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| Original file line number | Diff line number | Diff line change |
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| # JSON Output | ||
|
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||
| 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. | ||
|
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| JSON messages are emitted one per line to stderr. | ||
|
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| 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. | ||
|
|
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| ## Diagnostics | ||
|
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| Diagnostic messages provide errors or possible concerns generated during | ||
| compilation. `rustc` provides detailed information about where the diagnostic | ||
| originates, along with hints and suggestions. | ||
|
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||
| 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. | ||
|
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| Diagnostics have the following format: | ||
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||
| ```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. | ||
| For spans located within a macro expansion, this will be the | ||
| name of the expanded macro in the format "<MACRONAME macros>". | ||
| */ | ||
| "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": "consider prefixing 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: consider prefixing with an underscore: `_x`\n |\n = note: `#[warn(unused_variables)]` on by default\n\n" | ||
| } | ||
| ``` | ||
|
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||
| ## 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. | ||
| - "save-analysis": A JSON file emitted by the `-Zsave-analysis` feature. | ||
| */ | ||
| "emit": "link" | ||
| } | ||
| ``` | ||
|
|
||
| [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 |