Description
When working on the UI test suite, it is common to hit Ctrl-S in the IDE and then quickly switch to a terminal to run the test. Since RA triggers a check-on-save, this means that the tests can only start running once the check-build is completed. However, since nothing in the compiler changed, this check-build should be completed very quickly.
However, some time recently this seems to have gotten significantly slower. I now usually have to wait for multiple seconds before a test build can start, which was not the case in the past. This is on a system with an NVMe SSD and a pretty good CPU (pretty good for a laptop, anyway) -- it should be plenty fast enough for this.
To reproduce, run ./x check compiler, wait until it is completed, and then run time ./x check compiler. The result is:
$ time ./x check compiler
Building bootstrap
Finished `dev` profile [unoptimized] target(s) in 0.27s
Checking stage0 library artifacts (x86_64-unknown-linux-gnu)
Finished `release` profile [optimized + debuginfo] target(s) in 0.14s
Checking stage0 compiler artifacts {rustc-main, rustc_abi, rustc_arena, rustc_ast, rustc_ast_ir, rustc_ast_lowering, rustc_ast_passes, rustc_ast_pretty, rustc_attr, rustc_baked_icu_data, rustc_borrowck, rustc_builtin_macros, rustc_codegen_llvm, rustc_codegen_ssa, rustc_const_eval, rustc_data_structures, rustc_driver, rustc_driver_impl, rustc_error_codes, rustc_error_messages, rustc_errors, rustc_expand, rustc_feature, rustc_fluent_macro, rustc_fs_util, rustc_graphviz, rustc_hir, rustc_hir_analysis, rustc_hir_pretty, rustc_hir_typeck, rustc_incremental, rustc_index, rustc_index_macros, rustc_infer, rustc_interface, rustc_lexer, rustc_lint, rustc_lint_defs, rustc_llvm, rustc_log, rustc_macros, rustc_metadata, rustc_middle, rustc_mir_build, rustc_mir_dataflow, rustc_mir_transform, rustc_monomorphize, rustc_next_trait_solver, rustc_parse, rustc_parse_format, rustc_passes, rustc_pattern_analysis, rustc_privacy, rustc_query_impl, rustc_query_system, rustc_resolve, rustc_sanitizers, rustc_serialize, rustc_session, rustc_smir, rustc_span, rustc_symbol_mangling, rustc_target, rustc_trait_selection, rustc_traits, rustc_transmute, rustc_ty_utils, rustc_type_ir, rustc_type_ir_macros, stable_mir} (x86_64-unknown-linux-gnu)
Finished `release` profile [optimized + debuginfo] target(s) in 1.70s
Build completed successfully in 0:00:05
real 0m5,650s
user 0m5,324s
sys 0m0,334s
As you can see, there are less than 2s spent inside cargo. And yet overall this took more than 5s. This means that bootstrap spent 3s doing... something? Whatever it did takes two times longer than cargo took to verify that the entire compiler sources remain unchanged.
Would be nice to find a way to speed that up. :)
Cc @rust-lang/bootstrap