[ruff] implement useless if-else (RUF034) (#13218)

crates/ruff_linter/resources/test/fixtures/ruff/RUF034.py

@@ -0,0 +1,11 @@
+# Valid
+x = 1 if True else 2
+
+# Invalid
+x = 1 if True else 1
+
+# Invalid
+x = "a" if True else "a"
+
+# Invalid
+x = 0.1 if False else 0.1

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

@@ -1404,6 +1404,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::IfExpInsteadOfOrOperator) {
                 refurb::rules::if_exp_instead_of_or_operator(checker, if_exp);
             }
+            if checker.enabled(Rule::UselessIfElse) {
+                ruff::rules::useless_if_else(checker, if_exp);
+            }
         }
         Expr::ListComp(
             comp @ ast::ExprListComp {

crates/ruff_linter/src/codes.rs

@@ -961,6 +961,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "031") => (RuleGroup::Preview, rules::ruff::rules::IncorrectlyParenthesizedTupleInSubscript),
         (Ruff, "032") => (RuleGroup::Preview, rules::ruff::rules::DecimalFromFloatLiteral),
         (Ruff, "033") => (RuleGroup::Preview, rules::ruff::rules::PostInitDefault),
+        (Ruff, "034") => (RuleGroup::Preview, rules::ruff::rules::UselessIfElse),
         (Ruff, "100") => (RuleGroup::Stable, rules::ruff::rules::UnusedNOQA),
         (Ruff, "101") => (RuleGroup::Stable, rules::ruff::rules::RedirectedNOQA),
 

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

@@ -58,6 +58,7 @@ mod tests {
     #[test_case(Rule::AssertWithPrintMessage, Path::new("RUF030.py"))]
     #[test_case(Rule::IncorrectlyParenthesizedTupleInSubscript, Path::new("RUF031.py"))]
     #[test_case(Rule::DecimalFromFloatLiteral, Path::new("RUF032.py"))]
+    #[test_case(Rule::UselessIfElse, Path::new("RUF034.py"))]
     #[test_case(Rule::RedirectedNOQA, Path::new("RUF101.py"))]
     #[test_case(Rule::PostInitDefault, Path::new("RUF033.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {

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

@@ -29,6 +29,7 @@ pub(crate) use unnecessary_iterable_allocation_for_first_element::*;
 pub(crate) use unnecessary_key_check::*;
 pub(crate) use unused_async::*;
 pub(crate) use unused_noqa::*;
+pub(crate) use useless_if_else::*;
 pub(crate) use zip_instead_of_pairwise::*;
 
 mod ambiguous_unicode_character;
@@ -66,6 +67,7 @@ mod unnecessary_iterable_allocation_for_first_element;
 mod unnecessary_key_check;
 mod unused_async;
 mod unused_noqa;
+mod useless_if_else;
 mod zip_instead_of_pairwise;
 
 #[derive(Clone, Copy)]

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

@@ -0,0 +1,55 @@
+use crate::checkers::ast::Checker;
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast as ast;
+use ruff_python_ast::comparable::ComparableExpr;
+
+/// ## What it does
+/// Checks for useless if-else conditions with identical arms.
+///
+/// ## Why is this bad?
+/// Useless if-else conditions add unnecessary complexity to the code without
+/// providing any logical benefit.
+///
+/// Assigning the value directly is clearer and more explicit, and
+/// should be preferred.
+///
+/// ## Example
+/// ```python
+/// # Bad
+/// foo = x if y else x
+/// ```
+///
+/// Use instead:
+/// ```python
+/// # Good
+/// foo = x
+/// ```
+#[violation]
+pub struct UselessIfElse;
+
+impl Violation for UselessIfElse {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Useless if-else condition")
+    }
+}
+
+/// RUF031
+pub(crate) fn useless_if_else(checker: &mut Checker, if_expr: &ast::ExprIf) {
+    let ast::ExprIf {
+        body,
+        orelse,
+        range,
+        ..
+    } = if_expr;
+
+    // Skip if the body and orelse are not the same
+    if ComparableExpr::from(body) != ComparableExpr::from(orelse) {
+        return;
+    }
+
+    checker
+        .diagnostics
+        .push(Diagnostic::new(UselessIfElse, *range));
+}

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF034_RUF034.py.snap

@@ -0,0 +1,27 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF034.py:5:5: RUF034 Useless if-else condition
+  |
+4 | # Invalid
+5 | x = 1 if True else 1
+  |     ^^^^^^^^^^^^^^^^ RUF034
+6 | 
+7 | # Invalid
+  |
+
+RUF034.py:8:5: RUF034 Useless if-else condition
+   |
+ 7 | # Invalid
+ 8 | x = "a" if True else "a"
+   |     ^^^^^^^^^^^^^^^^^^^^ RUF034
+ 9 | 
+10 | # Invalid
+   |
+
+RUF034.py:11:5: RUF034 Useless if-else condition
+   |
+10 | # Invalid
+11 | x = 0.1 if False else 0.1
+   |     ^^^^^^^^^^^^^^^^^^^^^ RUF034
+   |