Stabilize let_chains

Author: c410-f3r

Diff for: compiler/rustc_ast/src/lib.rs

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

Diff for: compiler/rustc_ast_lowering/src/lib.rs

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

Diff for: compiler/rustc_ast_passes/src/ast_validation.rs

@@ -119,33 +119,23 @@ 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 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",
-                    );
-                }
+        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",
+                );
             }
-            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(

Diff for: compiler/rustc_ast_passes/src/feature_gate.rs

@@ -738,7 +738,6 @@ 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

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

Diff for: 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.
 
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 
 #[macro_use]

Diff for: compiler/rustc_borrowck/src/lib.rs

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

Diff for: compiler/rustc_builtin_macros/src/lib.rs

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

Diff for: 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)]
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(extern_types)]
 #![feature(once_cell)]

Diff for: 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)]
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

Diff for: compiler/rustc_error_messages/src/lib.rs

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

Diff for: compiler/rustc_expand/src/lib.rs

@@ -3,7 +3,7 @@
 #![feature(associated_type_bounds)]
 #![feature(associated_type_defaults)]
 #![feature(if_let_guard)]
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(macro_metavar_expr)]
 #![feature(proc_macro_diagnostic)]

Diff for: compiler/rustc_feature/src/accepted.rs

@@ -186,6 +186,8 @@ 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.

Diff for: compiler/rustc_feature/src/active.rs

@@ -424,8 +424,6 @@ 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(..))]`.

Diff for: compiler/rustc_infer/src/lib.rs

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

Diff for: compiler/rustc_lint/src/lib.rs

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

Diff for: compiler/rustc_metadata/src/lib.rs

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

Diff for: compiler/rustc_middle/src/lib.rs

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

Diff for: compiler/rustc_mir_build/src/lib.rs

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

Diff for: compiler/rustc_mir_transform/src/lib.rs

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

Diff for: compiler/rustc_parse/src/lib.rs

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

Diff for: compiler/rustc_parse/src/parser/expr.rs

@@ -2341,16 +2341,9 @@ impl<'a> Parser<'a> {
 
     /// Parses the condition of a `if` or `while` expression.
     fn parse_cond_expr(&mut self) -> PResult<'a, P<Expr>> {
-        let cond = self.with_let_management(true, |local_self| {
+        self.with_let_management(true, |local_self| {
             local_self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, 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)
+        })
     }
 
     // Checks if `let` is in an invalid position like `let x = let y = 1;` or
@@ -2389,7 +2382,6 @@ 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))
     }
 
@@ -2695,15 +2687,11 @@ 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, bool) {
+        fn check_let_expr(expr: &Expr) -> bool {
             match expr.kind {
-                ExprKind::Binary(_, ref lhs, ref 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, true),
-                _ => (false, true),
+                ExprKind::Binary(_, ref lhs, ref rhs) => check_let_expr(lhs) || check_let_expr(rhs),
+                ExprKind::Let(..) => true,
+                _ => false,
             }
         }
         let attrs = self.parse_outer_attributes()?;
@@ -2718,12 +2706,8 @@ impl<'a> Parser<'a> {
             let guard = if this.eat_keyword(kw::If) {
                 let if_span = this.prev_token.span;
                 let cond = this.with_let_management(true, |local_this| local_this.parse_expr())?;
-                let (has_let_expr, does_not_have_bin_op) = check_let_expr(&cond);
+                let has_let_expr = 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

@@ -7,7 +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)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

Diff for: compiler/rustc_resolve/src/lib.rs

@@ -10,7 +10,7 @@
 #![feature(box_patterns)]
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]

Diff for: compiler/rustc_session/src/lib.rs

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

Diff for: compiler/rustc_trait_selection/src/lib.rs

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

Diff for: 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)]
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(never_type)]

Diff for: library/std/src/lib.rs

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

Diff for: src/librustdoc/lib.rs

@@ -9,7 +9,7 @@
 #![feature(control_flow_enum)]
 #![feature(box_syntax)]
 #![feature(drain_filter)]
-#![feature(let_chains)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(let_else)]
 #![feature(test)]
 #![feature(never_type)]

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

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

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

@@ -4,7 +4,6 @@
 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
 }