New rule: Prevent assignment expressions in assert statements (#7856)

crates/ruff_linter/resources/test/fixtures/ruff/RUF018.py

@@ -0,0 +1,7 @@
+# RUF018
+assert (x := 0) == 0
+assert x, (y := "error")
+
+# OK
+if z := 0:
+    pass

crates/ruff_linter/src/checkers/ast/analyze/expression.rs

@@ -1413,6 +1413,11 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 pylint::rules::repeated_equality_comparison(checker, bool_op);
             }
         }
+        Expr::NamedExpr(..) => {
+            if checker.enabled(Rule::AssignmentInAssert) {
+                ruff::rules::assignment_in_assert(checker, expr);
+            }
+        }
         _ => {}
     };
 }

crates/ruff_linter/src/codes.rs

@@ -865,6 +865,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "016") => (RuleGroup::Unspecified, rules::ruff::rules::InvalidIndexType),
         #[allow(deprecated)]
         (Ruff, "017") => (RuleGroup::Nursery, rules::ruff::rules::QuadraticListSummation),
+        (Ruff, "018") => (RuleGroup::Preview, rules::ruff::rules::AssignmentInAssert),
         (Ruff, "100") => (RuleGroup::Unspecified, rules::ruff::rules::UnusedNOQA),
         (Ruff, "200") => (RuleGroup::Unspecified, rules::ruff::rules::InvalidPyprojectToml),
 

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

@@ -42,6 +42,7 @@ mod tests {
     )]
     #[test_case(Rule::QuadraticListSummation, Path::new("RUF017_1.py"))]
     #[test_case(Rule::QuadraticListSummation, Path::new("RUF017_0.py"))]
+    #[test_case(Rule::AssignmentInAssert, Path::new("RUF018.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(

crates/ruff_linter/src/rules/ruff/rules/assignment_in_assert.rs

@@ -0,0 +1,53 @@
+use ruff_python_ast::Expr;
+
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for named assignment expressions (e.g., `x := 0`) in `assert`
+/// statements.
+///
+/// ## Why is this bad?
+/// Named assignment expressions (also known as "walrus operators") are used to
+/// assign a value to a variable as part of a larger expression.
+///
+/// Named assignments are syntactically valid in `assert` statements. However,
+/// when the Python interpreter is run under the `-O` flag, `assert` statements
+/// are not executed. In this case, the named assignment will also be ignored,
+/// which may result in unexpected behavior (e.g., undefined variable
+/// accesses).
+///
+/// ## Examples
+/// ```python
+/// assert (x := 0) == 0
+/// ```
+///
+/// Use instead:
+/// ```python
+/// x = 0
+/// assert x == 0
+/// ```
+///
+/// ## References
+/// - [Python documentation: `-O`](https://docs.python.org/3/using/cmdline.html#cmdoption-O)
+#[violation]
+pub struct AssignmentInAssert;
+
+impl Violation for AssignmentInAssert {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Avoid assignment expressions in `assert` statements")
+    }
+}
+
+/// RUF018
+pub(crate) fn assignment_in_assert(checker: &mut Checker, value: &Expr) {
+    if checker.semantic().current_statement().is_assert_stmt() {
+        checker
+            .diagnostics
+            .push(Diagnostic::new(AssignmentInAssert, value.range()));
+    }
+}

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

@@ -1,4 +1,5 @@
 pub(crate) use ambiguous_unicode_character::*;
+pub(crate) use assignment_in_assert::*;
 pub(crate) use asyncio_dangling_task::*;
 pub(crate) use collection_literal_concatenation::*;
 pub(crate) use explicit_f_string_type_conversion::*;
@@ -16,6 +17,7 @@ pub(crate) use unreachable::*;
 pub(crate) use unused_noqa::*;
 
 mod ambiguous_unicode_character;
+mod assignment_in_assert;
 mod asyncio_dangling_task;
 mod collection_literal_concatenation;
 mod confusables;

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF018_RUF018.py.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF018.py:2:9: RUF018 Avoid assignment expressions in `assert` statements
+  |
+1 | # RUF018
+2 | assert (x := 0) == 0
+  |         ^^^^^^ RUF018
+3 | assert x, (y := "error")
+  |
+
+RUF018.py:3:12: RUF018 Avoid assignment expressions in `assert` statements
+  |
+1 | # RUF018
+2 | assert (x := 0) == 0
+3 | assert x, (y := "error")
+  |            ^^^^^^^^^^^^ RUF018
+4 | 
+5 | # OK
+  |
+
+

crates/ruff_workspace/src/configuration.rs

@@ -1047,6 +1047,7 @@ mod tests {
         Rule::TooManyPublicMethods,
         Rule::UndocumentedWarn,
         Rule::UnnecessaryEnumerate,
+        Rule::AssignmentInAssert,
     ];
 
     #[allow(clippy::needless_pass_by_value)]