Auto merge of rust-lang#100538 - Nilstrieb:revert-let-chains-beta, r=compiler-errors

Revert let_chains stabilization

This reverts commit 3266460.

It was discovered in rust-lang#100513 that they are not implemented correctly, which does not make them ready for stabilization.

The merge in the let parsing had a few conflicts, cc `@compiler-errors` and `@c410-f3r` to make sure I did it correctly (alternatively I could also revert `@compiler-errors'` let diagnostic improvement PR as well if a simpler revert is desired).

r? `@Mark-Simulacrum`

Author: bors

compiler/rustc_ast/src/lib.rs

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

compiler/rustc_ast_lowering/src/lib.rs

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

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -119,23 +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) {
-        let err = "`let` expressions are not supported here";
-        let mut diag = self.session.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",
-                );
+        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();
         }
-        diag.emit();
     }
 
     fn check_gat_where(

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -766,6 +766,7 @@ 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

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

compiler/rustc_attr/src/lib.rs

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

compiler/rustc_borrowck/src/lib.rs

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

compiler/rustc_builtin_macros/src/lib.rs

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

compiler/rustc_codegen_llvm/src/lib.rs

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

compiler/rustc_const_eval/src/lib.rs

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

compiler/rustc_error_messages/src/lib.rs

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

compiler/rustc_errors/src/lib.rs

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

compiler/rustc_expand/src/lib.rs

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

compiler/rustc_feature/src/accepted.rs

@@ -186,8 +186,6 @@ declare_features! (
     /// Allows some increased flexibility in the name resolution rules,
     /// especially around globs and shadowing (RFC 1560).
     (accepted, item_like_imports, "1.15.0", Some(35120), 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

@@ -424,6 +424,8 @@ declare_features! (
     (active, label_break_value, "1.28.0", Some(48594), 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

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

compiler/rustc_lint/src/lib.rs

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

compiler/rustc_metadata/src/lib.rs

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

compiler/rustc_middle/src/lib.rs

@@ -40,7 +40,7 @@
 #![feature(extern_types)]
 #![feature(new_uninit)]
 #![feature(once_cell)]
-#![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(trusted_len)]

compiler/rustc_mir_build/src/lib.rs

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

compiler/rustc_mir_transform/src/lib.rs

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

compiler/rustc_parse/src/lib.rs

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

compiler/rustc_parse/src/parser/expr.rs

@@ -2340,7 +2340,14 @@ 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.
@@ -2370,6 +2377,7 @@ 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), attrs))
     }
 
@@ -2675,13 +2683,15 @@ 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()?;
@@ -2696,7 +2706,12 @@ 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

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

compiler/rustc_resolve/src/lib.rs

@@ -10,8 +10,8 @@
 #![feature(box_patterns)]
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
-#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(iter_intersperse)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]

compiler/rustc_session/src/lib.rs

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

compiler/rustc_trait_selection/src/lib.rs

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

compiler/rustc_typeck/src/lib.rs

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

library/std/src/lib.rs

@@ -243,7 +243,7 @@
 #![feature(intra_doc_pointers)]
 #![feature(label_break_value)]
 #![feature(lang_items)]
-#![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(linkage)]
 #![feature(min_specialization)]

src/doc/reference

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

src/librustdoc/lib.rs

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

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