |
+LL |     #[cfg(FALSE)] (let 0 = 1);
+   |                    ^^^
+
+error: expected expression, found `let` statement
+  --> $DIR/feature-gate.rs:45:17
+   |
+LL |     noop_expr!((let 0 = 1));
+   |                 ^^^
+
+error: no rules expected the token `let`
+  --> $DIR/feature-gate.rs:58:15
+   |
+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 unstable
+  --> $DIR/feature-gate.rs:14:16
+   |
+LL |     if true && let 0 = 1 {}
+   |                ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:17:8
+   |
+LL |     if let 0 = 1 && true {}
+   |        ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:20:8
+   |
+LL |     if let Range { start: _, end: _ } = (true..true) && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:23:8
+   |
+LL |     if let 1 = 1 && let true = { true } && false {
+   |        ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:23:21
+   |
+LL |     if let 1 = 1 && let true = { true } && false {
+   |                     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:32:19
+   |
+LL |     while true && let 0 = 1 {}
+   |                   ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:35:11
+   |
+LL |     while let 0 = 1 && true {}
+   |           ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:38:11
+   |
+LL |     while let Range { start: _, end: _ } = (true..true) && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:55:20
+   |
+LL |     #[cfg(FALSE)] (let 0 = 1);
+   |                    ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/feature-gate.rs:45:17
+   |
+LL |     noop_expr!((let 0 = 1));
+   |                 ^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error: aborting due to 13 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2497-if-let-chains/invalid-let-in-a-valid-let-context.rs

@@ -1,3 +1,5 @@
+#![feature(let_chains)]
+
 fn main() {
     let _opt = Some(1i32);
 

src/test/ui/rfc-2497-if-let-chains/invalid-let-in-a-valid-let-context.stderr

@@ -1,35 +1,35 @@
 error: expected expression, found `let` statement
-  --> $DIR/invalid-let-in-a-valid-let-context.rs:6:19
+  --> $DIR/invalid-let-in-a-valid-let-context.rs:8:19
    |
 LL |         let _ = &&let Some(x) = Some(42);
    |                   ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/invalid-let-in-a-valid-let-context.rs:11:47
+  --> $DIR/invalid-let-in-a-valid-let-context.rs:13:47
    |
 LL |         if let Some(elem) = _opt && [1, 2, 3][let _ = &&let Some(x) = Some(42)] = 1 {
    |                                               ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/invalid-let-in-a-valid-let-context.rs:11:57
+  --> $DIR/invalid-let-in-a-valid-let-context.rs:13:57
    |
 LL |         if let Some(elem) = _opt && [1, 2, 3][let _ = &&let Some(x) = Some(42)] = 1 {
    |                                                         ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/invalid-let-in-a-valid-let-context.rs:21:23
+  --> $DIR/invalid-let-in-a-valid-let-context.rs:23:23
    |
 LL |             [1, 2, 3][let _ = ()];
    |                       ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/invalid-let-in-a-valid-let-context.rs:30:47
+  --> $DIR/invalid-let-in-a-valid-let-context.rs:32:47
    |
 LL |         if let Some(elem) = _opt && [1, 2, 3][let _ = ()] = 1 {
    |                                               ^^^
 
 error: expected expression, found `let` statement
-  --> $DIR/invalid-let-in-a-valid-let-context.rs:38:21
+  --> $DIR/invalid-let-in-a-valid-let-context.rs:40:21
    |
 LL |             let x = let y = 1;
    |                     ^^^

src/test/ui/rfc-2497-if-let-chains/irrefutable-lets.rs

@@ -1,7 +1,7 @@
 // revisions: allowed disallowed
 //[allowed] check-pass
 
-#![feature(if_let_guard)]
+#![feature(if_let_guard, let_chains)]
 #![cfg_attr(allowed, allow(irrefutable_let_patterns))]
 #![cfg_attr(disallowed, deny(irrefutable_let_patterns))]
 

src/test/ui/rfc-2497-if-let-chains/issue-90722.rs

@@ -1,5 +1,7 @@
 // check-pass
 
+#![feature(let_chains)]
+
 fn main() {
     let x = Some(vec!["test"]);
 

src/test/ui/rfc-2497-if-let-chains/issue-92145.rs

@@ -1,5 +1,7 @@
 // check-pass
 
+#![feature(let_chains)]
+
 fn main() {
     let opt = Some("foo bar");
 

src/test/ui/rfc-2497-if-let-chains/issue-93150.rs

@@ -2,6 +2,7 @@ fn main() {
     match true {
         _ if let true = true && true => {}
         //~^ ERROR `if let` guards are
+        //~| ERROR `let` expressions in this
         _ => {}
     }
 }

src/test/ui/rfc-2497-if-let-chains/issue-93150.stderr

@@ -8,6 +8,15 @@ LL |         _ if let true = true && true => {}
    = help: add `#![feature(if_let_guard)]` to the crate attributes to enable
    = help: you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`
 
-error: aborting due to previous error
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/issue-93150.rs:3:14
+   |
+LL |         _ if let true = true && true => {}
+   |              ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2497-if-let-chains/then-else-blocks.rs

@@ -1,6 +1,6 @@
 // run-pass
 
-#![feature(if_let_guard)]
+#![feature(if_let_guard, let_chains)]
 
 fn check_if_let(opt: Option<Option<Option<i32>>>, value: i32) -> bool {
     if let Some(first) = opt

src/tools/clippy/clippy_dev/src/lib.rs

@@ -1,3 +1,4 @@
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(once_cell)]
 #![feature(rustc_private)]

src/tools/clippy/clippy_lints/src/lib.rs

@@ -4,6 +4,7 @@
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]
 #![feature(iter_intersperse)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(lint_reasons)]
 #![feature(never_type)]

src/tools/clippy/clippy_utils/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(let_else)]
+#![feature(let_chains)]
 #![feature(lint_reasons)]
 #![feature(once_cell)]
 #![feature(rustc_private)]

src/tools/clippy/tests/ui/needless_late_init.fixed

@@ -1,4 +1,5 @@
 // run-rustfix
+#![feature(let_chains)]
 #![allow(
     unused,
     clippy::assign_op_pattern,

src/tools/clippy/tests/ui/needless_late_init.rs

@@ -1,4 +1,5 @@
 // run-rustfix
+#![feature(let_chains)]
 #![allow(
     unused,
     clippy::assign_op_pattern,

src/tools/clippy/tests/ui/needless_late_init.stderr

@@ -1,5 +1,5 @@
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:22:5
+  --> $DIR/needless_late_init.rs:23:5
    |
 LL |     let a;
    |     ^^^^^^ created here
@@ -13,7 +13,7 @@ LL |     let a = "zero";
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:25:5
+  --> $DIR/needless_late_init.rs:26:5
    |
 LL |     let b;
    |     ^^^^^^ created here
@@ -27,7 +27,7 @@ LL |     let b = 1;
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:26:5
+  --> $DIR/needless_late_init.rs:27:5
    |
 LL |     let c;
    |     ^^^^^^ created here
@@ -41,7 +41,7 @@ LL |     let c = 2;
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:30:5
+  --> $DIR/needless_late_init.rs:31:5
    |
 LL |     let d: usize;
    |     ^^^^^^^^^^^^^ created here
@@ -54,7 +54,7 @@ LL |     let d: usize = 1;
    |     ~~~~~~~~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:33:5
+  --> $DIR/needless_late_init.rs:34:5
    |
 LL |     let e;
    |     ^^^^^^ created here
@@ -67,7 +67,7 @@ LL |     let e = format!("{}", d);
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:38:5
+  --> $DIR/needless_late_init.rs:39:5
    |
 LL |     let a;
    |     ^^^^^^
@@ -88,7 +88,7 @@ LL |     };
    |      +
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:47:5
+  --> $DIR/needless_late_init.rs:48:5
    |
 LL |     let b;
    |     ^^^^^^
@@ -109,7 +109,7 @@ LL |     };
    |      +
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:54:5
+  --> $DIR/needless_late_init.rs:55:5
    |
 LL |     let d;
    |     ^^^^^^
@@ -130,7 +130,7 @@ LL |     };
    |      +
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:62:5
+  --> $DIR/needless_late_init.rs:63:5
    |
 LL |     let e;
    |     ^^^^^^
@@ -151,7 +151,7 @@ LL |     };
    |      +
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:69:5
+  --> $DIR/needless_late_init.rs:70:5
    |
 LL |     let f;
    |     ^^^^^^
@@ -167,7 +167,7 @@ LL +         1 => "three",
    |
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:75:5
+  --> $DIR/needless_late_init.rs:76:5
    |
 LL |     let g: usize;
    |     ^^^^^^^^^^^^^
@@ -187,7 +187,7 @@ LL |     };
    |      +
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:83:5
+  --> $DIR/needless_late_init.rs:84:5
    |
 LL |     let x;
    |     ^^^^^^ created here
@@ -201,7 +201,7 @@ LL |     let x = 1;
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:87:5
+  --> $DIR/needless_late_init.rs:88:5
    |
 LL |     let x;
    |     ^^^^^^ created here
@@ -215,7 +215,7 @@ LL |     let x = SignificantDrop;
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:91:5
+  --> $DIR/needless_late_init.rs:92:5
    |
 LL |     let x;
    |     ^^^^^^ created here
@@ -229,7 +229,7 @@ LL |     let x = SignificantDrop;
    |     ~~~~~
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:110:5
+  --> $DIR/needless_late_init.rs:111:5
    |
 LL |     let a;
    |     ^^^^^^
@@ -250,7 +250,7 @@ LL |     };
    |      +
 
 error: unneeded late initialization
-  --> $DIR/needless_late_init.rs:127:5
+  --> $DIR/needless_late_init.rs:128:5
    |
 LL |     let a;
    |     ^^^^^^