clippy_lints/src/mem_discriminant.rs

   1 use crate::utils::{match_def_path, paths, snippet, span_lint_and_then, walk_ptrs_ty_depth};
   2 use if_chain::if_chain;
   3 use rustc_errors::Applicability;
   4 use rustc_hir::{BorrowKind, Expr, ExprKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_session::{declare_lint_pass, declare_tool_lint};
   7
   8 use std::iter;
   9
  10 declare_clippy_lint! {
  11     /// **What it does:** Checks for calls of `mem::discriminant()` on a non-enum type.
  12     ///
  13     /// **Why is this bad?** The value of `mem::discriminant()` on non-enum types
  14     /// is unspecified.
  15     ///
  16     /// **Known problems:** None.
  17     ///
  18     /// **Example:**
  19     /// ```rust
  20     /// use std::mem;
  21     ///
  22     /// mem::discriminant(&"hello");
  23     /// mem::discriminant(&&Some(2));
  24     /// ```
  25     pub MEM_DISCRIMINANT_NON_ENUM,
  26     correctness,
  27     "calling `mem::descriminant` on non-enum type"
  28 }
  29
  30 declare_lint_pass!(MemDiscriminant => [MEM_DISCRIMINANT_NON_ENUM]);
  31
  32 impl<'tcx> LateLintPass<'tcx> for MemDiscriminant {
  33     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  34         if_chain! {
  35             if let ExprKind::Call(ref func, ref func_args) = expr.kind;
  36             // is `mem::discriminant`
  37             if let ExprKind::Path(ref func_qpath) = func.kind;
  38             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
  39             if match_def_path(cx, def_id, &paths::MEM_DISCRIMINANT);
  40             // type is non-enum
  41             let ty_param = cx.typeck_results().node_substs(func.hir_id).type_at(0);
  42             if !ty_param.is_enum();
  43
  44             then {
  45                 span_lint_and_then(
  46                     cx,
  47                     MEM_DISCRIMINANT_NON_ENUM,
  48                     expr.span,
  49                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  50                     |diag| {
  51                         // if this is a reference to an enum, suggest dereferencing
  52                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  53                         if ptr_depth >= 1 && base_ty.is_enum() {
  54                             let param = &func_args[0];
  55
  56                             // cancel out '&'s first
  57                             let mut derefs_needed = ptr_depth;
  58                             let mut cur_expr = param;
  59                             while derefs_needed > 0  {
  60                                 if let ExprKind::AddrOf(BorrowKind::Ref, _, ref inner_expr) = cur_expr.kind {
  61                                     derefs_needed -= 1;
  62                                     cur_expr = inner_expr;
  63                                 } else {
  64                                     break;
  65                                 }
  66                             }
  67
  68                             let derefs: String = iter::repeat('*').take(derefs_needed).collect();
  69                             diag.span_suggestion(
  70                                 param.span,
  71                                 "try dereferencing",
  72                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  73                                 Applicability::MachineApplicable,
  74                             );
  75                         }
  76                     },
  77                 )
  78             }
  79         }
  80     }
  81 }