[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,
-    whitespace_around_keywords, whitespace_around_named_parameter_equals,
+    missing_whitespace_around_operator, redundant_backslash, space_after_comma,
+    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,
+        }
+    }
+}