clippy_lints/src/mem_replace.rs

   1 use crate::utils::{match_def_path, match_qpath, opt_def_id, paths, snippet_with_applicability, span_lint_and_sugg};
   2 use if_chain::if_chain;
   3 use rustc::hir::{Expr, ExprKind, MutMutable, QPath};
   4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use rustc::{declare_tool_lint, lint_array};
   6 use rustc_errors::Applicability;
   7
   8 /// **What it does:** Checks for `mem::replace()` on an `Option` with
   9 /// `None`.
  10 ///
  11 /// **Why is this bad?** `Option` already has the method `take()` for
  12 /// taking its current value (Some(..) or None) and replacing it with
  13 /// `None`.
  14 ///
  15 /// **Known problems:** None.
  16 ///
  17 /// **Example:**
  18 /// ```rust
  19 /// let mut an_option = Some(0);
  20 /// let replaced = mem::replace(&mut an_option, None);
  21 /// ```
  22 /// Is better expressed with:
  23 /// ```rust
  24 /// let mut an_option = Some(0);
  25 /// let taken = an_option.take();
  26 /// ```
  27 declare_clippy_lint! {
  28     pub MEM_REPLACE_OPTION_WITH_NONE,
  29     style,
  30     "replacing an `Option` with `None` instead of `take()`"
  31 }
  32
  33 pub struct MemReplace;
  34
  35 impl LintPass for MemReplace {
  36     fn get_lints(&self) -> LintArray {
  37         lint_array![MEM_REPLACE_OPTION_WITH_NONE]
  38     }
  39 }
  40
  41 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemReplace {
  42     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  43         if_chain! {
  44             // Check that `expr` is a call to `mem::replace()`
  45             if let ExprKind::Call(ref func, ref func_args) = expr.node;
  46             if func_args.len() == 2;
  47             if let ExprKind::Path(ref func_qpath) = func.node;
  48             if let Some(def_id) = opt_def_id(cx.tables.qpath_def(func_qpath, func.hir_id));
  49             if match_def_path(cx.tcx, def_id, &paths::MEM_REPLACE);
  50
  51             // Check that second argument is `Option::None`
  52             if let ExprKind::Path(ref replacement_qpath) = func_args[1].node;
  53             if match_qpath(replacement_qpath, &paths::OPTION_NONE);
  54
  55             then {
  56                 // Since this is a late pass (already type-checked),
  57                 // and we already know that the second argument is an
  58                 // `Option`, we do not need to check the first
  59                 // argument's type. All that's left is to get
  60                 // replacee's path.
  61                 let replaced_path = match func_args[0].node {
  62                     ExprKind::AddrOf(MutMutable, ref replaced) => {
  63                         if let ExprKind::Path(QPath::Resolved(None, ref replaced_path)) = replaced.node {
  64                             replaced_path
  65                         } else {
  66                             return
  67                         }
  68                     },
  69                     ExprKind::Path(QPath::Resolved(None, ref replaced_path)) => replaced_path,
  70                     _ => return,
  71                 };
  72
  73                 let mut applicability = Applicability::MachineApplicable;
  74                 span_lint_and_sugg(
  75                     cx,
  76                     MEM_REPLACE_OPTION_WITH_NONE,
  77                     expr.span,
  78                     "replacing an `Option` with `None`",
  79                     "consider `Option::take()` instead",
  80                     format!("{}.take()", snippet_with_applicability(cx, replaced_path.span, "", &mut applicability)),
  81                     applicability,
  82                 );
  83             }
  84         }
  85     }
  86 }