rustc-dev-guide: update bootstrap tracing docs

src/doc/rustc-dev-guide/src/building/bootstrapping/debugging-bootstrap.md

@@ -1,6 +1,6 @@
 # Debugging bootstrap
 
-> FIXME: this page could be expanded 
+> FIXME: this section should be expanded
 
 ## `tracing` in bootstrap
 
@@ -10,21 +10,69 @@ Bootstrap has conditional [`tracing`][tracing] setup to provide structured loggi
 
 ### Enabling `tracing` output
 
-Bootstrap will conditionally enable `tracing` output if the `BOOTSTRAP_TRACING` env var is set.
+Bootstrap will conditionally build `tracing` support and enable `tracing` output if the `BOOTSTRAP_TRACING` env var is set.
 
-Example usage:
+#### Basic usage
+
+Example basic usage[^just-trace]:
+
+[^just-trace]: It is not recommend to use *just* `BOOTSTRAP_TRACING=TRACE` because that will dump *everything* at `TRACE` level, including logs intentionally gated behind custom targets as they are too verbose even for `TRACE` level by default.
 
 ```bash
-$ BOOTSTRAP_TRACING=TRACE ./x build library --stage 1
+$ BOOTSTRAP_TRACING=bootstrap=TRACE ./x build library --stage 1
 ```
 
-Example output[^experimental]:
+Example output[^unstable]:
 
-![Example bootstrap tracing output](./debugging-bootstrap/tracing-output-example.png)
+```
+$ BOOTSTRAP_TRACING=bootstrap=TRACE ./x check src/bootstrap/
+Building bootstrap
+   Compiling bootstrap v0.0.0 (/home/joe/repos/rust/src/bootstrap)
+    Finished `dev` profile [unoptimized] target(s) in 2.74s
+ DEBUG bootstrap parsing flags
+ bootstrap::core::config::flags::Flags::parse args=["check", "src/bootstrap/"]
+ DEBUG bootstrap parsing config based on flags
+ DEBUG bootstrap creating new build based on config
+ bootstrap::Build::build
+  TRACE bootstrap setting up job management
+  TRACE bootstrap downloading rustfmt early
+   bootstrap::handling hardcoded subcommands (Format, Suggest, Perf)
+    DEBUG bootstrap not a hardcoded subcommand; returning to normal handling, cmd=Check { all_targets: false }
+  DEBUG bootstrap handling subcommand normally
+   bootstrap::executing real run
+     bootstrap::(1) executing dry-run sanity-check
+     bootstrap::(2) executing actual run
+Checking stage0 library artifacts (x86_64-unknown-linux-gnu)
+    Finished `release` profile [optimized + debuginfo] target(s) in 0.04s
+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)
+    Finished `release` profile [optimized + debuginfo] target(s) in 0.23s
+Checking stage0 bootstrap artifacts (x86_64-unknown-linux-gnu)
+    Checking bootstrap v0.0.0 (/home/joe/repos/rust/src/bootstrap)
+    Finished `release` profile [optimized + debuginfo] target(s) in 0.64s
+  DEBUG bootstrap checking for postponed test failures from `test  --no-fail-fast`
+Build completed successfully in 0:00:08
+```
 
-[^experimental]: This shows what's *possible* with the infra in an experimental implementation.
+#### Controlling log output
 
-The env var `BOOTSTRAP_TRACING` accepts a [`tracing` env-filter][tracing-env-filter]. The `TRACE` filter will enable *all* `trace` level or less verbose level 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 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):
+
+```bash
+$ BOOTSTRAP_TRACING=CONFIG_HANDLING=TRACE ./x build library --stage 1
+```
+
+[^unstable]: This output is always subject to further changes.
 
 [tracing-env-filter]: https://docs.rs/tracing-subscriber/0.3.19/tracing_subscriber/filter/struct.EnvFilter.html
 
@@ -73,28 +121,6 @@ For `#[instrument]`, it's recommended to:
 - Explicitly pick an instrumentation name via `name = ".."` to distinguish between e.g. `run` of different steps.
 - Take care to not cause diverging behavior via tracing, e.g. building extra things only when tracing infra is enabled.
 
-### Enabling `tracing` bootstrap feature in rust-analyzer
+### rust-analyzer integration?
 
-You can adjust your `settings.json`'s `rust-analyzer.check.overrideCommand` and `rust-analyzer.cargo.buildScripts.overrideCommand` if you want to also enable `logging` cargo feature by default in your editor. This is mostly useful if you want proper r-a completions and such when working on bootstrap itself.
-
-```json
-"rust-analyzer.check.overrideCommand": [
-    "BOOTSTRAP_TRACING=1", // <- BOOTSTRAP_TRACING=1 won't enable tracing filter, but it will activate bootstrap's `tracing` feature
-    "python3",
-    "x.py",
-    "check",
-    "--json-output",
-    "--build-dir=build-rust-analyzer"
-],
-```
-
-```json
-"rust-analyzer.cargo.buildScripts.overrideCommand": [
-    "BOOTSTRAP_TRACING=1", // <- note this
-    "python3",
-    "x.py",
-    "check",
-    "--json-output",
-    "--build-dir=build-rust-analyzer"
-],
-```
+Unfortunately, because bootstrap is a `rust-analyzer.linkedProjects`, you can't ask r-a to check/build bootstrap itself with `tracing` feature enabled to get relevant completions, due to lack of support as described in <https://github.com/rust-lang/rust-analyzer/issues/8521>.