add json emit mode

README.md

@@ -220,6 +220,7 @@ needs to be specified in `rustfmt.toml`, e.g., with `edition = "2018"`.
   | stdout | writes output to stdout | No |
   | coverage | displays how much of the input file was processed | Yes |
   | checkstyle | emits in a checkstyle format | Yes |
+  | json | emits diffs in a json format | Yes |
 
 ## License
 

src/bin/main.rs

@@ -98,7 +98,7 @@ fn make_opts() -> Options {
     );
     let is_nightly = is_nightly();
     let emit_opts = if is_nightly {
-        "[files|stdout|coverage|checkstyle]"
+        "[files|stdout|coverage|checkstyle|json]"
     } else {
         "[files|stdout]"
     };
@@ -631,6 +631,7 @@ fn emit_mode_from_emit_str(emit_str: &str) -> Result<EmitMode, FailureError> {
         "stdout" => Ok(EmitMode::Stdout),
         "coverage" => Ok(EmitMode::Coverage),
         "checkstyle" => Ok(EmitMode::Checkstyle),
+        "json" => Ok(EmitMode::Json),
         _ => Err(format_err!("Invalid value for `--emit`")),
     }
 }

src/config/options.rs

@@ -119,6 +119,9 @@ pub enum EmitMode {
     Coverage,
     /// Unfancy stdout
     Checkstyle,
+    /// Writes the resulting diffs in a JSON format. Returns an empty array
+    /// `[]` if there were no diffs.
+    Json,
     /// Output the changed lines (for internal value only)
     ModifiedLines,
     /// Checks if a diff can be generated. If so, rustfmt outputs a diff and

src/emitter.rs

@@ -2,6 +2,7 @@ pub(crate) use self::checkstyle::*;
 pub(crate) use self::diff::*;
 pub(crate) use self::files::*;
 pub(crate) use self::files_with_backup::*;
+pub(crate) use self::json::*;
 pub(crate) use self::modified_lines::*;
 pub(crate) use self::stdout::*;
 use crate::FileName;
@@ -12,6 +13,7 @@ mod checkstyle;
 mod diff;
 mod files;
 mod files_with_backup;
+mod json;
 mod modified_lines;
 mod stdout;
 
@@ -28,7 +30,7 @@ pub(crate) struct EmitterResult {
 
 pub(crate) trait Emitter {
     fn emit_formatted_file(
-        &self,
+        &mut self,
         output: &mut dyn Write,
         formatted_file: FormattedFile<'_>,
     ) -> Result<EmitterResult, io::Error>;

src/emitter/checkstyle.rs

@@ -21,7 +21,7 @@ impl Emitter for CheckstyleEmitter {
     }
 
     fn emit_formatted_file(
-        &self,
+        &mut self,
         output: &mut dyn Write,
         FormattedFile {
             filename,

src/emitter/diff.rs

@@ -14,7 +14,7 @@ impl DiffEmitter {
 
 impl Emitter for DiffEmitter {
     fn emit_formatted_file(
-        &self,
+        &mut self,
         _output: &mut dyn Write,
         FormattedFile {
             filename,

src/emitter/files.rs

@@ -6,7 +6,7 @@ pub(crate) struct FilesEmitter;
 
 impl Emitter for FilesEmitter {
     fn emit_formatted_file(
-        &self,
+        &mut self,
         _output: &mut dyn Write,
         FormattedFile {
             filename,

src/emitter/files_with_backup.rs

@@ -6,7 +6,7 @@ pub(crate) struct FilesWithBackupEmitter;
 
 impl Emitter for FilesWithBackupEmitter {
     fn emit_formatted_file(
-        &self,
+        &mut self,
         _output: &mut dyn Write,
         FormattedFile {
             filename,