diff --git a/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py b/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py
index cedd7ba170..4b70bae72e 100644
--- a/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py
+++ b/crates/ruff/resources/test/fixtures/flake8_pytest_style/PT018.py
@@ -43,3 +43,12 @@ message
 assert something  # OK
 assert something and something_else  # Error
 assert something and something_else and something_third  # Error
+
+
+def test_multiline():
+    assert something and something_else; x = 1
+
+    x = 1; assert something and something_else
+
+    x = 1; \
+        assert something and something_else
diff --git a/crates/ruff/src/autofix/edits.rs b/crates/ruff/src/autofix/edits.rs
index 0a41c8bfc2..4f02de6c32 100644
--- a/crates/ruff/src/autofix/edits.rs
+++ b/crates/ruff/src/autofix/edits.rs
@@ -226,25 +226,25 @@ fn trailing_semicolon(offset: TextSize, locator: &Locator) -> Option<TextSize> {
 fn next_stmt_break(semicolon: TextSize, locator: &Locator) -> TextSize {
     let start_location = semicolon + TextSize::from(1);
 
-    let contents = &locator.contents()[usize::from(start_location)..];
-    for line in NewlineWithTrailingNewline::from(contents) {
+    for line in
+        NewlineWithTrailingNewline::with_offset(locator.after(start_location), start_location)
+    {
         let trimmed = line.trim_whitespace();
         // Skip past any continuations.
         if trimmed.starts_with('\\') {
             continue;
         }
 
-        return start_location
-            + if trimmed.is_empty() {
-                // If the line is empty, then despite the previous statement ending in a
-                // semicolon, we know that it's not a multi-statement line.
-                line.start()
-            } else {
-                // Otherwise, find the start of the next statement. (Or, anything that isn't
-                // whitespace.)
-                let relative_offset = line.find(|c: char| !is_python_whitespace(c)).unwrap();
-                line.start() + TextSize::try_from(relative_offset).unwrap()
-            };
+        return if trimmed.is_empty() {
+            // If the line is empty, then despite the previous statement ending in a
+            // semicolon, we know that it's not a multi-statement line.
+            line.start()
+        } else {
+            // Otherwise, find the start of the next statement. (Or, anything that isn't
+            // whitespace.)
+            let relative_offset = line.find(|c: char| !is_python_whitespace(c)).unwrap();
+            line.start() + TextSize::try_from(relative_offset).unwrap()
+        };
     }
 
     locator.line_end(start_location)
diff --git a/crates/ruff/src/noqa.rs b/crates/ruff/src/noqa.rs
index 8dbfa4b61a..4711149a9a 100644
--- a/crates/ruff/src/noqa.rs
+++ b/crates/ruff/src/noqa.rs
@@ -562,7 +562,7 @@ fn add_noqa_inner(
     let mut prev_end = TextSize::default();
 
     for (offset, (rules, directive)) in matches_by_line {
-        output.push_str(&locator.contents()[TextRange::new(prev_end, offset)]);
+        output.push_str(locator.slice(TextRange::new(prev_end, offset)));
 
         let line = locator.full_line(offset);
 
@@ -619,7 +619,7 @@ fn add_noqa_inner(
         prev_end = offset + line.text_len();
     }
 
-    output.push_str(&locator.contents()[usize::from(prev_end)..]);
+    output.push_str(locator.after(prev_end));
 
     (count, output)
 }
diff --git a/crates/ruff/src/rules/flake8_bugbear/rules/mutable_argument_default.rs b/crates/ruff/src/rules/flake8_bugbear/rules/mutable_argument_default.rs
index 0ea4f69059..f00944d1e8 100644
--- a/crates/ruff/src/rules/flake8_bugbear/rules/mutable_argument_default.rs
+++ b/crates/ruff/src/rules/flake8_bugbear/rules/mutable_argument_default.rs
@@ -140,7 +140,7 @@ fn move_initialization(
     let mut body = function_def.body.iter();
 
     let statement = body.next()?;
-    if indexer.preceded_by_multi_statement_line(statement, locator) {
+    if indexer.in_multi_statement_line(statement, locator) {
         return None;
     }
 
@@ -170,7 +170,7 @@ fn move_initialization(
         if let Some(statement) = body.next() {
             // If there's a second statement, insert _before_ it, but ensure this isn't a
             // multi-statement line.
-            if indexer.preceded_by_multi_statement_line(statement, locator) {
+            if indexer.in_multi_statement_line(statement, locator) {
                 return None;
             }
             Edit::insertion(content, locator.line_start(statement.start()))
diff --git a/crates/ruff/src/rules/flake8_implicit_str_concat/rules/implicit.rs b/crates/ruff/src/rules/flake8_implicit_str_concat/rules/implicit.rs
index 60d8ba496f..e516cd56e8 100644
--- a/crates/ruff/src/rules/flake8_implicit_str_concat/rules/implicit.rs
+++ b/crates/ruff/src/rules/flake8_implicit_str_concat/rules/implicit.rs
@@ -126,8 +126,8 @@ pub(crate) fn implicit(
 }
 
 fn concatenate_strings(a_range: TextRange, b_range: TextRange, locator: &Locator) -> Option<Fix> {
-    let a_text = &locator.contents()[a_range];
-    let b_text = &locator.contents()[b_range];
+    let a_text = locator.slice(a_range);
+    let b_text = locator.slice(b_range);
 
     let a_leading_quote = leading_quote(a_text)?;
     let b_leading_quote = leading_quote(b_text)?;
diff --git a/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs b/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs
index 2a1ea637aa..0201096a05 100644
--- a/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs
+++ b/crates/ruff/src/rules/flake8_pytest_style/rules/assertion.rs
@@ -1,7 +1,7 @@
 use std::borrow::Cow;
 
-use anyhow::bail;
 use anyhow::Result;
+use anyhow::{bail, Context};
 use libcst_native::{
     self, Assert, BooleanOp, CompoundStatement, Expression, ParenthesizableWhitespace,
     ParenthesizedNode, SimpleStatementLine, SimpleWhitespace, SmallStatement, Statement,
@@ -635,9 +635,8 @@ fn parenthesize<'a>(expression: Expression<'a>, parent: &Expression<'a>) -> Expr
 /// `assert a == "hello"` and `assert b == "world"`.
 fn fix_composite_condition(stmt: &Stmt, locator: &Locator, stylist: &Stylist) -> Result<Edit> {
     // Infer the indentation of the outer block.
-    let Some(outer_indent) = whitespace::indentation(locator, stmt) else {
-        bail!("Unable to fix multiline statement");
-    };
+    let outer_indent =
+        whitespace::indentation(locator, stmt).context("Unable to fix multiline statement")?;
 
     // Extract the module text.
     let contents = locator.lines(stmt.range());
@@ -672,11 +671,11 @@ fn fix_composite_condition(stmt: &Stmt, locator: &Locator, stylist: &Stylist) ->
         &mut indented_block.body
     };
 
-    let [Statement::Simple(simple_statement_line)] = &statements[..] else {
+    let [Statement::Simple(simple_statement_line)] = statements.as_slice() else {
         bail!("Expected one simple statement")
     };
 
-    let [SmallStatement::Assert(assert_statement)] = &simple_statement_line.body[..] else {
+    let [SmallStatement::Assert(assert_statement)] = simple_statement_line.body.as_slice() else {
         bail!("Expected simple statement to be an assert")
     };
 
@@ -754,6 +753,9 @@ pub(crate) fn composite_condition(
             if matches!(composite, CompositionKind::Simple)
                 && msg.is_none()
                 && !checker.indexer().comment_ranges().intersects(stmt.range())
+                && !checker
+                    .indexer()
+                    .in_multi_statement_line(stmt, checker.locator())
             {
                 diagnostic.try_set_fix(|| {
                     fix_composite_condition(stmt, checker.locator(), checker.stylist())
diff --git a/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap b/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap
index cbfde1160d..d9c9771a7d 100644
--- a/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap
+++ b/crates/ruff/src/rules/flake8_pytest_style/snapshots/ruff__rules__flake8_pytest_style__tests__PT018.snap
@@ -299,6 +299,8 @@ PT018.py:44:1: PT018 [*] Assertion should be broken down into multiple parts
    44 |+assert something
    45 |+assert something_else
 45 46 | assert something and something_else and something_third  # Error
+46 47 | 
+47 48 | 
 
 PT018.py:45:1: PT018 [*] Assertion should be broken down into multiple parts
    |
@@ -316,5 +318,37 @@ PT018.py:45:1: PT018 [*] Assertion should be broken down into multiple parts
 45    |-assert something and something_else and something_third  # Error
    45 |+assert something and something_else
    46 |+assert something_third
+46 47 | 
+47 48 | 
+48 49 | def test_multiline():
+
+PT018.py:49:5: PT018 Assertion should be broken down into multiple parts
+   |
+48 | def test_multiline():
+49 |     assert something and something_else; x = 1
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PT018
+50 | 
+51 |     x = 1; assert something and something_else
+   |
+   = help: Break down assertion into multiple parts
+
+PT018.py:51:12: PT018 Assertion should be broken down into multiple parts
+   |
+49 |     assert something and something_else; x = 1
+50 | 
+51 |     x = 1; assert something and something_else
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PT018
+52 | 
+53 |     x = 1; \
+   |
+   = help: Break down assertion into multiple parts
+
+PT018.py:54:9: PT018 Assertion should be broken down into multiple parts
+   |
+53 |     x = 1; \
+54 |         assert something and something_else
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PT018
+   |
+   = help: Break down assertion into multiple parts
 
 
diff --git a/crates/ruff/src/rules/isort/rules/organize_imports.rs b/crates/ruff/src/rules/isort/rules/organize_imports.rs
index 8b72f149ab..62c7ba47d7 100644
--- a/crates/ruff/src/rules/isort/rules/organize_imports.rs
+++ b/crates/ruff/src/rules/isort/rules/organize_imports.rs
@@ -1,16 +1,16 @@
 use std::path::Path;
 
 use itertools::{EitherOrBoth, Itertools};
-use ruff_python_ast::{PySourceType, Ranged, Stmt};
-use ruff_text_size::TextRange;
 
 use ruff_diagnostics::{AutofixKind, Diagnostic, Edit, Fix, Violation};
 use ruff_macros::{derive_message_formats, violation};
-use ruff_python_ast::whitespace::{followed_by_multi_statement_line, trailing_lines_end};
+use ruff_python_ast::whitespace::trailing_lines_end;
+use ruff_python_ast::{PySourceType, Ranged, Stmt};
 use ruff_python_codegen::Stylist;
 use ruff_python_index::Indexer;
 use ruff_python_trivia::{leading_indentation, textwrap::indent, PythonWhitespace};
 use ruff_source_file::{Locator, UniversalNewlines};
+use ruff_text_size::TextRange;
 
 use crate::line_width::LineWidth;
 use crate::registry::AsRule;
@@ -97,7 +97,7 @@ pub(crate) fn organize_imports(
     // Special-cases: there's leading or trailing content in the import block. These
     // are too hard to get right, and relatively rare, so flag but don't fix.
     if indexer.preceded_by_multi_statement_line(block.imports.first().unwrap(), locator)
-        || followed_by_multi_statement_line(block.imports.last().unwrap(), locator)
+        || indexer.followed_by_multi_statement_line(block.imports.last().unwrap(), locator)
     {
         return Some(Diagnostic::new(UnsortedImports, range));
     }
diff --git a/crates/ruff/src/rules/pyflakes/rules/f_string_missing_placeholders.rs b/crates/ruff/src/rules/pyflakes/rules/f_string_missing_placeholders.rs
index fd2982c11a..5606cb74c3 100644
--- a/crates/ruff/src/rules/pyflakes/rules/f_string_missing_placeholders.rs
+++ b/crates/ruff/src/rules/pyflakes/rules/f_string_missing_placeholders.rs
@@ -62,8 +62,7 @@ fn find_useless_f_strings<'a>(
                 kind: StringKind::FString | StringKind::RawFString,
                 ..
             } => {
-                let first_char =
-                    &locator.contents()[TextRange::at(range.start(), TextSize::from(1))];
+                let first_char = locator.slice(TextRange::at(range.start(), TextSize::from(1)));
                 // f"..."  => f_position = 0
                 // fr"..." => f_position = 0
                 // rf"..." => f_position = 1
diff --git a/crates/ruff_diagnostics/src/diagnostic.rs b/crates/ruff_diagnostics/src/diagnostic.rs
index 5d505877bc..19a5d3ec45 100644
--- a/crates/ruff_diagnostics/src/diagnostic.rs
+++ b/crates/ruff_diagnostics/src/diagnostic.rs
@@ -60,6 +60,17 @@ impl Diagnostic {
         }
     }
 
+    /// Set the [`Fix`] used to fix the diagnostic, if the provided function returns `Ok`.
+    /// Otherwise, log the error.
+    #[inline]
+    pub fn try_set_optional_fix(&mut self, func: impl FnOnce() -> Result<Option<Fix>>) {
+        match func() {
+            Ok(None) => {}
+            Ok(Some(fix)) => self.fix = Some(fix),
+            Err(err) => error!("Failed to create fix for {}: {}", self.kind.name, err),
+        }
+    }
+
     pub const fn range(&self) -> TextRange {
         self.range
     }
diff --git a/crates/ruff_python_ast/src/whitespace.rs b/crates/ruff_python_ast/src/whitespace.rs
index 282076ee29..6f89365efb 100644
--- a/crates/ruff_python_ast/src/whitespace.rs
+++ b/crates/ruff_python_ast/src/whitespace.rs
@@ -1,10 +1,8 @@
-use crate::{Ranged, Stmt};
+use ruff_python_trivia::{indentation_at_offset, is_python_whitespace, PythonWhitespace};
+use ruff_source_file::{newlines::UniversalNewlineIterator, Locator};
 use ruff_text_size::{TextRange, TextSize};
 
-use ruff_python_trivia::{
-    has_trailing_content, indentation_at_offset, is_python_whitespace, PythonWhitespace,
-};
-use ruff_source_file::{newlines::UniversalNewlineIterator, Locator};
+use crate::{Ranged, Stmt};
 
 /// Extract the leading indentation from a line.
 #[inline]
@@ -18,20 +16,12 @@ where
 /// Return the end offset at which the empty lines following a statement.
 pub fn trailing_lines_end(stmt: &Stmt, locator: &Locator) -> TextSize {
     let line_end = locator.full_line_end(stmt.end());
-    let rest = &locator.contents()[usize::from(line_end)..];
-
-    UniversalNewlineIterator::with_offset(rest, line_end)
+    UniversalNewlineIterator::with_offset(locator.after(line_end), line_end)
         .take_while(|line| line.trim_whitespace().is_empty())
         .last()
         .map_or(line_end, |line| line.full_end())
 }
 
-/// Return `true` if a `Stmt` appears to be part of a multi-statement line, with
-/// other statements following it.
-pub fn followed_by_multi_statement_line(stmt: &Stmt, locator: &Locator) -> bool {
-    has_trailing_content(stmt.end(), locator)
-}
-
 /// If a [`Ranged`] has a trailing comment, return the index of the hash.
 pub fn trailing_comment_start_offset<T>(located: &T, locator: &Locator) -> Option<TextSize>
 where
@@ -39,7 +29,7 @@ where
 {
     let line_end = locator.line_end(located.end());
 
-    let trailing = &locator.contents()[TextRange::new(located.end(), line_end)];
+    let trailing = locator.slice(TextRange::new(located.end(), line_end));
 
     for (index, char) in trailing.char_indices() {
         if char == '#' {
diff --git a/crates/ruff_python_index/src/indexer.rs b/crates/ruff_python_index/src/indexer.rs
index f6cc333423..536661d04d 100644
--- a/crates/ruff_python_index/src/indexer.rs
+++ b/crates/ruff_python_index/src/indexer.rs
@@ -39,7 +39,7 @@ impl Indexer {
         let mut line_start = TextSize::default();
 
         for (tok, range) in tokens.iter().flatten() {
-            let trivia = &locator.contents()[TextRange::new(prev_end, range.start())];
+            let trivia = locator.slice(TextRange::new(prev_end, range.start()));
 
             // Get the trivia between the previous and the current token and detect any newlines.
             // This is necessary because `RustPython` doesn't emit `[Tok::Newline]` tokens
@@ -250,14 +250,26 @@ impl Indexer {
         Some(continuation)
     }
 
-    /// Return `true` if a `Stmt` appears to be part of a multi-statement line, with
-    /// other statements preceding it.
+    /// Return `true` if a [`Stmt`] appears to be preceded by other statements in a multi-statement
+    /// line.
     pub fn preceded_by_multi_statement_line(&self, stmt: &Stmt, locator: &Locator) -> bool {
         has_leading_content(stmt.start(), locator)
             || self
                 .preceded_by_continuations(stmt.start(), locator)
                 .is_some()
     }
+
+    /// Return `true` if a [`Stmt`] appears to be followed by other statements in a multi-statement
+    /// line.
+    pub fn followed_by_multi_statement_line(&self, stmt: &Stmt, locator: &Locator) -> bool {
+        has_trailing_content(stmt.end(), locator)
+    }
+
+    /// Return `true` if a [`Stmt`] appears to be part of a multi-statement line.
+    pub fn in_multi_statement_line(&self, stmt: &Stmt, locator: &Locator) -> bool {
+        self.followed_by_multi_statement_line(stmt, locator)
+            || self.preceded_by_multi_statement_line(stmt, locator)
+    }
 }
 
 #[cfg(test)]
diff --git a/crates/ruff_python_trivia/src/whitespace.rs b/crates/ruff_python_trivia/src/whitespace.rs
index 7e23c4f3e7..5209164870 100644
--- a/crates/ruff_python_trivia/src/whitespace.rs
+++ b/crates/ruff_python_trivia/src/whitespace.rs
@@ -4,7 +4,7 @@ use ruff_text_size::{TextRange, TextSize};
 /// Extract the leading indentation from a line.
 pub fn indentation_at_offset<'a>(offset: TextSize, locator: &'a Locator) -> Option<&'a str> {
     let line_start = locator.line_start(offset);
-    let indentation = &locator.contents()[TextRange::new(line_start, offset)];
+    let indentation = locator.slice(TextRange::new(line_start, offset));
 
     if indentation.chars().all(is_python_whitespace) {
         Some(indentation)
@@ -16,14 +16,14 @@ pub fn indentation_at_offset<'a>(offset: TextSize, locator: &'a Locator) -> Opti
 /// Return `true` if the node starting the given [`TextSize`] has leading content.
 pub fn has_leading_content(offset: TextSize, locator: &Locator) -> bool {
     let line_start = locator.line_start(offset);
-    let leading = &locator.contents()[TextRange::new(line_start, offset)];
+    let leading = locator.slice(TextRange::new(line_start, offset));
     leading.chars().any(|char| !is_python_whitespace(char))
 }
 
 /// Return `true` if the node ending at the given [`TextSize`] has trailing content.
 pub fn has_trailing_content(offset: TextSize, locator: &Locator) -> bool {
     let line_end = locator.line_end(offset);
-    let trailing = &locator.contents()[TextRange::new(offset, line_end)];
+    let trailing = locator.slice(TextRange::new(offset, line_end));
 
     for char in trailing.chars() {
         if char == '#' {