Only enable the let_else feature on bootstrap

On later stages, the feature is already stable.

Result of running:

rg -l "feature.let_else" compiler/ src/librustdoc/ library/ | xargs sed -s -i "s#\\[feature.let_else#\\[cfg_attr\\(bootstrap, feature\\(let_else\\)#"

compiler/rustc_ast/src/lib.rs

@@ -15,7 +15,7 @@
 #![feature(if_let_guard)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(slice_internals)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -32,7 +32,7 @@
 
 #![feature(box_patterns)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_ast_passes/src/lib.rs

@@ -9,7 +9,7 @@
 #![feature(if_let_guard)]
 #![feature(iter_is_partitioned)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_attr/src/lib.rs

@@ -5,7 +5,7 @@
 //! to this crate.
 
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 

compiler/rustc_borrowck/src/lib.rs

@@ -3,7 +3,7 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

compiler/rustc_builtin_macros/src/lib.rs

@@ -9,7 +9,7 @@
 #![feature(if_let_guard)]
 #![feature(is_sorted)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
 #![recursion_limit = "256"]

compiler/rustc_codegen_llvm/src/lib.rs

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

compiler/rustc_codegen_ssa/src/lib.rs

@@ -1,7 +1,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(box_patterns)]
 #![feature(try_blocks)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(once_cell)]
 #![feature(associated_type_bounds)]
 #![feature(strict_provenance)]

compiler/rustc_const_eval/src/lib.rs

@@ -10,7 +10,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(decl_macro)]
 #![feature(exact_size_is_empty)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(slice_ptr_get)]

compiler/rustc_data_structures/src/lib.rs

@@ -13,7 +13,7 @@
 #![feature(cell_leak)]
 #![feature(control_flow_enum)]
 #![feature(extend_one)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(hash_raw_entry)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(maybe_uninit_uninit_array)]

compiler/rustc_driver/src/lib.rs

@@ -5,7 +5,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(once_cell)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_errors/src/lib.rs

@@ -7,7 +7,7 @@
 #![feature(if_let_guard)]
 #![feature(adt_const_params)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![feature(result_option_inspect)]
 #![feature(rustc_attrs)]

compiler/rustc_expand/src/lib.rs

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

compiler/rustc_hir/src/lib.rs

@@ -5,7 +5,7 @@
 #![feature(associated_type_defaults)]
 #![feature(closure_track_caller)]
 #![feature(const_btree_new)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(once_cell)]
 #![feature(min_specialization)]
 #![feature(never_type)]

compiler/rustc_incremental/src/lib.rs

@@ -2,7 +2,7 @@
 
 #![deny(missing_docs)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 

compiler/rustc_index/src/lib.rs

@@ -3,7 +3,7 @@
 #![feature(allow_internal_unstable)]
 #![feature(bench_black_box)]
 #![feature(extend_one)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(new_uninit)]
 #![feature(step_trait)]

compiler/rustc_infer/src/lib.rs

@@ -19,7 +19,7 @@
 #![feature(extend_one)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(try_blocks)]

compiler/rustc_interface/src/lib.rs

@@ -1,5 +1,5 @@
 #![feature(box_patterns)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(internal_output_capture)]
 #![feature(thread_spawn_unchecked)]
 #![feature(once_cell)]

compiler/rustc_lint/src/lib.rs

@@ -34,7 +34,7 @@
 #![feature(iter_intersperse)]
 #![feature(iter_order_by)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![recursion_limit = "256"]

compiler/rustc_macros/src/lib.rs

@@ -1,5 +1,5 @@
 #![feature(allow_internal_unstable)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![feature(proc_macro_diagnostic)]
 #![feature(proc_macro_span)]

compiler/rustc_metadata/src/lib.rs

@@ -5,7 +5,7 @@
 #![cfg_attr(bootstrap, feature(generic_associated_types))]
 #![feature(iter_from_generator)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(once_cell)]
 #![feature(proc_macro_internals)]
 #![feature(macro_metavar_expr)]

compiler/rustc_middle/src/lib.rs

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

compiler/rustc_mir_build/src/lib.rs

@@ -6,7 +6,7 @@
 #![feature(control_flow_enum)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(once_cell)]
 #![recursion_limit = "256"]

compiler/rustc_mir_dataflow/src/lib.rs

@@ -1,7 +1,7 @@
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(exact_size_is_empty)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(once_cell)]
 #![feature(stmt_expr_attributes)]

compiler/rustc_mir_transform/src/lib.rs

@@ -1,7 +1,7 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(never_type)]

compiler/rustc_monomorphize/src/lib.rs

@@ -1,6 +1,6 @@
 #![feature(array_windows)]
 #![feature(control_flow_enum)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]

compiler/rustc_parse/src/lib.rs

@@ -4,7 +4,7 @@
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![feature(rustc_attrs)]
 #![recursion_limit = "256"]

compiler/rustc_passes/src/lib.rs

@@ -8,7 +8,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(try_blocks)]

compiler/rustc_privacy/src/lib.rs

@@ -1,7 +1,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(associated_type_defaults)]
 #![feature(control_flow_enum)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(rustc_private)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]

compiler/rustc_query_system/src/lib.rs

@@ -1,7 +1,7 @@
 #![feature(assert_matches)]
 #![feature(core_intrinsics)]
 #![feature(hash_raw_entry)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(extern_types)]
 #![allow(rustc::potential_query_instability)]

compiler/rustc_resolve/src/lib.rs

@@ -12,7 +12,7 @@
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![recursion_limit = "256"]
 #![allow(rustdoc::private_intra_doc_links)]

compiler/rustc_save_analysis/src/lib.rs

@@ -1,6 +1,6 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(if_let_guard)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 #![feature(never_type)]

compiler/rustc_serialize/src/lib.rs

@@ -14,7 +14,7 @@ Core encoding and decoding interfaces.
 #![feature(min_specialization)]
 #![feature(core_intrinsics)]
 #![feature(maybe_uninit_slice)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(new_uninit)]
 #![feature(allocator_api)]
 #![cfg_attr(test, feature(test))]

compiler/rustc_session/src/lib.rs

@@ -1,6 +1,6 @@
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(once_cell)]

compiler/rustc_span/src/lib.rs

@@ -15,7 +15,7 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(array_windows)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(if_let_guard)]
 #![feature(negative_impls)]
 #![feature(min_specialization)]

compiler/rustc_target/src/lib.rs

@@ -11,7 +11,7 @@
 #![feature(assert_matches)]
 #![feature(associated_type_bounds)]
 #![feature(exhaustive_patterns)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

compiler/rustc_trait_selection/src/lib.rs

@@ -18,7 +18,7 @@
 #![feature(hash_drain_filter)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(if_let_guard)]
 #![feature(never_type)]
 #![feature(type_alias_impl_trait)]

compiler/rustc_traits/src/lib.rs

@@ -3,7 +3,7 @@
 
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_ty_utils/src/lib.rs

@@ -6,7 +6,7 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(control_flow_enum)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(never_type)]
 #![feature(box_patterns)]
 #![recursion_limit = "256"]

compiler/rustc_typeck/src/lib.rs

@@ -66,7 +66,7 @@ This API is completely unstable and subject to change.
 #![feature(iter_intersperse)]
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(never_type)]
 #![feature(once_cell)]

library/alloc/src/lib.rs

@@ -169,7 +169,7 @@
 #![cfg_attr(not(test), feature(generator_trait))]
 #![feature(hashmap_internals)]
 #![feature(lang_items)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]

library/std/src/lib.rs

@@ -255,7 +255,7 @@
 #![cfg_attr(bootstrap, feature(label_break_value))]
 #![feature(lang_items)]
 #![feature(let_chains)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(linkage)]
 #![feature(link_cfg)]
 #![feature(min_specialization)]

src/librustdoc/lib.rs

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