src/tools/clippy/clippy_lints/src/mem_discriminant.rs

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::source::snippet;
   3 use clippy_utils::ty::walk_ptrs_ty_depth;
   4 use clippy_utils::{match_def_path, paths};
   5 use if_chain::if_chain;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::{BorrowKind, Expr, ExprKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10 use std::iter;
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for calls of `mem::discriminant()` on a non-enum type.
  14     ///
  15     /// **Why is this bad?** The value of `mem::discriminant()` on non-enum types
  16     /// is unspecified.
  17     ///
  18     /// **Known problems:** None.
  19     ///
  20     /// **Example:**
  21     /// ```rust
  22     /// use std::mem;
  23     ///
  24     /// mem::discriminant(&"hello");
  25     /// mem::discriminant(&&Some(2));
  26     /// ```
  27     pub MEM_DISCRIMINANT_NON_ENUM,
  28     correctness,
  29     "calling `mem::descriminant` on non-enum type"
  30 }
  31
  32 declare_lint_pass!(MemDiscriminant => [MEM_DISCRIMINANT_NON_ENUM]);
  33
  34 impl<'tcx> LateLintPass<'tcx> for MemDiscriminant {
  35     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  36         if_chain! {
  37             if let ExprKind::Call(func, func_args) = expr.kind;
  38             // is `mem::discriminant`
  39             if let ExprKind::Path(ref func_qpath) = func.kind;
  40             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
  41             if match_def_path(cx, def_id, &paths::MEM_DISCRIMINANT);
  42             // type is non-enum
  43             let ty_param = cx.typeck_results().node_substs(func.hir_id).type_at(0);
  44             if !ty_param.is_enum();
  45
  46             then {
  47                 span_lint_and_then(
  48                     cx,
  49                     MEM_DISCRIMINANT_NON_ENUM,
  50                     expr.span,
  51                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  52                     |diag| {
  53                         // if this is a reference to an enum, suggest dereferencing
  54                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  55                         if ptr_depth >= 1 && base_ty.is_enum() {
  56                             let param = &func_args[0];
  57
  58                             // cancel out '&'s first
  59                             let mut derefs_needed = ptr_depth;
  60                             let mut cur_expr = param;
  61                             while derefs_needed > 0  {
  62                                 if let ExprKind::AddrOf(BorrowKind::Ref, _, inner_expr) = cur_expr.kind {
  63                                     derefs_needed -= 1;
  64                                     cur_expr = inner_expr;
  65                                 } else {
  66                                     break;
  67                                 }
  68                             }
  69
  70                             let derefs: String = iter::repeat('*').take(derefs_needed).collect();
  71                             diag.span_suggestion(
  72                                 param.span,
  73                                 "try dereferencing",
  74                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  75                                 Applicability::MachineApplicable,
  76                             );
  77                         }
  78                     },
  79                 )
  80             }
  81         }
  82     }
  83 }