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