Write summary messages to stderr when fixing via stdin (instead of omitting them) (#7838)

Previously we just omitted diagnostic summaries when using `--fix` or
`--diff` with a stdin file. Now, we still write the summaries to stderr
instead of the main writer (which is generally stdout but could be
changed by `--output-file`).

crates/ruff_cli/src/lib.rs

@@ -208,6 +208,7 @@ pub fn check(args: CheckCommand, log_level: LogLevel) -> Result<ExitStatus> {
         }
         _ => Box::new(BufWriter::new(io::stdout())),
     };
+    let stderr_writer = Box::new(BufWriter::new(io::stderr()));
 
     if cli.show_settings {
         commands::show_settings::show_settings(
@@ -392,15 +393,18 @@ pub fn check(args: CheckCommand, log_level: LogLevel) -> Result<ExitStatus> {
             )?
         };
 
-        // Always try to print violations (the printer itself may suppress output),
-        // unless we're writing fixes via stdin (in which case, the transformed
-        // source code goes to stdout).
-        if !(is_stdin && matches!(fix_mode, FixMode::Apply | FixMode::Diff)) {
-            if cli.statistics {
-                printer.write_statistics(&diagnostics, &mut writer)?;
-            } else {
-                printer.write_once(&diagnostics, &mut writer)?;
-            }
+        // Always try to print violations (though the printer itself may suppress output)
+        // If we're writing fixes via stdin, the transformed source code goes to the writer
+        // so send the summary to stderr instead
+        let mut summary_writer = if is_stdin && matches!(fix_mode, FixMode::Apply | FixMode::Diff) {
+            stderr_writer
+        } else {
+            writer
+        };
+        if cli.statistics {
+            printer.write_statistics(&diagnostics, &mut summary_writer)?;
+        } else {
+            printer.write_once(&diagnostics, &mut summary_writer)?;
         }
 
         if !cli.exit_zero {

crates/ruff_cli/tests/integration_test.rs

@@ -150,6 +150,7 @@ fn stdin_fix_py() {
     print(sys.version)
 
     ----- stderr -----
+    Found 1 error (1 fixed, 0 remaining).
     "###);
 }
 
@@ -317,6 +318,7 @@ fn stdin_fix_jupyter() {
      "nbformat_minor": 5
     }
     ----- stderr -----
+    Found 2 errors (2 fixed, 0 remaining).
     "###);
 }
 
@@ -336,6 +338,8 @@ fn stdin_fix_when_not_fixable_should_still_print_contents() {
          print(sys.version)
 
     ----- stderr -----
+    -:3:4: F634 If test is a tuple, which is always `True`
+    Found 2 errors (1 fixed, 1 remaining).
     "###);
 }
 
@@ -961,6 +965,9 @@ fn fix_applies_safe_fixes_by_default() {
     print('foo')
 
     ----- stderr -----
+    -:1:14: F601 Dictionary key literal `'a'` repeated
+    Found 2 errors (1 fixed, 1 remaining).
+    1 hidden fix can be enabled with the `--unsafe-fixes` option.
     "###);
 }
 
@@ -988,6 +995,7 @@ fn fix_applies_unsafe_fixes_with_opt_in() {
     print('foo')
 
     ----- stderr -----
+    Found 2 errors (2 fixed, 0 remaining).
     "###);
 }
 
@@ -1014,6 +1022,7 @@ fn fix_only_flag_applies_safe_fixes_by_default() {
     print('foo')
 
     ----- stderr -----
+    Fixed 1 error.
     "###);
 }
 
@@ -1041,6 +1050,7 @@ fn fix_only_flag_applies_unsafe_fixes_with_opt_in() {
     print('foo')
 
     ----- stderr -----
+    Fixed 2 errors.
     "###);
 }
 
@@ -1070,6 +1080,7 @@ fn diff_shows_safe_fixes_by_default() {
 
 
     ----- stderr -----
+    Would fix 1 error.
     "###
     );
 }
@@ -1102,6 +1113,7 @@ fn diff_shows_unsafe_fixes_with_opt_in() {
 
 
     ----- stderr -----
+    Would fix 2 errors.
     "###
     );
 }