# 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. 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 { /* 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 non-existent 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. ```javascript { /* 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. */ "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 { /* 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": {...}, } ] } ``` [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