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

Revert let_chains stabilization

This reverts commit 3266460.

It was discovered in #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)]