Add $message_type field to distinguish json diagnostic outputs

compiler/rustc_errors/src/json.rs

@@ -145,6 +145,25 @@ impl JsonEmitter {
     pub fn ignored_directories_in_source_blocks(self, value: Vec<String>) -> Self {
         Self { ignored_directories_in_source_blocks: value, ..self }
     }
+
+    fn emit(&mut self, val: EmitTyped<'_>) -> io::Result<()> {
+        if self.pretty {
+            serde_json::to_writer_pretty(&mut *self.dst, &val)?
+        } else {
+            serde_json::to_writer(&mut *self.dst, &val)?
+        };
+        self.dst.write_all(b"\n")?;
+        self.dst.flush()
+    }
+}
+
+#[derive(Serialize)]
+#[serde(tag = "$message_type", rename_all = "snake_case")]
+enum EmitTyped<'a> {
+    Diagnostic(Diagnostic),
+    Artifact(ArtifactNotification<'a>),
+    FutureIncompat(FutureIncompatReport<'a>),
+    UnusedExtern(UnusedExterns<'a, 'a, 'a>),
 }
 
 impl Translate for JsonEmitter {
@@ -160,47 +179,36 @@ impl Translate for JsonEmitter {
 impl Emitter for JsonEmitter {
     fn emit_diagnostic(&mut self, diag: &crate::Diagnostic) {
         let data = Diagnostic::from_errors_diagnostic(diag, self);
-        let result = if self.pretty {
-            writeln!(&mut self.dst, "{}", serde_json::to_string_pretty(&data).unwrap())
-        } else {
-            writeln!(&mut self.dst, "{}", serde_json::to_string(&data).unwrap())
-        }
-        .and_then(|_| self.dst.flush());
+        let result = self.emit(EmitTyped::Diagnostic(data));
         if let Err(e) = result {
             panic!("failed to print diagnostics: {e:?}");
         }
     }
 
     fn emit_artifact_notification(&mut self, path: &Path, artifact_type: &str) {
         let data = ArtifactNotification { artifact: path, emit: artifact_type };
-        let result = if self.pretty {
-            writeln!(&mut self.dst, "{}", serde_json::to_string_pretty(&data).unwrap())
-        } else {
-            writeln!(&mut self.dst, "{}", serde_json::to_string(&data).unwrap())
-        }
-        .and_then(|_| self.dst.flush());
+        let result = self.emit(EmitTyped::Artifact(data));
         if let Err(e) = result {
             panic!("failed to print notification: {e:?}");
         }
     }
 
     fn emit_future_breakage_report(&mut self, diags: Vec<crate::Diagnostic>) {
-        let data: Vec<FutureBreakageItem> = diags
+        let data: Vec<FutureBreakageItem<'_>> = diags
             .into_iter()
             .map(|mut diag| {
                 if diag.level == crate::Level::Allow {
                     diag.level = crate::Level::Warning(None);
                 }
-                FutureBreakageItem { diagnostic: Diagnostic::from_errors_diagnostic(&diag, self) }
+                FutureBreakageItem {
+                    diagnostic: EmitTyped::Diagnostic(Diagnostic::from_errors_diagnostic(
+                        &diag, self,
+                    )),
+                }
             })
             .collect();
         let report = FutureIncompatReport { future_incompat_report: data };
-        let result = if self.pretty {
-            writeln!(&mut self.dst, "{}", serde_json::to_string_pretty(&report).unwrap())
-        } else {
-            writeln!(&mut self.dst, "{}", serde_json::to_string(&report).unwrap())
-        }
-        .and_then(|_| self.dst.flush());
+        let result = self.emit(EmitTyped::FutureIncompat(report));
         if let Err(e) = result {
             panic!("failed to print future breakage report: {e:?}");
         }
@@ -209,12 +217,7 @@ impl Emitter for JsonEmitter {
     fn emit_unused_externs(&mut self, lint_level: rustc_lint_defs::Level, unused_externs: &[&str]) {
         let lint_level = lint_level.as_str();
         let data = UnusedExterns { lint_level, unused_extern_names: unused_externs };
-        let result = if self.pretty {
-            writeln!(&mut self.dst, "{}", serde_json::to_string_pretty(&data).unwrap())
-        } else {
-            writeln!(&mut self.dst, "{}", serde_json::to_string(&data).unwrap())
-        }
-        .and_then(|_| self.dst.flush());
+        let result = self.emit(EmitTyped::UnusedExtern(data));
         if let Err(e) = result {
             panic!("failed to print unused externs: {e:?}");
         }
@@ -313,13 +316,15 @@ struct ArtifactNotification<'a> {
 }
 
 #[derive(Serialize)]
-struct FutureBreakageItem {
-    diagnostic: Diagnostic,
+struct FutureBreakageItem<'a> {
+    // Always EmitTyped::Diagnostic, but we want to make sure it gets serialized
+    // with "$message_type".
+    diagnostic: EmitTyped<'a>,
 }
 
 #[derive(Serialize)]
-struct FutureIncompatReport {
-    future_incompat_report: Vec<FutureBreakageItem>,
+struct FutureIncompatReport<'a> {
+    future_incompat_report: Vec<FutureBreakageItem<'a>>,
 }
 
 // NOTE: Keep this in sync with the equivalent structs in rustdoc's

src/doc/rustc/src/json.md

@@ -11,9 +11,11 @@ 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.
+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
 
@@ -29,6 +31,8 @@ Diagnostics have the following format:
 
 ```javascript
 {
+    /* Type of this message */
+    "$message_type": "diagnostic",
     /* The primary message. */
     "message": "unused variable: `x`",
     /* The diagnostic code.
@@ -217,6 +221,8 @@ flag][option-emit] documentation.
 
 ```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:
@@ -239,6 +245,8 @@ information, even if the diagnostics have been suppressed (such as with an
 
 ```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.
     */

tests/ui/diagnostic-width/flag-json.stderr

@@ -1,4 +1,4 @@
-{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
+{"$message_type":"diagnostic","message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
 
 Erroneous code examples:
 
@@ -33,8 +33,8 @@ LL | ..._: () = 42;
    |       expected due to this
 
 "}
-{"message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error
+{"$message_type":"diagnostic","message":"aborting due to previous error","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to previous error
 
 "}
-{"message":"For more information about this error, try `rustc --explain E0308`.","code":null,"level":"failure-note","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0308`.
+{"$message_type":"diagnostic","message":"For more information about this error, try `rustc --explain E0308`.","code":null,"level":"failure-note","spans":[],"children":[],"rendered":"For more information about this error, try `rustc --explain E0308`.
 "}

tests/ui/json/json-bom-plus-crlf-multifile.stderr

@@ -1,4 +1,4 @@
-{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
+{"$message_type":"diagnostic","message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
 
 Erroneous code examples:
 
@@ -26,7 +26,7 @@ most common being when calling a function and passing an argument which has a
 different type than the matching type in the function declaration.
 "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":621,"byte_end":622,"line_start":17,"line_end":17,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1;  // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":"expected `String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":612,"byte_end":618,"line_start":17,"line_end":17,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String = 1;  // Error in the middle of line.","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":622,"byte_end":622,"line_start":17,"line_end":17,"column_start":23,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1;  // Error in the middle of line.","highlight_start":23,"highlight_end":23}],"label":null,"suggested_replacement":".to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:17:22: error[E0308]: mismatched types
 "}
-{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
+{"$message_type":"diagnostic","message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
 
 Erroneous code examples:
 
@@ -54,7 +54,7 @@ most common being when calling a function and passing an argument which has a
 different type than the matching type in the function declaration.
 "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":681,"byte_end":682,"line_start":19,"line_end":19,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1","highlight_start":22,"highlight_end":23}],"label":"expected `String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":672,"byte_end":678,"line_start":19,"line_end":19,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String = 1","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":682,"byte_end":682,"line_start":19,"line_end":19,"column_start":23,"column_end":23,"is_primary":true,"text":[{"text":"    let s : String = 1","highlight_start":23,"highlight_end":23}],"label":null,"suggested_replacement":".to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:19:22: error[E0308]: mismatched types
 "}
-{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
+{"$message_type":"diagnostic","message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
 
 Erroneous code examples:
 
@@ -82,7 +82,7 @@ most common being when calling a function and passing an argument which has a
 different type than the matching type in the function declaration.
 "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":745,"byte_end":746,"line_start":23,"line_end":23,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1;  // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":"expected `String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":735,"byte_end":741,"line_start":22,"line_end":22,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String =","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":746,"byte_end":746,"line_start":23,"line_end":23,"column_start":2,"column_end":2,"is_primary":true,"text":[{"text":"1;  // Error after the newline.","highlight_start":2,"highlight_end":2}],"label":null,"suggested_replacement":".to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:23:1: error[E0308]: mismatched types
 "}
-{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
+{"$message_type":"diagnostic","message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
 
 Erroneous code examples:
 
@@ -110,5 +110,5 @@ most common being when calling a function and passing an argument which has a
 different type than the matching type in the function declaration.
 "},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":801,"byte_end":809,"line_start":25,"line_end":26,"column_start":22,"column_end":6,"is_primary":true,"text":[{"text":"    let s : String = (","highlight_start":22,"highlight_end":23},{"text":"    );  // Error spanning the newline.","highlight_start":1,"highlight_end":6}],"label":"expected `String`, found `()`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":792,"byte_end":798,"line_start":25,"line_end":25,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":"    let s : String = (","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:25:22: error[E0308]: mismatched types
 "}
-{"message":"aborting due to 4 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 4 previous errors
+{"$message_type":"diagnostic","message":"aborting due to 4 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 4 previous errors
 "}