clippy_lints/src/mem_replace.rs

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