clippy_lints/src/question_mark.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::utils::sugg::Sugg;
  11 use if_chain::if_chain;
  12 use rustc::hir::def::Def;
  13 use rustc::hir::*;
  14 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  15 use rustc::{declare_tool_lint, lint_array};
  16 use syntax::ptr::P;
  17
  18 use crate::utils::paths::*;
  19 use crate::utils::{match_def_path, match_type, span_lint_and_then, SpanlessEq};
  20 use rustc_errors::Applicability;
  21
  22 /// **What it does:** Checks for expressions that could be replaced by the question mark operator
  23 ///
  24 /// **Why is this bad?** Question mark usage is more idiomatic
  25 ///
  26 /// **Known problems:** None
  27 ///
  28 /// **Example:**
  29 /// ```rust
  30 /// if option.is_none() {
  31 ///     return None;
  32 /// }
  33 /// ```
  34 ///
  35 /// Could be written:
  36 ///
  37 /// ```rust
  38 /// option?;
  39 /// ```
  40 declare_clippy_lint! {
  41     pub QUESTION_MARK,
  42     style,
  43     "checks for expressions that could be replaced by the question mark operator"
  44 }
  45
  46 #[derive(Copy, Clone)]
  47 pub struct Pass;
  48
  49 impl LintPass for Pass {
  50     fn get_lints(&self) -> LintArray {
  51         lint_array!(QUESTION_MARK)
  52     }
  53 }
  54
  55 impl Pass {
  56     /// Check if the given expression on the given context matches the following structure:
  57     ///
  58     /// ```ignore
  59     /// if option.is_none() {
  60     ///    return None;
  61     /// }
  62     /// ```
  63     ///
  64     /// If it matches, it will suggest to use the question mark operator instead
  65     fn check_is_none_and_early_return_none(cx: &LateContext<'_, '_>, expr: &Expr) {
  66         if_chain! {
  67             if let ExprKind::If(if_expr, body, else_) = &expr.node;
  68             if let ExprKind::MethodCall(segment, _, args) = &if_expr.node;
  69             if segment.ident.name == "is_none";
  70             if Self::expression_returns_none(cx, body);
  71             if let Some(subject) = args.get(0);
  72             if Self::is_option(cx, subject);
  73
  74             then {
  75                 let receiver_str = &Sugg::hir(cx, subject, "..");
  76                 let mut replacement: Option<String> = None;
  77                 if let Some(else_) = else_ {
  78                     if_chain! {
  79                         if let ExprKind::Block(block, None) = &else_.node;
  80                         if block.stmts.len() == 0;
  81                         if let Some(block_expr) = &block.expr;
  82                         if SpanlessEq::new(cx).ignore_fn().eq_expr(subject, block_expr);
  83                         then {
  84                             replacement = Some(format!("Some({}?)", receiver_str));
  85                         }
  86                     }
  87                 } else if Self::moves_by_default(cx, subject) {
  88                         replacement = Some(format!("{}.as_ref()?;", receiver_str));
  89                 } else {
  90                         replacement = Some(format!("{}?;", receiver_str));
  91                 }
  92
  93                 if let Some(replacement_str) = replacement {
  94                     span_lint_and_then(
  95                         cx,
  96                         QUESTION_MARK,
  97                         expr.span,
  98                         "this block may be rewritten with the `?` operator",
  99                         |db| {
 100                             db.span_suggestion_with_applicability(
 101                                 expr.span,
 102                                 "replace_it_with",
 103                                 replacement_str,
 104                                 Applicability::MaybeIncorrect, // snippet
 105                             );
 106                         }
 107                     )
 108                }
 109             }
 110         }
 111     }
 112
 113     fn moves_by_default(cx: &LateContext<'_, '_>, expression: &Expr) -> bool {
 114         let expr_ty = cx.tables.expr_ty(expression);
 115
 116         !expr_ty.is_copy_modulo_regions(cx.tcx, cx.param_env, expression.span)
 117     }
 118
 119     fn is_option(cx: &LateContext<'_, '_>, expression: &Expr) -> bool {
 120         let expr_ty = cx.tables.expr_ty(expression);
 121
 122         match_type(cx, expr_ty, &OPTION)
 123     }
 124
 125     fn expression_returns_none(cx: &LateContext<'_, '_>, expression: &Expr) -> bool {
 126         match expression.node {
 127             ExprKind::Block(ref block, _) => {
 128                 if let Some(return_expression) = Self::return_expression(block) {
 129                     return Self::expression_returns_none(cx, &return_expression);
 130                 }
 131
 132                 false
 133             },
 134             ExprKind::Ret(Some(ref expr)) => Self::expression_returns_none(cx, expr),
 135             ExprKind::Path(ref qp) => {
 136                 if let Def::VariantCtor(def_id, _) = cx.tables.qpath_def(qp, expression.hir_id) {
 137                     return match_def_path(cx.tcx, def_id, &OPTION_NONE);
 138                 }
 139
 140                 false
 141             },
 142             _ => false,
 143         }
 144     }
 145
 146     fn return_expression(block: &Block) -> Option<P<Expr>> {
 147         // Check if last expression is a return statement. Then, return the expression
 148         if_chain! {
 149             if block.stmts.len() == 1;
 150             if let Some(expr) = block.stmts.iter().last();
 151             if let StmtKind::Semi(ref expr, _) = expr.node;
 152             if let ExprKind::Ret(ref ret_expr) = expr.node;
 153             if let &Some(ref ret_expr) = ret_expr;
 154
 155             then {
 156                 return Some(ret_expr.clone());
 157             }
 158         }
 159
 160         // Check for `return` without a semicolon.
 161         if_chain! {
 162             if block.stmts.len() == 0;
 163             if let Some(ExprKind::Ret(Some(ret_expr))) = block.expr.as_ref().map(|e| &e.node);
 164             then {
 165                 return Some(ret_expr.clone());
 166             }
 167         }
 168
 169         None
 170     }
 171 }
 172
 173 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
 174     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
 175         Self::check_is_none_and_early_return_none(cx, expr);
 176     }
 177 }