Show fixes by default (astral-sh#19919)

## Summary

This PR fixes astral-sh#7352 by exposing the `show_fix_diff` option used in our
snapshot tests in the CLI. As the issue suggests, we plan to make this
the default output format in the future, so this is added to the `full`
output format in preview for now.

This turned out to be pretty straightforward. I just used our existing
`Applicability` settings to determine whether or not to print the diff.

The snapshot differences are because we now set
`Applicability::DisplayOnly` for our snapshot tests. This
`Applicability` is also used to determine whether or not the fix icon
(`[*]`) is rendered, so this is now shown for display-only fixes in our
snapshots. This was already the case previously, but we were only
setting `Applicability::Unsafe` in these tests and ignoring the
`Applicability` when rendering fix diffs. CLI users can't enable
display-only fixes, so this is only a test change for now, but this
should work smoothly if we decide to expose a `--display-only-fixes`
flag or similar in the future.

I also deleted the `PrinterFlags::SHOW_FIX_DIFF` flag. This was
completely unused before, and it seemed less confusing just to delete it
than to enable it in the right place and check it along with the
`OutputFormat` and `preview`.

## Test Plan

I only added one CLI test for now. I'm kind of assuming that we have
decent coverage of the cases where this shouldn't be firing, especially
the `output_format` CLI test, which shows that this definitely doesn't
affect non-preview `full` output. I'm happy to add more tests with
different combinations of options, if we're worried about any in
particular. I did try `--diff` and `--preview` and a few other
combinations manually.

And here's a screenshot using our trusty UP049 example from the design
discussion confirming that all the colors and other formatting still
look as expected:

<img width="786" height="629" alt="image"
src="https://github.com/user-attachments/assets/94e408bc-af7b-4573-b546-a5ceac2620f2"
/>

And one with an unsafe fix to see the footer:

<img width="782" height="367" alt="image"
src="https://github.com/user-attachments/assets/bbb29e47-310b-4293-b2c2-cc7aee3baff4"
/>


## Related issues and PR
- astral-sh#7352
- astral-sh#12595
- astral-sh#12598
- astral-sh#12599
- astral-sh#12600

I think we could probably close all of these issues now. I think we've
either resolved or avoided most of them, and if we encounter them again
with the new output format, it would probably make sense to open new
ones anyway.

Author: ntBre

crates/ruff/src/printer.rs

@@ -30,8 +30,6 @@ bitflags! {
         const SHOW_VIOLATIONS = 1 << 0;
         /// Whether to show a summary of the fixed violations when emitting diagnostics.
         const SHOW_FIX_SUMMARY = 1 << 1;
-        /// Whether to show a diff of each fixed violation when emitting diagnostics.
-        const SHOW_FIX_DIFF = 1 << 2;
     }
 }
 
@@ -260,9 +258,9 @@ impl Printer {
             OutputFormat::Concise | OutputFormat::Full => {
                 TextEmitter::default()
                     .with_show_fix_status(show_fix_status(self.fix_mode, fixables.as_ref()))
-                    .with_show_fix_diff(self.flags.intersects(Flags::SHOW_FIX_DIFF))
+                    .with_show_fix_diff(self.format == OutputFormat::Full && preview)
                     .with_show_source(self.format == OutputFormat::Full)
-                    .with_unsafe_fixes(self.unsafe_fixes)
+                    .with_fix_applicability(self.unsafe_fixes.required_applicability())
                     .with_preview(preview)
                     .emit(writer, &diagnostics.inner, &context)?;
 
@@ -464,7 +462,7 @@ impl Printer {
             TextEmitter::default()
                 .with_show_fix_status(show_fix_status(self.fix_mode, fixables.as_ref()))
                 .with_show_source(preview)
-                .with_unsafe_fixes(self.unsafe_fixes)
+                .with_fix_applicability(self.unsafe_fixes.required_applicability())
                 .emit(writer, &diagnostics.inner, &context)?;
         }
         writer.flush()?;

crates/ruff/tests/lint.rs

@@ -5830,3 +5830,33 @@ nested_optional: Optional[Optional[Optional[str]]] = None
     ",
     );
 }
+
+#[test]
+fn show_fixes_in_full_output_with_preview_enabled() {
+    assert_cmd_snapshot!(
+        Command::new(get_cargo_bin(BIN_NAME))
+            .args(["check", "--no-cache", "--output-format", "full"])
+            .args(["--select", "F401"])
+            .arg("--preview")
+            .arg("-")
+            .pass_stdin("import math"),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    F401 [*] `math` imported but unused
+     --> -:1:8
+      |
+    1 | import math
+      |        ^^^^
+      |
+    help: Remove unused import: `math`
+      - import math
+
+    Found 1 error.
+    [*] 1 fixable with the `--fix` option.
+
+    ----- stderr -----
+    ",
+    );
+}

crates/ruff_db/src/diagnostic/mod.rs

@@ -349,6 +349,13 @@ impl Diagnostic {
         self.fix().is_some()
     }
 
+    /// Returns `true` if the diagnostic is [`fixable`](Diagnostic::fixable) and applies at the
+    /// configured applicability level.
+    pub fn has_applicable_fix(&self, config: &DisplayDiagnosticConfig) -> bool {
+        self.fix()
+            .is_some_and(|fix| fix.applies(config.fix_applicability))
+    }
+
     /// Returns the offset of the parent statement for this diagnostic if it exists.
     ///
     /// This is primarily used for checking noqa/secondary code suppressions.

crates/ruff_db/src/diagnostic/render.rs

@@ -254,9 +254,7 @@ impl<'a> ResolvedDiagnostic<'a> {
             id,
             message: diag.inner.message.as_str().to_string(),
             annotations,
-            is_fixable: diag
-                .fix()
-                .is_some_and(|fix| fix.applies(config.fix_applicability)),
+            is_fixable: diag.has_applicable_fix(config),
         }
     }
 

crates/ruff_db/src/diagnostic/render/concise.rs

@@ -77,11 +77,9 @@ impl<'a> ConciseRenderer<'a> {
                     )?;
                 }
                 if self.config.show_fix_status {
-                    if let Some(fix) = diag.fix() {
-                        // Do not display an indicator for inapplicable fixes
-                        if fix.applies(self.config.fix_applicability) {
-                            write!(f, "[{fix}] ", fix = fmt_styled("*", stylesheet.separator))?;
-                        }
+                    // Do not display an indicator for inapplicable fixes
+                    if diag.has_applicable_fix(self.config) {
+                        write!(f, "[{fix}] ", fix = fmt_styled("*", stylesheet.separator))?;
                     }
                 }
             } else {

crates/ruff_db/src/diagnostic/render/full.rs

@@ -58,7 +58,7 @@ impl<'a> FullRenderer<'a> {
                 writeln!(f, "{}", renderer.render(diag.to_annotate()))?;
             }
 
-            if self.config.show_fix_diff {
+            if self.config.show_fix_diff && diag.has_applicable_fix(self.config) {
                 if let Some(diff) = Diff::from_diagnostic(diag, &stylesheet, self.resolver) {
                     write!(f, "{diff}")?;
                 }
@@ -697,6 +697,8 @@ print()
     fn notebook_output_with_diff() {
         let (mut env, diagnostics) = create_notebook_diagnostics(DiagnosticFormat::Full);
         env.show_fix_diff(true);
+        env.fix_applicability(Applicability::DisplayOnly);
+
         insta::assert_snapshot!(env.render_diagnostics(&diagnostics), @r"
         error[unused-import][*]: `os` imported but unused
          --> notebook.ipynb:cell 1:2:8
@@ -726,7 +728,7 @@ print()
         2 | 
         3 | print('hello world')
 
-        error[unused-variable]: Local variable `x` is assigned to but never used
+        error[unused-variable][*]: Local variable `x` is assigned to but never used
          --> notebook.ipynb:cell 3:4:5
           |
         2 | def foo():
@@ -749,6 +751,7 @@ print()
     fn notebook_output_with_diff_spanning_cells() {
         let (mut env, mut diagnostics) = create_notebook_diagnostics(DiagnosticFormat::Full);
         env.show_fix_diff(true);
+        env.fix_applicability(Applicability::DisplayOnly);
 
         // Move all of the edits from the later diagnostics to the first diagnostic to simulate a
         // single diagnostic with edits in different cells.
@@ -761,7 +764,7 @@ print()
         *fix = Fix::unsafe_edits(edits.remove(0), edits);
 
         insta::assert_snapshot!(env.render(&diagnostic), @r"
-        error[unused-import]: `os` imported but unused
+        error[unused-import][*]: `os` imported but unused
          --> notebook.ipynb:cell 1:2:8
           |
         1 | # cell 1
@@ -924,6 +927,7 @@ line 10
         env.add("example.py", contents);
         env.format(DiagnosticFormat::Full);
         env.show_fix_diff(true);
+        env.fix_applicability(Applicability::DisplayOnly);
 
         let mut diagnostic = env.err().primary("example.py", "3", "3", "label").build();
         diagnostic.help("Start of diff:");
@@ -936,7 +940,7 @@ line 10
         )));
 
         insta::assert_snapshot!(env.render(&diagnostic), @r"
-        error[test-diagnostic]: main diagnostic message
+        error[test-diagnostic][*]: main diagnostic message
          --> example.py:3:1
           |
         1 | line 1

crates/ruff_linter/src/message/text.rs

@@ -3,9 +3,9 @@ use std::io::Write;
 use ruff_db::diagnostic::{
     Diagnostic, DiagnosticFormat, DisplayDiagnosticConfig, DisplayDiagnostics,
 };
+use ruff_diagnostics::Applicability;
 
 use crate::message::{Emitter, EmitterContext};
-use crate::settings::types::UnsafeFixes;
 
 pub struct TextEmitter {
     config: DisplayDiagnosticConfig,
@@ -46,10 +46,8 @@ impl TextEmitter {
     }
 
     #[must_use]
-    pub fn with_unsafe_fixes(mut self, unsafe_fixes: UnsafeFixes) -> Self {
-        self.config = self
-            .config
-            .fix_applicability(unsafe_fixes.required_applicability());
+    pub fn with_fix_applicability(mut self, applicability: Applicability) -> Self {
+        self.config = self.config.fix_applicability(applicability);
         self
     }
 
@@ -86,13 +84,13 @@ impl Emitter for TextEmitter {
 #[cfg(test)]
 mod tests {
     use insta::assert_snapshot;
+    use ruff_diagnostics::Applicability;
 
     use crate::message::TextEmitter;
     use crate::message::tests::{
         capture_emitter_notebook_output, capture_emitter_output, create_diagnostics,
         create_notebook_diagnostics, create_syntax_error_diagnostics,
     };
-    use crate::settings::types::UnsafeFixes;
 
     #[test]
     fn default() {
@@ -117,7 +115,7 @@ mod tests {
         let mut emitter = TextEmitter::default()
             .with_show_fix_status(true)
             .with_show_source(true)
-            .with_unsafe_fixes(UnsafeFixes::Enabled);
+            .with_fix_applicability(Applicability::Unsafe);
         let content = capture_emitter_output(&mut emitter, &create_diagnostics());
 
         assert_snapshot!(content);
@@ -128,7 +126,7 @@ mod tests {
         let mut emitter = TextEmitter::default()
             .with_show_fix_status(true)
             .with_show_source(true)
-            .with_unsafe_fixes(UnsafeFixes::Enabled);
+            .with_fix_applicability(Applicability::Unsafe);
         let (messages, notebook_indexes) = create_notebook_diagnostics();
         let content = capture_emitter_notebook_output(&mut emitter, &messages, &notebook_indexes);
 

crates/ruff_linter/src/rules/eradicate/snapshots/ruff_linter__rules__eradicate__tests__ERA001_ERA001.py.snap

@@ -1,7 +1,7 @@
 ---
 source: crates/ruff_linter/src/rules/eradicate/mod.rs
 ---
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
  --> ERA001.py:1:1
   |
 1 | #import os
@@ -16,7 +16,7 @@ help: Remove commented-out code
 3 | a = 4
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
  --> ERA001.py:2:1
   |
 1 | #import os
@@ -33,7 +33,7 @@ help: Remove commented-out code
 4 | #foo(1, 2, 3)
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
  --> ERA001.py:3:1
   |
 1 | #import os
@@ -52,7 +52,7 @@ help: Remove commented-out code
 5 | 
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
  --> ERA001.py:5:1
   |
 3 | #a = 3
@@ -72,7 +72,7 @@ help: Remove commented-out code
 7 |     content = 1 # print('hello')
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:13:5
    |
 11 |     # This is a real comment.
@@ -91,7 +91,7 @@ help: Remove commented-out code
 15 | #import os  # noqa: ERA001
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:21:5
    |
 19 | class A():
@@ -109,7 +109,7 @@ help: Remove commented-out code
 23 | dictionary = {
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:26:5
    |
 24 | dictionary = {
@@ -129,7 +129,7 @@ help: Remove commented-out code
 28 | 
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:27:5
    |
 25 |     # "key1": 123,  # noqa: ERA001
@@ -148,7 +148,7 @@ help: Remove commented-out code
 29 | #import os  # noqa
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:32:1
    |
 30 | #import os  # noqa
@@ -168,7 +168,7 @@ help: Remove commented-out code
 34 | # try: print()
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:33:1
    |
 32 | # case 1:
@@ -187,7 +187,7 @@ help: Remove commented-out code
 35 | # except:
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:34:1
    |
 32 | # case 1:
@@ -207,7 +207,7 @@ help: Remove commented-out code
 36 | # except Foo:
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:35:1
    |
 33 | # try:
@@ -227,7 +227,7 @@ help: Remove commented-out code
 37 | # except Exception as e: print(e)
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:36:1
    |
 34 | # try:  # with comment
@@ -247,7 +247,7 @@ help: Remove commented-out code
 38 | 
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:37:1
    |
 35 | # try: print()
@@ -266,7 +266,7 @@ help: Remove commented-out code
 39 | 
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:38:1
    |
 36 | # except:
@@ -284,7 +284,7 @@ help: Remove commented-out code
 40 | # Script tag without an opening tag (Error)
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:44:1
    |
 43 | # requires-python = ">=3.11"
@@ -303,7 +303,7 @@ help: Remove commented-out code
 46 | # ]
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:47:1
    |
 45 | #   "requests<3",
@@ -322,7 +322,7 @@ help: Remove commented-out code
 49 | # Script tag (OK)
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:75:1
    |
 73 | # /// script
@@ -342,7 +342,7 @@ help: Remove commented-out code
 77 | # ]
 note: This is a display-only fix and is likely to be incorrect
 
-ERA001 Found commented-out code
+ERA001 [*] Found commented-out code
   --> ERA001.py:78:1
    |
 76 | #   "requests<3",