Implement pylint's else-if-used rule (PLR5501)

crates/ruff/resources/test/fixtures/pylint/else_if_used.py

@@ -0,0 +1,47 @@
+"""
+Test for else-if-used
+"""
+def ok0():
+    """Should not trigger on elif"""
+    if 1:
+        pass
+    elif 2:
+        pass
+
+
+def ok1():
+    """If the orelse has more than 1 item in it, shouldn't trigger"""
+    if 1:
+        pass
+    else:
+        print()
+        if 1:
+            pass
+
+
+def ok2():
+    """If the orelse has more than 1 item in it, shouldn't trigger"""
+    if 1:
+        pass
+    else:
+        if 1:
+            pass
+        print()
+
+
+def not_ok0():
+    if 1:
+        pass
+    else:
+        if 2:
+            pass
+
+
+def not_ok1():
+    if 1:
+        pass
+    else:
+        if 2:
+            pass
+        else:
+            pass

crates/ruff/src/checkers/ast.rs

@@ -1604,6 +1604,13 @@ where
                 if self.settings.rules.enabled(&Rule::OutdatedVersionBlock) {
                     pyupgrade::rules::outdated_version_block(self, stmt, test, body, orelse);
                 }
+                if self.settings.rules.enabled(&Rule::ElseIfUsed) {
+                    if let Some(diagnostic) =
+                        pylint::rules::else_if_used(stmt, body, orelse, self.locator)
+                    {
+                        self.diagnostics.push(diagnostic);
+                    }
+                }
             }
             StmtKind::Assert { test, msg } => {
                 if self.settings.rules.enabled(&Rule::AssertTuple) {

crates/ruff/src/codes.rs

@@ -143,6 +143,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<Rule> {
         (Pylint, "R0133") => Rule::ComparisonOfConstant,
         (Pylint, "R1701") => Rule::ConsiderMergingIsinstance,
         (Pylint, "R1722") => Rule::ConsiderUsingSysExit,
+        (Pylint, "R5501") => Rule::ElseIfUsed,
         (Pylint, "R2004") => Rule::MagicValueComparison,
         (Pylint, "W0120") => Rule::UselessElseOnLoop,
         (Pylint, "W0602") => Rule::GlobalVariableNotAssigned,

crates/ruff/src/registry.rs

@@ -133,6 +133,7 @@ ruff_macros::register_rules!(
     rules::pylint::rules::BadStringFormatType,
     rules::pylint::rules::BidirectionalUnicode,
     rules::pylint::rules::BadStrStripCall,
+    rules::pylint::rules::ElseIfUsed,
     rules::pylint::rules::UselessImportAlias,
     rules::pylint::rules::UnnecessaryDirectLambdaCall,
     rules::pylint::rules::NonlocalWithoutBinding,

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

@@ -45,6 +45,7 @@ mod tests {
     #[test_case(Rule::TooManyArguments, Path::new("too_many_arguments.py"); "PLR0913")]
     #[test_case(Rule::TooManyBranches, Path::new("too_many_branches.py"); "PLR0912")]
     #[test_case(Rule::TooManyStatements, Path::new("too_many_statements.py"); "PLR0915")]
+    #[test_case(Rule::ElseIfUsed, Path::new("else_if_used.py"); "PLR5501")]
     #[test_case(Rule::BadStringFormatType, Path::new("bad_string_format_type.py"); "PLE1307")]
     #[test_case(Rule::BidirectionalUnicode, Path::new("bidirectional_unicode.py"); "PLE2502")]
     #[test_case(Rule::BadStrStripCall, Path::new("bad_str_strip_call.py"); "PLE01310")]

crates/ruff/src/rules/pylint/rules/else_if_used.rs

@@ -0,0 +1,45 @@
+use ruff_macros::{define_violation, derive_message_formats};
+use rustpython_parser::ast::{Stmt, StmtKind};
+
+use crate::ast::helpers::identifier_range;
+use crate::registry::Diagnostic;
+use crate::source_code::Locator;
+use crate::violation::Violation;
+
+define_violation!(
+    pub struct ElseIfUsed {}
+);
+
+impl Violation for ElseIfUsed {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Consider using \"elif\" instead of \"else\" then \"if\" to remove one indentation level")
+    }
+}
+
+/// PLR5501
+pub fn else_if_used(
+    stmt: &Stmt,
+    body: &[Stmt],
+    _orelse: &[Stmt],
+    locator: &Locator,
+) -> Option<Diagnostic> {
+    if let StmtKind::If { orelse, .. } = &stmt.node {
+        // If the body contains more than just the orelse, can't apply.
+        if body.len() == 1 {
+            if let [first, ..] = &orelse[..] {
+                if matches!(first.node, StmtKind::If { .. }) {
+                    // check the source if this is else then if or elif
+                    // we do this to see if they are on the same line
+                    if stmt.location.row() != first.location.row() {
+                        return Some(Diagnostic::new(
+                            ElseIfUsed {},
+                            identifier_range(stmt, locator),
+                        ));
+                    }
+                }
+            }
+        }
+    }
+    None
+}

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

@@ -4,6 +4,7 @@ pub use bad_string_format_type::{bad_string_format_type, BadStringFormatType};
 pub use bidirectional_unicode::{bidirectional_unicode, BidirectionalUnicode};
 pub use comparison_of_constant::{comparison_of_constant, ComparisonOfConstant};
 pub use consider_using_sys_exit::{consider_using_sys_exit, ConsiderUsingSysExit};
+pub use else_if_used::{else_if_used, ElseIfUsed};
 pub use global_variable_not_assigned::GlobalVariableNotAssigned;
 pub use invalid_all_format::{invalid_all_format, InvalidAllFormat};
 pub use invalid_all_object::{invalid_all_object, InvalidAllObject};
@@ -35,6 +36,7 @@ mod bad_string_format_type;
 mod bidirectional_unicode;
 mod comparison_of_constant;
 mod consider_using_sys_exit;
+mod else_if_used;
 mod global_variable_not_assigned;
 mod invalid_all_format;
 mod invalid_all_object;

crates/ruff/src/rules/pylint/snapshots/ruff__rules__pylint__tests__PLR5501_else_if_used.py.snap

@@ -0,0 +1,45 @@
+---
+source: crates/ruff/src/rules/pylint/mod.rs
+expression: diagnostics
+---
+- kind:
+    ElseIfUsed: {}
+  location:
+    row: 6
+    column: 4
+  end_location:
+    row: 9
+    column: 12
+  fix: ~
+  parent: ~
+- kind:
+    ElseIfUsed: {}
+  location:
+    row: 24
+    column: 4
+  end_location:
+    row: 29
+    column: 15
+  fix: ~
+  parent: ~
+- kind:
+    ElseIfUsed: {}
+  location:
+    row: 33
+    column: 4
+  end_location:
+    row: 37
+    column: 16
+  fix: ~
+  parent: ~
+- kind:
+    ElseIfUsed: {}
+  location:
+    row: 41
+    column: 4
+  end_location:
+    row: 47
+    column: 16
+  fix: ~
+  parent: ~
+