useless_return.rs

use ruff_python_ast::{self as ast, Constant, Expr, Stmt};

use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Fix};
use ruff_macros::{derive_message_formats, violation};
use ruff_python_ast::helpers::{is_const_none, ReturnStatementVisitor};
use ruff_python_ast::statement_visitor::StatementVisitor;
use ruff_text_size::Ranged;

use crate::checkers::ast::Checker;
use crate::fix;
use crate::registry::AsRule;

/// ## What it does
/// Checks for functions that end with an unnecessary `return` or
/// `return None`, and contain no other `return` statements.
///
/// ## Why is this bad?
/// Python implicitly assumes a `None` return at the end of a function, making
/// it unnecessary to explicitly write `return None`.
///
/// ## Example
/// ```python
/// def f():
///     print(5)
///     return None
/// ```
///
/// Use instead:
/// ```python
/// def f():
///     print(5)
/// ```
#[violation]
pub struct UselessReturn;

impl AlwaysFixableViolation for UselessReturn {
    #[derive_message_formats]
    fn message(&self) -> String {
        format!("Useless `return` statement at end of function")
    }

    fn fix_title(&self) -> String {
        format!("Remove useless `return` statement")
    }
}

/// PLR1711
pub(crate) fn useless_return(
    checker: &mut Checker,
    stmt: &Stmt,
    body: &[Stmt],
    returns: Option<&Expr>,
) {
    // Skip functions that have a return annotation that is not `None`.
    if !returns.map_or(true, is_const_none) {
        return;
    }

    // Find the last statement in the function.
    let Some(last_stmt) = body.last() else {
        // Skip empty functions.
        return;
    };
    if !last_stmt.is_return_stmt() {
        return;
    }

    // Skip functions that consist of a single return statement.
    if body.len() == 1 {
        return;
    }

    // Skip functions that consist of a docstring and a return statement.
    if body.len() == 2 {
        if let Stmt::Expr(ast::StmtExpr { value, range: _ }) = &body[0] {
            if matches!(
                value.as_ref(),
                Expr::Constant(ast::ExprConstant {
                    value: Constant::Str(_),
                    ..
                })
            ) {
                return;
            }
        }
    }

    // Verify that the last statement is a return statement.
    let Stmt::Return(ast::StmtReturn { value, range: _ }) = &last_stmt else {
        return;
    };

    // Verify that the return statement is either bare or returns `None`.
    if !value.as_ref().map_or(true, |expr| is_const_none(expr)) {
        return;
    };

    // Finally: verify that there are no _other_ return statements in the function.
    let mut visitor = ReturnStatementVisitor::default();
    visitor.visit_body(body);
    if visitor.returns.len() > 1 {
        return;
    }

    let mut diagnostic = Diagnostic::new(UselessReturn, last_stmt.range());
    if checker.patch(diagnostic.kind.rule()) {
        let edit =
            fix::edits::delete_stmt(last_stmt, Some(stmt), checker.locator(), checker.indexer());
        diagnostic.set_fix(Fix::automatic(edit).isolate(Checker::isolation(
            checker.semantic().current_statement_id(),
        )));
    }
    checker.diagnostics.push(diagnostic);
}