Revert let_chains stabilization

This reverts commit 3266460.

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

Author: Noratrieb

Diff for: 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)]

Diff for: 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"]

Diff for: 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(

Diff for: 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",

Diff for: 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"]
 

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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.

Diff for: 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(..))]`.

Diff for: 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)]

Diff for: 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"]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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,
-                _ => false,
+                ExprKind::Let(..) => (true, true),
+                _ => (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);
                 }

Diff for: 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)]

Diff for: 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"]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

Diff for: 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)]

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

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

Diff for: 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
 }

Diff for: 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
@@ -40,21 +40,30 @@ 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
 ...
 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`.

Diff for: 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
 }