| `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.
`apt-get install graphviz`) and then run the following commands:

```bash
$ dot -T pdf maybe_init_suffix.dot > maybe_init_suffix.pdf
$ firefox maybe_init_suffix.pdf # Or your favorite pdf viewer
```

### Debugging type layouts

The internal attribute `#[rustc_layout]` can be used to dump the [`Layout`] of
the type it is attached to. For example:

```rust
#![feature(rustc_attrs)]

#[rustc_layout(debug)]
type T<'a> = &'a u32;
```

Will emit the following:

```text
error: layout_of(&'a u32) = Layout {
    fields: Primitive,
    variants: Single {
        index: 0,
    },
    abi: Scalar(
        Scalar {
            value: Pointer,
            valid_range: 1..=18446744073709551615,
        },
    ),
    largest_niche: Some(
        Niche {
            offset: Size {
                raw: 0,
            },
            scalar: Scalar {
                value: Pointer,
                valid_range: 1..=18446744073709551615,
            },
        },
    ),
    align: AbiAndPrefAlign {
        abi: Align {
            pow2: 3,
        },
        pref: Align {
            pow2: 3,
        },
    },
    size: Size {
        raw: 8,
    },
}
 --> src/lib.rs:4:1
  |
4 | type T<'a> = &'a u32;
  | ^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error
```



## Configuring CodeLLDB for debugging `rustc`

If you are using VSCode, and have edited your `bootstrap.toml` to request debugging
level 1 or 2 for the parts of the code you're interested in, then you should be
able to use the [CodeLLDB] extension in VSCode to debug it.

Here is a sample `launch.json` file, being used to run a stage 1 compiler direct
from the directory where it is built (does not have to be "installed"):

```javascript
// .vscode/launch.json
{
    "version": "0.2.0",
    "configurations": [
      {
        "type": "lldb",
        "request": "launch",
        "name": "Launch",
        "args": [],  // array of string command-line arguments to pass to compiler
        "program": "${workspaceFolder}/build/host/stage1/bin/rustc",
        "windows": {  // applicable if using windows
            "program": "${workspaceFolder}/build/host/stage1/bin/rustc.exe"
        },
        "cwd": "${workspaceFolder}",  // current working directory at program start
        "stopOnEntry": false,
        "sourceLanguages": ["rust"]
      }
    ]
  }
```