Advanced Debugging Techniques: CI Artifacts, Internal Attributes, and Graphviz Output

on *why* it was changed. See [this tutorial][bisect-tutorial] on how to use it. ## Downloading Artifacts from Rust's CI The [rustup-toolchain-install-master][rtim] tool by kennytm can be used to download the artifacts produced by Rust's CI for a specific SHA1 -- this basically corresponds to the successful landing of some PR -- and then sets them up for your local use. This also works for artifacts produced by `@bors try`. This is helpful when you want to examine the resulting build of a PR without doing the build yourself. ## `#[rustc_*]` TEST attributes The compiler defines a whole lot of internal (perma-unstable) attributes some of which are useful for debugging by dumping extra compiler-internal information. These are prefixed with `rustc_` and are gated behind the internal feature `rustc_attrs` (enabled via e.g. `#![feature(rustc_attrs)]`). For a complete and up to date list, see [`builtin_attrs`]. More specifically, the ones marked `TEST`. Here are some notable ones: | Attribute | Description | |----------------|-------------| | `rustc_def_path` | Dumps the [`def_path_str`] of an item. | | `rustc_dump_def_parents` | Dumps the chain of `DefId` parents of certain definitions. | | `rustc_dump_item_bounds` | Dumps the [`item_bounds`] of an item. | | `rustc_dump_predicates` | Dumps the [`predicates_of`] an item. | | `rustc_dump_vtable` | Dumps the vtable layout of an impl, or a type alias of a dyn type. | | `rustc_hidden_type_of_opaques` | Dumps the [hidden type of each opaque types][opaq] in the crate. | | `rustc_layout` | [See this section](#debugging-type-layouts). | | `rustc_object_lifetime_default` | Dumps the [object lifetime defaults] of an item. | | `rustc_outlives` | Dumps implied bounds of an item. More precisely, the [`inferred_outlives_of`] an item. | | `rustc_regions` | Dumps NLL closure region requirements. | | `rustc_symbol_name` | Dumps the mangled & demangled [`symbol_name`] of an item. | | `rustc_variances` | Dumps the [variances] of an item. | Right below you can find elaborate explainers on a selected few. ### Formatting Graphviz output (.dot files) Some compiler options for debugging specific features yield graphviz graphs - e.g. the `#[rustc_mir(borrowck_graphviz_postflow="suffix.dot")]` attribute on a function dumps various borrow-checker dataflow graphs in conjunction with `-Zdump-mir-dataflow`. These all produce `.dot` files. To view these files, install graphviz (e.g.

This section details more advanced debugging tools for the Rust compiler. It explains how to download artifacts from Rust's CI using `rustup-toolchain-install-master`. It then introduces `#[rustc_*]` TEST attributes (gated behind the `rustc_attrs` feature) for dumping compiler-internal information, listing several notable ones like `rustc_def_path`, `rustc_dump_item_bounds`, and `rustc_layout`. The section concludes by explaining how to format and view Graphviz output (`.dot` files) generated by some compiler debugging options using the graphviz tool.