rust-clippy/clippy_lints/src/question_mark.rs

use rustc::lint::*;
use rustc::hir::*;
use rustc::hir::def::Def;
use utils::sugg::Sugg;
use syntax::ptr::P;

use utils::{match_def_path, match_type, span_lint_and_then};
use utils::paths::*;

/// **What it does:** Checks for expressions that could be replaced by the question mark operator
///
/// **Why is this bad?** Question mark usage is more idiomatic
///
/// **Known problems:** None
///
/// **Example:**
/// ```rust
/// if option.is_none() {
///     return None;
/// }
/// ```
///
/// Could be written:
///
/// ```rust
/// option?;
/// ```
declare_clippy_lint!{
    pub QUESTION_MARK,
    style,
    "checks for expressions that could be replaced by the question mark operator"
}

#[derive(Copy, Clone)]
pub struct QuestionMarkPass;

impl LintPass for QuestionMarkPass {
    fn get_lints(&self) -> LintArray {
        lint_array!(QUESTION_MARK)
    }
}

impl QuestionMarkPass {
    /// Check if the given expression on the given context matches the following structure:
    ///
    /// ```
    /// if option.is_none() {
    ///    return None;
    /// }
    /// ```
    ///
    /// If it matches, it will suggest to use the question mark operator instead
    fn check_is_none_and_early_return_none(cx: &LateContext, expr: &Expr) {
        if_chain! {
            if let ExprIf(ref if_expr, ref body, _) = expr.node;
            if let ExprMethodCall(ref segment, _, ref args) = if_expr.node;
            if segment.name == "is_none";
            if Self::expression_returns_none(cx, body);
            if let Some(subject) = args.get(0);
            if Self::is_option(cx, subject);

            then {
                span_lint_and_then(
                    cx,
                    QUESTION_MARK,
                    expr.span,
                    "this block may be rewritten with the `?` operator",
                    |db| {
                        let receiver_str = &Sugg::hir(cx, subject, "..");

                        db.span_suggestion(
                            expr.span,
                            "replace_it_with",
                            format!("{}?;", receiver_str),
                        );
                    }
                )
            }
        }
    }

    fn is_option(cx: &LateContext, expression: &Expr) -> bool {
        let expr_ty = cx.tables.expr_ty(expression);

        match_type(cx, expr_ty, &OPTION)
    }

    fn expression_returns_none(cx: &LateContext, expression: &Expr) -> bool {
        match expression.node {
            ExprBlock(ref block) => {
                if let Some(return_expression) = Self::return_expression(block) {
                    return Self::expression_returns_none(cx, &return_expression);
                }

                false
            },
            ExprRet(Some(ref expr)) => {
                Self::expression_returns_none(cx, expr)
            },
            ExprPath(ref qp) => {
                if let Def::VariantCtor(def_id, _) = cx.tables.qpath_def(qp, expression.hir_id) {
                    return match_def_path(cx.tcx, def_id,  &OPTION_NONE);
                }

                false
            },
            _ => false
        }
    }

    fn return_expression(block: &Block) -> Option<P<Expr>> {
        // Check if last expression is a return statement. Then, return the expression
        if_chain! {
            if block.stmts.len() == 1;
            if let Some(expr) = block.stmts.iter().last();
            if let StmtSemi(ref expr, _) = expr.node;
            if let ExprRet(ref ret_expr) = expr.node;
            if let &Some(ref ret_expr) = ret_expr;

            then {
                return Some(ret_expr.clone());
            }
        }

        // Check if the block has an implicit return expression
        if let Some(ref ret_expr) = block.expr {
            return Some(ret_expr.clone());
        }

        None
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for QuestionMarkPass {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        Self::check_is_none_and_early_return_none(cx, expr);
    }
}