clippy_lints/src/mem_discriminant.rs

   1 use crate::utils::{match_def_path, opt_def_id, paths, snippet, span_lint_and_then, walk_ptrs_ty_depth};
   2 use if_chain::if_chain;
   3 use rustc::hir::{Expr, ExprKind};
   4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   5 use rustc::{declare_tool_lint, lint_array};
   6 use rustc_errors::Applicability;
   7
   8 use std::iter;
   9
  10 /// **What it does:** Checks for calls of `mem::discriminant()` on a non-enum type.
  11 ///
  12 /// **Why is this bad?** The value of `mem::discriminant()` on non-enum types
  13 /// is unspecified.
  14 ///
  15 /// **Known problems:** None.
  16 ///
  17 /// **Example:**
  18 /// ```rust
  19 /// mem::discriminant(&"hello");
  20 /// mem::discriminant(&&Some(2));
  21 /// ```
  22 declare_clippy_lint! {
  23     pub MEM_DISCRIMINANT_NON_ENUM,
  24     correctness,
  25     "calling mem::descriminant on non-enum type"
  26 }
  27
  28 pub struct MemDiscriminant;
  29
  30 impl LintPass for MemDiscriminant {
  31     fn get_lints(&self) -> LintArray {
  32         lint_array![MEM_DISCRIMINANT_NON_ENUM]
  33     }
  34
  35     fn name(&self) -> &'static str {
  36         "MemDiscriminant"
  37     }
  38 }
  39
  40 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemDiscriminant {
  41     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  42         if_chain! {
  43             if let ExprKind::Call(ref func, ref func_args) = expr.node;
  44             // is `mem::discriminant`
  45             if let ExprKind::Path(ref func_qpath) = func.node;
  46             if let Some(def_id) = opt_def_id(cx.tables.qpath_def(func_qpath, func.hir_id));
  47             if match_def_path(cx.tcx, def_id, &paths::MEM_DISCRIMINANT);
  48             // type is non-enum
  49             let ty_param = cx.tables.node_substs(func.hir_id).type_at(0);
  50             if !ty_param.is_enum();
  51
  52             then {
  53                 span_lint_and_then(
  54                     cx,
  55                     MEM_DISCRIMINANT_NON_ENUM,
  56                     expr.span,
  57                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  58                     |db| {
  59                         // if this is a reference to an enum, suggest dereferencing
  60                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  61                         if ptr_depth >= 1 && base_ty.is_enum() {
  62                             let param = &func_args[0];
  63
  64                             // cancel out '&'s first
  65                             let mut derefs_needed = ptr_depth;
  66                             let mut cur_expr = param;
  67                             while derefs_needed > 0  {
  68                                 if let ExprKind::AddrOf(_, ref inner_expr) = cur_expr.node {
  69                                     derefs_needed -= 1;
  70                                     cur_expr = inner_expr;
  71                                 } else {
  72                                     break;
  73                                 }
  74                             }
  75
  76                             let derefs: String = iter::repeat('*').take(derefs_needed).collect();
  77                             db.span_suggestion_with_applicability(
  78                                 param.span,
  79                                 "try dereferencing",
  80                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  81                                 Applicability::MachineApplicable,
  82                             );
  83                         }
  84                     },
  85                 )
  86             }
  87         }
  88     }
  89 }