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
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for calls of `mem::discriminant()` on a non-enum type.
  14     ///
  15     /// ### Why is this bad?
  16     /// The value of `mem::discriminant()` on non-enum types
  17     /// is unspecified.
  18     ///
  19     /// ### Example
  20     /// ```rust
  21     /// use std::mem;
  22     ///
  23     /// mem::discriminant(&"hello");
  24     /// mem::discriminant(&&Some(2));
  25     /// ```
  26     pub MEM_DISCRIMINANT_NON_ENUM,
  27     correctness,
  28     "calling `mem::descriminant` on non-enum type"
  29 }
  30
  31 declare_lint_pass!(MemDiscriminant => [MEM_DISCRIMINANT_NON_ENUM]);
  32
  33 impl<'tcx> LateLintPass<'tcx> for MemDiscriminant {
  34     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  35         if_chain! {
  36             if let ExprKind::Call(func, func_args) = expr.kind;
  37             // is `mem::discriminant`
  38             if let ExprKind::Path(ref func_qpath) = func.kind;
  39             if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
  40             if match_def_path(cx, def_id, &paths::MEM_DISCRIMINANT);
  41             // type is non-enum
  42             let ty_param = cx.typeck_results().node_substs(func.hir_id).type_at(0);
  43             if !ty_param.is_enum();
  44
  45             then {
  46                 span_lint_and_then(
  47                     cx,
  48                     MEM_DISCRIMINANT_NON_ENUM,
  49                     expr.span,
  50                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  51                     |diag| {
  52                         // if this is a reference to an enum, suggest dereferencing
  53                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  54                         if ptr_depth >= 1 && base_ty.is_enum() {
  55                             let param = &func_args[0];
  56
  57                             // cancel out '&'s first
  58                             let mut derefs_needed = ptr_depth;
  59                             let mut cur_expr = param;
  60                             while derefs_needed > 0  {
  61                                 if let ExprKind::AddrOf(BorrowKind::Ref, _, inner_expr) = cur_expr.kind {
  62                                     derefs_needed -= 1;
  63                                     cur_expr = inner_expr;
  64                                 } else {
  65                                     break;
  66                                 }
  67                             }
  68
  69                             let derefs = "*".repeat(derefs_needed);
  70                             diag.span_suggestion(
  71                                 param.span,
  72                                 "try dereferencing",
  73                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  74                                 Applicability::MachineApplicable,
  75                             );
  76                         }
  77                     },
  78                 )
  79             }
  80         }
  81     }
  82 }