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3rd chunk of `src/building/bootstrapping/debugging-bootstrap.md`
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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_data_structures, rustc_attr_parsing, 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)
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Checking stage0 bootstrap artifacts (x86_64-unknown-linux-gnu)
    Checking bootstrap v0.0.0 (/home/joe/repos/rust/src/bootstrap)
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  DEBUG bootstrap checking for postponed test failures from `test  --no-fail-fast`
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```

#### Controlling tracing output

The env var `BOOTSTRAP_TRACING` accepts a [`tracing` env-filter][tracing-env-filter].

There are two orthogonal ways to control which kind of tracing logs you want:

1. You can specify the log **level**, e.g. `DEBUG` or `TRACE`.
2. You can also control the log **target**, e.g. `bootstrap` or `bootstrap::core::config` vs custom targets like `CONFIG_HANDLING`.
    - Custom targets are used to limit what is output when `BOOTSTRAP_TRACING=bootstrap=TRACE` is used, as they can be too verbose even for `TRACE` level by default. Currently used custom targets:
        - `CONFIG_HANDLING`

The `TRACE` filter will enable *all* `trace` level or less verbose level tracing output.

You can of course combine them (custom target logs are typically gated behind `TRACE` log level additionally):
Title: Controlling Tracing Output with `BOOTSTRAP_TRACING`
Summary
This section explains how to control the output of the `tracing` feature in the Rust bootstrap process using the `BOOTSTRAP_TRACING` environment variable and `tracing` env-filter. It describes how to specify log levels such as `DEBUG` or `TRACE`, and how to target specific modules like `bootstrap` or `bootstrap::core::config`, as well as custom targets like `CONFIG_HANDLING`, to fine-tune the verbosity and scope of the logs.