# JSON Output

This chapter documents the JSON structures emitted by `rustc`. JSON may be
enabled with the [`--error-format=json` flag][option-error-format]. Additional
options may be specified with the [`--json` flag][option-json] which can
change which messages are generated, and the format of the messages.

JSON messages are emitted one per line to stderr.

If parsing the output with Rust, the
[`cargo_metadata`](https://crates.io/crates/cargo_metadata) crate provides
some support for parsing the messages.

Each type of message has a `$message_type` field which can be used to
distinguish the different formats. When parsing, care should be taken
to be forwards-compatible with future changes to the format. Optional
values may be `null`. New fields may be added. Enumerated fields like
"level" or "suggestion_applicability" may add new values.

## Diagnostics

Diagnostic messages provide errors or possible concerns generated during
compilation. `rustc` provides detailed information about where the diagnostic
originates, along with hints and suggestions.

Diagnostics are arranged in a parent/child relationship where the parent
diagnostic value is the core of the diagnostic, and the attached children
provide additional context, help, and information.

Diagnostics have the following format:

```javascript
{
    /* Type of this message */
    "$message_type": "diagnostic",
    /* The primary message. */
    "message": "unused variable: `x`",
    /* The diagnostic code.
       Some messages may set this value to null.
    */
    "code": {
        /* A unique string identifying which diagnostic triggered. */
        "code": "unused_variables",
        /* An optional string explaining more detail about the diagnostic code. */
        "explanation": null
    },
    /* The severity of the diagnostic.
       Values may be:
       - "error": A fatal error that prevents compilation.
       - "warning": A possible error or concern.
       - "note": Additional information or context about the diagnostic.
       - "help": A suggestion on how to resolve the diagnostic.
       - "failure-note": A note attached to the message for further information.
       - "error: internal compiler error": Indicates a bug within the compiler.
    */
    "level": "warning",
    /* An array of source code locations to point out specific details about
       where the diagnostic originates from. This may be empty, for example
       for some global messages, or child messages attached to a parent.

       Character offsets are offsets of Unicode Scalar Values.
    */
    "spans": [
        {
            /* The file where the span is located.
               Note that this path may not exist. For example, if the path
               points to the standard library, and the rust src is not
               available in the sysroot, then it may point to a nonexistent
               file. Beware that this may also point to the source of an
               external crate.
            */
            "file_name": "lib.rs",
            /* The byte offset where the span starts (0-based, inclusive). */
            "byte_start": 21,
            /* The byte offset where the span ends (0-based, exclusive). */
            "byte_end": 22,
            /* The first line number of the span (1-based, inclusive). */
            "line_start": 2,
            /* The last line number of the span (1-based, inclusive). */
            "line_end": 2,
            /* The first character offset of the line_start (1-based, inclusive). */
            "column_start": 9,
            /* The last character offset of the line_end (1-based, exclusive). */
            "column_end": 10,
            /* Whether or not this is the "primary" span.

               This indicates that this span is the focal point of the
               diagnostic.

               There are rare cases where multiple spans may be marked as
               primary. For example, "immutable borrow occurs here" and
               "mutable borrow ends here" can be two separate primary spans.

               The top (parent) message should always have at least one
               primary span, unless it has zero spans. Child messages may have
               zero or more primary spans.
            */
            "is_primary": true,
            /* An array of objects showing the original source code for this
               span. This shows the entire lines of text where the span is
               located. A span across multiple lines will have a separate
               value for each line.
            */
            "text": [
                {
                    /* The entire line of the original source code. */
                    "text": "    let x = 123;",
                    /* The first character offset of the line of
                       where the span covers this line (1-based, inclusive). */
                    "highlight_start": 9,
                    /* The last character offset of the line of
                       where the span covers this line (1-based, exclusive). */
                    "highlight_end": 10
                }
            ],
            /* An optional message to display at this span location.
               This is typically null for primary spans.
            */
            "label": null,
            /* An optional string of a suggested replacement for this span to
               solve the issue. Tools may try to replace the contents of the
               span with this text.
            */
            "suggested_replacement": null,
            /* An optional string that indicates the confidence of the
               "suggested_replacement". Tools may use this value to determine
               whether or not suggestions should be automatically applied.

               Possible values may be:
               - "MachineApplicable": The suggestion is definitely what the
                 user intended. This suggestion should be automatically
                 applied.
               - "MaybeIncorrect": The suggestion may be what the user
                 intended, but it is uncertain. The suggestion should result
                 in valid Rust code if it is applied.
               - "HasPlaceholders": The suggestion contains placeholders like
                 `(...)`. The suggestion cannot be applied automatically
                 because it will not result in valid Rust code. The user will
                 need to fill in the placeholders.
               - "Unspecified": The applicability of the suggestion is unknown.
            */
            "suggestion_applicability": null,
            /* An optional object indicating the expansion of a macro within
               this span.

               If a message occurs within a macro invocation, this object will
               provide details of where within the macro expansion the message
               is located.
            */
            "expansion": {
                /* The span of the macro invocation.
                   Uses the same span definition as the "spans" array.
                */
                "span": {/*...*/}
                /* Name of the macro, such as "foo!" or "#[derive(Eq)]". */
                "macro_decl_name": "some_macro!",
                /* Optional span where the relevant part of the macro is
                  defined. */
                "def_site_span": {/*...*/},
            }
        }
    ],
    /* Array of attached diagnostic messages.
       This is an array of objects using the same format as the parent
       message. Children are not nested (children do not themselves
       contain "children" definitions).
    */
    "children": [
        {
            "message": "`#[warn(unused_variables)]` on by default",
            "code": null,
            "level": "note",
            "spans": [],
            "children": [],
            "rendered": null
        },
        {
            "message": "if this is intentional, prefix it with an underscore",
            "code": null,
            "level": "help",
            "spans": [
                {
                    "file_name": "lib.rs",
                    "byte_start": 21,
                    "byte_end": 22,
                    "line_start": 2,
                    "line_end": 2,
                    "column_start": 9,
                    "column_end": 10,
                    "is_primary": true,
                    "text": [
                        {
                            "text": "    let x = 123;",
                            "highlight_start": 9,
                            "highlight_end": 10
                        }
                    ],
                    "label": null,
                    "suggested_replacement": "_x",
                    "suggestion_applicability": "MachineApplicable",
                    "expansion": null
                }
            ],
            "children": [],
            "rendered": null
        }
    ],
    /* Optional string of the rendered version of the diagnostic as displayed
       by rustc. Note that this may be influenced by the `--json` flag.
    */
    "rendered": "warning: unused variable: `x`\n --> lib.rs:2:9\n  |\n2 |     let x = 123;\n  |         ^ help: if this is intentional, prefix it with an underscore: `_x`\n  |\n  = note: `#[warn(unused_variables)]` on by default\n\n"
}
```

## Artifact notifications

Artifact notifications are emitted when the [`--json=artifacts`
flag][option-json] is used. They indicate that a file artifact has been saved
to disk. More information about emit kinds may be found in the [`--emit`
flag][option-emit] documentation. Notifications can contain more than one file
for each type, for example when using multiple codegen units.

```javascript
{
    /* Type of this message */
    "$message_type": "artifact",
    /* The filename that was generated. */
    "artifact": "libfoo.rlib",
    /* The kind of artifact that was generated. Possible values:
       - "link": The generated crate as specified by the crate-type.
       - "dep-info": The `.d` file with dependency information in a Makefile-like syntax.
       - "metadata": The Rust `.rmeta` file containing metadata about the crate.
       - "asm": The `.s` file with generated assembly
       - "llvm-ir": The `.ll` file with generated textual LLVM IR
       - "llvm-bc": The `.bc` file with generated LLVM bitcode
       - "mir": The `.mir` file with rustc's mid-level intermediate representation.
       - "obj": The `.o` file with generated native object code
    */
    "emit": "link"
}
```

## Future-incompatible reports

If the [`--json=future-incompat`][option-json] flag is used, then a separate
JSON structure will be emitted if the crate may stop compiling in the future.
This contains diagnostic information about the particular warnings that may be
turned into a hard error in the future. This will include the diagnostic
information, even if the diagnostics have been suppressed (such as with an
`#[allow]` attribute or the `--cap-lints` option).

```javascript
{
    /* Type of this message */
    "$message_type": "future_incompat",
    /* An array of objects describing a warning that will become a hard error
       in the future.
    */
    "future_incompat_report":
    [
        {
            /* A diagnostic structure as defined in
               https://doc.rust-lang.org/rustc/json.html#diagnostics
            */
            "diagnostic": {...},
        }
    ]
}
```

## Unused Dependency Notifications

The options `--json=unused-externs` and `--json=unused-externs-silent` in
conjunction with the `unused-crate-dependencies` lint will emit JSON structures
reporting any crate dependencies (specified with `--extern`) which never had any
symbols referenced. These are intended to be consumed by the build system which
can then emit diagnostics telling the user to remove the unused dependencies
from `Cargo.toml` (or whatever build-system file defines dependencies).

The JSON structure is:
```json
{
    "lint_level": "deny", /* Level of the warning */
    "unused_names": [
        "foo"  /* Names of unused crates, as specified with --extern foo=libfoo.rlib */
    ],
}
```

The warn/deny/forbid lint level (as defined either on the command line or in the
source) dictates the `lint_level` in the JSON. With `unused-externs`, a
`deny` or `forbid` level diagnostic will also cause `rustc` to exit with a
failure exit code.

`unused-externs-silent` will report the diagnostic the same way, but will not
cause `rustc` to exit with failure - it's up to the consumer to flag failure
appropriately. (This is needed by Cargo which shares the same dependencies
across multiple build targets, so it should only report an unused dependency if
its not used by any of the targets.)

[option-emit]: command-line-arguments.md#option-emit
[option-error-format]: command-line-arguments.md#option-error-format
[option-json]: command-line-arguments.md#option-json
