clippy_lints/src/mem_replace.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
  11 use crate::rustc::hir::{Expr, ExprKind, MutMutable, QPath};
  12 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  13 use crate::rustc::{declare_tool_lint, lint_array};
  14 use crate::rustc_errors::Applicability;
  15 use crate::utils::{match_def_path, match_qpath, opt_def_id, paths, snippet_with_applicability, span_lint_and_sugg};
  16 use if_chain::if_chain;
  17
  18 /// **What it does:** Checks for `mem::replace()` on an `Option` with
  19 /// `None`.
  20 ///
  21 /// **Why is this bad?** `Option` already has the method `take()` for
  22 /// taking its current value (Some(..) or None) and replacing it with
  23 /// `None`.
  24 ///
  25 /// **Known problems:** None.
  26 ///
  27 /// **Example:**
  28 /// ```rust
  29 /// let mut an_option = Some(0);
  30 /// let replaced = mem::replace(&mut an_option, None);
  31 /// ```
  32 /// Is better expressed with:
  33 /// ```rust
  34 /// let mut an_option = Some(0);
  35 /// let taken = an_option.take();
  36 /// ```
  37 declare_clippy_lint! {
  38     pub MEM_REPLACE_OPTION_WITH_NONE,
  39     style,
  40     "replacing an `Option` with `None` instead of `take()`"
  41 }
  42
  43 pub struct MemReplace;
  44
  45 impl LintPass for MemReplace {
  46     fn get_lints(&self) -> LintArray {
  47         lint_array![MEM_REPLACE_OPTION_WITH_NONE]
  48     }
  49 }
  50
  51 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
  52     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  53         if_chain! {
  54             // Check that `expr` is a call to `mem::replace()`
  55             if let ExprKind::Call(ref func, ref func_args) = expr.node;
  56             if func_args.len() == 2;
  57             if let ExprKind::Path(ref func_qpath) = func.node;
  58             if let Some(def_id) = opt_def_id(cx.tables.qpath_def(func_qpath, func.hir_id));
  59             if match_def_path(cx.tcx, def_id, &paths::MEM_REPLACE);
  60
  61             // Check that second argument is `Option::None`
  62             if let ExprKind::Path(ref replacement_qpath) = func_args[1].node;
  63             if match_qpath(replacement_qpath, &paths::OPTION_NONE);
  64
  65             then {
  66                 // Since this is a late pass (already type-checked),
  67                 // and we already know that the second argument is an
  68                 // `Option`, we do not need to check the first
  69                 // argument's type. All that's left is to get
  70                 // replacee's path.
  71                 let replaced_path = match func_args[0].node {
  72                     ExprKind::AddrOf(MutMutable, ref replaced) => {
  73                         if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.node {
  74                             replaced_path
  75                         } else {
  76                             return
  77                         }
  78                     },
  79                     ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
  80                     _ => return,
  81                 };
  82
  83                 let mut applicability = Applicability::MachineApplicable;
  84                 span_lint_and_sugg(
  85                     cx,
  86                     MEM_REPLACE_OPTION_WITH_NONE,
  87                     expr.span,
  88                     "replacing an `Option` with `None`",
  89                     "consider `Option::take()` instead",
  90                     format!("{}.take()", snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)),
  91                     applicability,
  92                 );
  93             }
  94         }
  95     }
  96 }