Auto merge of #100812 - Nilstrieb:revert-let-chains-nightly, r=Mark-Simulacrum

Revert let_chains stabilization

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

Bumps the stage0 compiler which already has it reverted.

Author: bors

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)]

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"]

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -119,7 +119,12 @@ 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() {
+            sess.emit_err(ForbiddenLet { span: expr.span, reason: forbidden_let_reason });
+        } else {
+            sess.emit_err(ForbiddenLetStable { span: expr.span });
+        }
     }
 
     fn check_gat_where(

compiler/rustc_ast_passes/src/errors.rs

@@ -30,6 +30,14 @@ impl AddSubdiagnostic for ForbiddenLetReason {
     }
 }
 
+#[derive(SessionDiagnostic)]
+#[diag(ast_passes::forbidden_let_stable)]
+#[note]
+pub struct ForbiddenLetStable {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(SessionDiagnostic)]
 #[diag(ast_passes::forbidden_assoc_constraint)]
 pub struct ForbiddenAssocConstraint {

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",

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"]
 

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)]

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)]

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)]

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)]