Fix formatter instability for lines only consisting of zero-width characters (#11748)

Cargo.lock

@@ -2215,7 +2215,6 @@ dependencies = [
  "static_assertions",
  "thiserror",
  "tracing",
- "unicode-width",
 ]
 
 [[package]]

crates/ruff_formatter/src/printer/mod.rs

@@ -124,7 +124,7 @@ impl<'a> Printer<'a> {
                     self.flush_line_suffixes(queue, stack, Some(element));
                 } else {
                     // Only print a newline if the current line isn't already empty
-                    if self.state.line_width > 0 {
+                    if !self.state.buffer[self.state.line_start..].is_empty() {
                         self.push_marker();
                         self.print_char('\n');
                     }
@@ -830,6 +830,7 @@ impl<'a> Printer<'a> {
                 .push_str(self.options.line_ending.as_str());
 
             self.state.line_width = 0;
+            self.state.line_start = self.state.buffer.len();
 
             // Fit's only tests if groups up to the first line break fit.
             // The next group must re-measure if it still fits.
@@ -872,12 +873,29 @@ enum FillPairLayout {
 /// position the printer currently is.
 #[derive(Default, Debug)]
 struct PrinterState<'a> {
+    /// The formatted output.
     buffer: String,
+
+    /// The source markers that map source positions to formatted positions.
     source_markers: Vec<SourceMarker>,
+
+    /// The next source position that should be flushed when writing the next text.
     pending_source_position: Option<TextSize>,
+
+    /// The current indentation that should be written before the next text.
     pending_indent: Indention,
+
+    /// Caches if the code up to the next newline has been measured to fit on a single line.
+    /// This is used to avoid re-measuring the same content multiple times.
     measured_group_fits: bool,
+
+    /// The offset at which the current line in `buffer` starts.
+    line_start: usize,
+
+    /// The accumulated unicode-width of all characters on the current line.
     line_width: u32,
+
+    /// The line suffixes that should be printed at the end of the line.
     line_suffixes: LineSuffixes<'a>,
     verbatim_markers: Vec<TextRange>,
     group_modes: GroupModes,

crates/ruff_python_formatter/Cargo.toml

@@ -37,7 +37,6 @@ smallvec = { workspace = true }
 static_assertions = { workspace = true }
 thiserror = { workspace = true }
 tracing = { workspace = true }
-unicode-width = { workspace = true }
 
 [dev-dependencies]
 ruff_formatter = { workspace = true }

crates/ruff_python_formatter/resources/test/fixtures/ruff/docstring_non_visible_characters.py

@@ -0,0 +1,6 @@
+# Regresssion test for https://github.com/astral-sh/ruff/issues/11724
+
+'''
+
+
+'''

crates/ruff_python_formatter/src/verbatim.rs

@@ -2,8 +2,6 @@ use std::borrow::Cow;
 use std::iter::FusedIterator;
 use std::slice::Iter;
 
-use unicode_width::UnicodeWidthStr;
-
 use ruff_formatter::{write, FormatError};
 use ruff_python_ast::AnyNodeRef;
 use ruff_python_ast::Stmt;
@@ -760,19 +758,9 @@ impl Format<PyFormatContext<'_>> for FormatVerbatimStatementRange {
 
                 // Write the line separator that terminates the line, except if it is the last line (that isn't separated by a hard line break).
                 if logical_line.has_trailing_newline {
-                    // Insert an empty line if the text is non-empty but all characters have a width of zero.
-                    // This is necessary to work around the fact that the Printer omits hard line breaks if the line width is 0.
-                    // The alternative is to "fix" the printer and explicitly track the width and whether the line is empty.
-                    // There's currently no use case for zero-width content outside of the verbatim context (and, form feeds are a Python specific speciality).
-                    // It, therefore, feels wrong to add additional complexity to the very hot `Printer::print_char` function,
-                    // to work around this special case. Therefore, work around the Printer behavior here, in the cold verbatim-formatting.
-                    if f.context().source()[trimmed_line_range].width() == 0 {
-                        empty_line().fmt(f)?;
-                    } else {
                     hard_line_break().fmt(f)?;
                 }
             }
-            }
 
             first = false;
         }

crates/ruff_python_formatter/tests/snapshots/format@docstring_non_visible_characters.py.snap

@@ -0,0 +1,23 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/docstring_non_visible_characters.py
+---
+## Input
+```python
+# Regresssion test for https://github.com/astral-sh/ruff/issues/11724
+
+'''
+
+
+'''
+```
+
+## Output
+```python
+# Regresssion test for https://github.com/astral-sh/ruff/issues/11724
+
+"""
+
+
+"""
+```