[pycodestyle] Implement redundant-backslash (E502) (#10292)

## Summary

Implements the
[redundant-backslash](https://pycodestyle.pycqa.org/en/latest/intro.html#error-codes)
rule (E502) from pycodestyle.

## Test Plan

New fixture has been added

Part of #2402

crates/ruff_linter/resources/test/fixtures/pycodestyle/E502.py

@@ -0,0 +1,88 @@
+a = 2 + 2
+
+a = (2 + 2)
+
+a = 2 + \
+    3 \
+    + 4
+
+a = (3 -\
+    2 +   \
+    7)
+
+z = 5 + \
+    (3 -\
+    2 +   \
+    7) + \
+    4
+
+b = [2 +
+    2]
+
+b = [
+    2 + 4 + 5 + \
+    44 \
+    - 5
+]
+
+c = (True and
+    False \
+    or False \
+    and True \
+)
+
+c = (True and
+    False)
+
+d = True and \
+    False or \
+    False \
+    and not True
+
+
+s = {
+    'x': 2 + \
+    2
+}
+
+
+s = {
+    'x': 2 +
+    2
+}
+
+
+x = {2 + 4 \
+  + 3}
+
+y = (
+    2 + 2  # \
+    + 3  # \
+    + 4 \
+    + 3
+)
+
+
+x = """
+    (\\
+    )
+"""
+
+
+("""hello \
+""")
+
+("hello \
+")
+
+
+x = "abc" \
+    "xyz"
+
+x = ("abc" \
+    "xyz")
+
+
+def foo():
+    x = (a + \
+        2)

crates/ruff_linter/src/checkers/logical_lines.rs

@@ -1,6 +1,7 @@
 use crate::line_width::IndentWidth;
 use ruff_diagnostics::Diagnostic;
 use ruff_python_codegen::Stylist;
+use ruff_python_index::Indexer;
 use ruff_python_parser::lexer::LexResult;
 use ruff_python_parser::TokenKind;
 use ruff_source_file::Locator;
@@ -9,8 +10,8 @@ use ruff_text_size::{Ranged, TextRange};
 use crate::registry::AsRule;
 use crate::rules::pycodestyle::rules::logical_lines::{
     extraneous_whitespace, indentation, missing_whitespace, missing_whitespace_after_keyword,
-    missing_whitespace_around_operator, space_after_comma, space_around_operator,
+    missing_whitespace_around_operator, redundant_backslash, space_after_comma,
-    whitespace_around_keywords, whitespace_around_named_parameter_equals,
+    space_around_operator, whitespace_around_keywords, whitespace_around_named_parameter_equals,
     whitespace_before_comment, whitespace_before_parameters, LogicalLines, TokenFlags,
 };
 use crate::settings::LinterSettings;
@@ -35,6 +36,7 @@ pub(crate) fn expand_indent(line: &str, indent_width: IndentWidth) -> usize {
 pub(crate) fn check_logical_lines(
     tokens: &[LexResult],
     locator: &Locator,
+    indexer: &Indexer,
     stylist: &Stylist,
     settings: &LinterSettings,
 ) -> Vec<Diagnostic> {
@@ -73,6 +75,7 @@ pub(crate) fn check_logical_lines(
 
         if line.flags().contains(TokenFlags::BRACKET) {
             whitespace_before_parameters(&line, &mut context);
+            redundant_backslash(&line, locator, indexer, &mut context);
         }
 
         // Extract the indentation level.

crates/ruff_linter/src/codes.rs

@@ -146,6 +146,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pycodestyle, "E401") => (RuleGroup::Stable, rules::pycodestyle::rules::MultipleImportsOnOneLine),
         (Pycodestyle, "E402") => (RuleGroup::Stable, rules::pycodestyle::rules::ModuleImportNotAtTopOfFile),
         (Pycodestyle, "E501") => (RuleGroup::Stable, rules::pycodestyle::rules::LineTooLong),
+        (Pycodestyle, "E502") => (RuleGroup::Preview, rules::pycodestyle::rules::logical_lines::RedundantBackslash),
         (Pycodestyle, "E701") => (RuleGroup::Stable, rules::pycodestyle::rules::MultipleStatementsOnOneLineColon),
         (Pycodestyle, "E702") => (RuleGroup::Stable, rules::pycodestyle::rules::MultipleStatementsOnOneLineSemicolon),
         (Pycodestyle, "E703") => (RuleGroup::Stable, rules::pycodestyle::rules::UselessSemicolon),

crates/ruff_linter/src/linter.rs

@@ -132,7 +132,7 @@ pub fn check_path(
         .any(|rule_code| rule_code.lint_source().is_logical_lines())
     {
         diagnostics.extend(crate::checkers::logical_lines::check_logical_lines(
-            &tokens, locator, stylist, settings,
+            &tokens, locator, indexer, stylist, settings,
         ));
     }
 

crates/ruff_linter/src/registry.rs

@@ -327,6 +327,7 @@ impl Rule {
             | Rule::NoSpaceAfterBlockComment
             | Rule::NoSpaceAfterInlineComment
             | Rule::OverIndented
+            | Rule::RedundantBackslash
             | Rule::TabAfterComma
             | Rule::TabAfterKeyword
             | Rule::TabAfterOperator

crates/ruff_linter/src/rules/pycodestyle/mod.rs

@@ -71,6 +71,7 @@ mod tests {
     #[test_case(Rule::IsLiteral, Path::new("constant_literals.py"))]
     #[test_case(Rule::TypeComparison, Path::new("E721.py"))]
     #[test_case(Rule::ModuleImportNotAtTopOfFile, Path::new("E402_2.py"))]
+    #[test_case(Rule::RedundantBackslash, Path::new("E502.py"))]
     fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!(
             "preview__{}_{}",

crates/ruff_linter/src/rules/pycodestyle/rules/logical_lines/mod.rs

@@ -3,6 +3,7 @@ pub(crate) use indentation::*;
 pub(crate) use missing_whitespace::*;
 pub(crate) use missing_whitespace_after_keyword::*;
 pub(crate) use missing_whitespace_around_operator::*;
+pub(crate) use redundant_backslash::*;
 pub(crate) use space_around_operator::*;
 pub(crate) use whitespace_around_keywords::*;
 pub(crate) use whitespace_around_named_parameter_equals::*;
@@ -25,6 +26,7 @@ mod indentation;
 mod missing_whitespace;
 mod missing_whitespace_after_keyword;
 mod missing_whitespace_around_operator;
+mod redundant_backslash;
 mod space_around_operator;
 mod whitespace_around_keywords;
 mod whitespace_around_named_parameter_equals;

crates/ruff_linter/src/rules/pycodestyle/rules/logical_lines/redundant_backslash.rs

@@ -0,0 +1,92 @@
+use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_index::Indexer;
+use ruff_python_parser::TokenKind;
+use ruff_source_file::Locator;
+use ruff_text_size::{Ranged, TextRange, TextSize};
+
+use crate::checkers::logical_lines::LogicalLinesContext;
+
+use super::LogicalLine;
+
+/// ## What it does
+/// Checks for redundant backslashes between brackets.
+///
+/// ## Why is this bad?
+/// Explicit line joins using a backslash are redundant between brackets.
+///
+/// ## Example
+/// ```python
+/// x = (2 + \
+///     2)
+/// ```
+///
+/// Use instead:
+/// ```python
+/// x = (2 +
+///     2)
+/// ```
+///
+/// [PEP 8]: https://peps.python.org/pep-0008/#maximum-line-length
+#[violation]
+pub struct RedundantBackslash;
+
+impl AlwaysFixableViolation for RedundantBackslash {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Redundant backslash")
+    }
+
+    fn fix_title(&self) -> String {
+        "Remove redundant backslash".to_string()
+    }
+}
+
+/// E502
+pub(crate) fn redundant_backslash(
+    line: &LogicalLine,
+    locator: &Locator,
+    indexer: &Indexer,
+    context: &mut LogicalLinesContext,
+) {
+    let mut parens = 0;
+    let continuation_lines = indexer.continuation_line_starts();
+    let mut start_index = 0;
+
+    for token in line.tokens() {
+        match token.kind() {
+            TokenKind::Lpar | TokenKind::Lsqb | TokenKind::Lbrace => {
+                if parens == 0 {
+                    let start = locator.line_start(token.start());
+                    start_index = continuation_lines
+                        .binary_search(&start)
+                        .map_or_else(|err_index| err_index, |ok_index| ok_index);
+                }
+                parens += 1;
+            }
+            TokenKind::Rpar | TokenKind::Rsqb | TokenKind::Rbrace => {
+                parens -= 1;
+                if parens == 0 {
+                    let end = locator.line_start(token.start());
+                    let end_index = continuation_lines
+                        .binary_search(&end)
+                        .map_or_else(|err_index| err_index, |ok_index| ok_index);
+                    for continuation_line in &continuation_lines[start_index..end_index] {
+                        let backslash_end = locator.line_end(*continuation_line);
+                        let backslash_start = backslash_end - TextSize::new(1);
+                        let mut diagnostic = Diagnostic::new(
+                            RedundantBackslash,
+                            TextRange::new(backslash_start, backslash_end),
+                        );
+                        diagnostic.set_fix(Fix::safe_edit(Edit::deletion(
+                            backslash_start,
+                            backslash_end,
+                        )));
+                        context.push_diagnostic(diagnostic);
+                    }
+                }
+            }
+            _ => continue,
+        }
+    }
+}

crates/ruff_linter/src/rules/pycodestyle/snapshots/ruff_linter__rules__pycodestyle__tests__preview__E502_E502.py.snap

@@ -0,0 +1,281 @@
+---
+source: crates/ruff_linter/src/rules/pycodestyle/mod.rs
+---
+E502.py:9:9: E502 [*] Redundant backslash
+   |
+ 7 |     + 4
+ 8 | 
+ 9 | a = (3 -\
+   |         ^ E502
+10 |     2 +   \
+11 |     7)
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+6  6  |     3 \
+7  7  |     + 4
+8  8  | 
+9     |-a = (3 -\
+   9  |+a = (3 -
+10 10 |     2 +   \
+11 11 |     7)
+12 12 | 
+
+E502.py:10:11: E502 [*] Redundant backslash
+   |
+ 9 | a = (3 -\
+10 |     2 +   \
+   |           ^ E502
+11 |     7)
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+7  7  |     + 4
+8  8  | 
+9  9  | a = (3 -\
+10    |-    2 +   \
+   10 |+    2 +   
+11 11 |     7)
+12 12 | 
+13 13 | z = 5 + \
+
+E502.py:14:9: E502 [*] Redundant backslash
+   |
+13 | z = 5 + \
+14 |     (3 -\
+   |         ^ E502
+15 |     2 +   \
+16 |     7) + \
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+11 11 |     7)
+12 12 | 
+13 13 | z = 5 + \
+14    |-    (3 -\
+   14 |+    (3 -
+15 15 |     2 +   \
+16 16 |     7) + \
+17 17 |     4
+
+E502.py:15:11: E502 [*] Redundant backslash
+   |
+13 | z = 5 + \
+14 |     (3 -\
+15 |     2 +   \
+   |           ^ E502
+16 |     7) + \
+17 |     4
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+12 12 | 
+13 13 | z = 5 + \
+14 14 |     (3 -\
+15    |-    2 +   \
+   15 |+    2 +   
+16 16 |     7) + \
+17 17 |     4
+18 18 | 
+
+E502.py:23:17: E502 [*] Redundant backslash
+   |
+22 | b = [
+23 |     2 + 4 + 5 + \
+   |                 ^ E502
+24 |     44 \
+25 |     - 5
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+20 20 |     2]
+21 21 | 
+22 22 | b = [
+23    |-    2 + 4 + 5 + \
+   23 |+    2 + 4 + 5 + 
+24 24 |     44 \
+25 25 |     - 5
+26 26 | ]
+
+E502.py:24:8: E502 [*] Redundant backslash
+   |
+22 | b = [
+23 |     2 + 4 + 5 + \
+24 |     44 \
+   |        ^ E502
+25 |     - 5
+26 | ]
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+21 21 | 
+22 22 | b = [
+23 23 |     2 + 4 + 5 + \
+24    |-    44 \
+   24 |+    44 
+25 25 |     - 5
+26 26 | ]
+27 27 | 
+
+E502.py:29:11: E502 [*] Redundant backslash
+   |
+28 | c = (True and
+29 |     False \
+   |           ^ E502
+30 |     or False \
+31 |     and True \
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+26 26 | ]
+27 27 | 
+28 28 | c = (True and
+29    |-    False \
+   29 |+    False 
+30 30 |     or False \
+31 31 |     and True \
+32 32 | )
+
+E502.py:30:14: E502 [*] Redundant backslash
+   |
+28 | c = (True and
+29 |     False \
+30 |     or False \
+   |              ^ E502
+31 |     and True \
+32 | )
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+27 27 | 
+28 28 | c = (True and
+29 29 |     False \
+30    |-    or False \
+   30 |+    or False 
+31 31 |     and True \
+32 32 | )
+33 33 | 
+
+E502.py:31:14: E502 [*] Redundant backslash
+   |
+29 |     False \
+30 |     or False \
+31 |     and True \
+   |              ^ E502
+32 | )
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+28 28 | c = (True and
+29 29 |     False \
+30 30 |     or False \
+31    |-    and True \
+   31 |+    and True 
+32 32 | )
+33 33 | 
+34 34 | c = (True and
+
+E502.py:44:14: E502 [*] Redundant backslash
+   |
+43 | s = {
+44 |     'x': 2 + \
+   |              ^ E502
+45 |     2
+46 | }
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+41 41 | 
+42 42 | 
+43 43 | s = {
+44    |-    'x': 2 + \
+   44 |+    'x': 2 + 
+45 45 |     2
+46 46 | }
+47 47 | 
+
+E502.py:55:12: E502 [*] Redundant backslash
+   |
+55 | x = {2 + 4 \
+   |            ^ E502
+56 |   + 3}
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+52 52 | }
+53 53 | 
+54 54 | 
+55    |-x = {2 + 4 \
+   55 |+x = {2 + 4 
+56 56 |   + 3}
+57 57 | 
+58 58 | y = (
+
+E502.py:61:9: E502 [*] Redundant backslash
+   |
+59 |     2 + 2  # \
+60 |     + 3  # \
+61 |     + 4 \
+   |         ^ E502
+62 |     + 3
+63 | )
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+58 58 | y = (
+59 59 |     2 + 2  # \
+60 60 |     + 3  # \
+61    |-    + 4 \
+   61 |+    + 4 
+62 62 |     + 3
+63 63 | )
+64 64 | 
+
+E502.py:82:12: E502 [*] Redundant backslash
+   |
+80 |     "xyz"
+81 | 
+82 | x = ("abc" \
+   |            ^ E502
+83 |     "xyz")
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+79 79 | x = "abc" \
+80 80 |     "xyz"
+81 81 | 
+82    |-x = ("abc" \
+   82 |+x = ("abc" 
+83 83 |     "xyz")
+84 84 | 
+85 85 | 
+
+E502.py:87:14: E502 [*] Redundant backslash
+   |
+86 | def foo():
+87 |     x = (a + \
+   |              ^ E502
+88 |         2)
+   |
+   = help: Remove redundant backslash
+
+ℹ Safe fix
+84 84 | 
+85 85 | 
+86 86 | def foo():
+87    |-    x = (a + \
+   87 |+    x = (a + 
+88 88 |         2)

crates/ruff_workspace/src/pyproject.rs

@@ -380,7 +380,7 @@ per-file-ignores = { "__init__.py" = ["F401"] }
         assert!(result.is_err());
         let result = PatternPrefixPair::from_str("**/bar:E501");
         assert!(result.is_ok());
-        let result = PatternPrefixPair::from_str("bar:E502");
+        let result = PatternPrefixPair::from_str("bar:E503");
         assert!(result.is_err());
     }
 }

ruff.schema.json

@@ -2867,6 +2867,7 @@
         "E5",
         "E50",
         "E501",
+        "E502",
         "E7",
         "E70",
         "E701",

scripts/check_docs_formatted.py

@@ -69,6 +69,7 @@ KNOWN_FORMATTING_VIOLATIONS = [
     "over-indented",
     "pass-statement-stub-body",
     "prohibited-trailing-comma",
+    "redundant-backslash",
     "shebang-leading-whitespace",
     "surrounding-whitespace",
     "tab-indentation",