Add let_else feature gate

compiler/rustc_ast_lowering/src/block.rs

@@ -1,6 +1,7 @@
 use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
 use rustc_ast::{AttrVec, Block, BlockCheckMode, Expr, Local, LocalKind, Stmt, StmtKind};
 use rustc_hir as hir;
+use rustc_session::parse::feature_err;
 use rustc_span::symbol::Ident;
 use rustc_span::{sym, DesugaringKind};
 
@@ -170,6 +171,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             span,
             kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
         });
+        if !self.sess.features_untracked().let_else {
+            feature_err(
+                &self.sess.parse_sess,
+                sym::let_else,
+                local.span,
+                "`let...else` statements are unstable",
+            )
+            .emit();
+        }
         (stmt, if_expr)
     }
 }

compiler/rustc_feature/src/active.rs

@@ -676,6 +676,9 @@ declare_features! (
     /// Allows additional const parameter types, such as `&'static str` or user defined types
     (incomplete, adt_const_params, "1.56.0", Some(44580), None),
 
+    /// Allows `let...else` statements.
+    (active, let_else, "1.56.0", Some(87335), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_span/src/symbol.rs

@@ -744,6 +744,7 @@ symbols! {
         le,
         len,
         let_chains,
+        let_else,
         lhs,
         lib,
         libc,

src/test/ui/feature-gates/feature-gate-let_else.rs

@@ -0,0 +1,5 @@
+fn main() {
+    let Some(x) = Some(1) else { //~ ERROR `let...else` statements are unstable
+        return;
+    };
+}

src/test/ui/feature-gates/feature-gate-let_else.stderr

@@ -0,0 +1,14 @@
+error[E0658]: `let...else` statements are unstable
+  --> $DIR/feature-gate-let_else.rs:2:5
+   |
+LL | /     let Some(x) = Some(1) else {
+LL | |         return;
+LL | |     };
+   | |______^
+   |
+   = note: see issue #87335 <https://github.com/rust-lang/rust/issues/87335> for more information
+   = help: add `#![feature(let_else)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.