clippy_lints/src/mem_replace.rs

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