Revert let chains stabilization (feature(let_chains) again)

Undo the language stabilization of let chains from rust-lang#132833 / 1.88.0 by:
- moving `let_chains` from accepted features back to unstable features
- restoring the ast feature gate for `let_chains`
- re-gating chained `let` expressions in conditions behind the feature

Plain `if let` / `while let` remains stable; only chains such as
`if a && let Some(x) = y` require `#![feature(let_chains)]`.

This reintroduces the pre-stabilization language surface for let chains
(tracking issue rust-lang#53667).

Author: Icelain

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -492,6 +492,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(global_registration, "global registration is experimental");
     gate_all!(guard_patterns, "guard patterns are experimental", "consider using match arm guards");
     gate_all!(impl_restriction, "`impl` restrictions are experimental");
+    gate_all!(let_chains, "`let` expressions in this position are unstable");
     gate_all!(min_generic_const_args, "unbraced const blocks as const args are experimental");
     gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
     gate_all!(move_expr, "`move(expr)` syntax is experimental");

compiler/rustc_feature/src/accepted.rs

@@ -275,8 +275,6 @@ declare_features! (
     (accepted, keylocker_x86, "1.89.0", Some(134813)),
     /// Allows `'a: { break 'a; }`.
     (accepted, label_break_value, "1.65.0", Some(48594)),
-    /// Allows `if/while p && let q = r && ...` chains.
-    (accepted, let_chains, "1.88.0", Some(53667)),
     /// Allows `let...else` statements.
     (accepted, let_else, "1.65.0", Some(87335)),
     /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.

compiler/rustc_feature/src/unstable.rs

@@ -607,6 +607,8 @@ declare_features! (
     (unstable, large_assignments, "1.52.0", Some(83518)),
     /// Allow to have type alias types for inter-crate use.
     (incomplete, lazy_type_alias, "1.72.0", Some(112792)),
+    /// Allows `if/while p && let q = r && ...` chains.
+    (unstable, let_chains, "1.37.0", Some(53667)),
     /// Allows using `#[link(kind = "link-arg", name = "...")]`
     /// to pass custom arguments to the linker.
     (unstable, link_arg_attribute, "1.76.0", Some(99427)),

compiler/rustc_parse/src/parser/expr.rs

@@ -4353,15 +4353,9 @@ impl MutVisitor for CondChecker<'_> {
                     };
                     *recovered = Recovered::Yes(error);
                 } else if self.depth > 1 {
-                    // Top level `let` is always allowed; only gate chains
-                    match self.let_chains_policy {
-                        LetChainsPolicy::AlwaysAllowed => (),
-                        LetChainsPolicy::EditionDependent { current_edition } => {
-                            if !current_edition.at_least_rust_2024() || !span.at_least_rust_2024() {
-                                self.parser.dcx().emit_err(errors::LetChainPre2024 { span });
-                            }
-                        }
-                    }
+                    // Top-level `if let` / `while let` is stable; only chained `let`s are gated.
+                    // Re-introduces the `let_chains` feature gate (undoes #132833 stabilization).
+                    self.parser.psess.gated_spans.gate(sym::let_chains, span);
                 }
             }
             ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, _, _) => {