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::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 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 pub struct MemDiscriminant;
  31
  32 impl LintPass for MemDiscriminant {
  33     fn get_lints(&self) -> LintArray {
  34         lint_array![MEM_DISCRIMINANT_NON_ENUM]
  35     }
  36
  37     fn name(&self) -> &'static str {
  38         "MemDiscriminant"
  39     }
  40 }
  41
  42 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemDiscriminant {
  43     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  44         if_chain! {
  45             if let ExprKind::Call(ref func, ref func_args) = expr.node;
  46             // is `mem::discriminant`
  47             if let ExprKind::Path(ref func_qpath) = func.node;
  48             if let Some(def_id) = cx.tables.qpath_def(func_qpath, func.hir_id).opt_def_id();
  49             if match_def_path(cx.tcx, def_id, &paths::MEM_DISCRIMINANT);
  50             // type is non-enum
  51             let ty_param = cx.tables.node_substs(func.hir_id).type_at(0);
  52             if !ty_param.is_enum();
  53
  54             then {
  55                 span_lint_and_then(
  56                     cx,
  57                     MEM_DISCRIMINANT_NON_ENUM,
  58                     expr.span,
  59                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  60                     |db| {
  61                         // if this is a reference to an enum, suggest dereferencing
  62                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  63                         if ptr_depth >= 1 && base_ty.is_enum() {
  64                             let param = &func_args[0];
  65
  66                             // cancel out '&'s first
  67                             let mut derefs_needed = ptr_depth;
  68                             let mut cur_expr = param;
  69                             while derefs_needed > 0  {
  70                                 if let ExprKind::AddrOf(_, ref inner_expr) = cur_expr.node {
  71                                     derefs_needed -= 1;
  72                                     cur_expr = inner_expr;
  73                                 } else {
  74                                     break;
  75                                 }
  76                             }
  77
  78                             let derefs: String = iter::repeat('*').take(derefs_needed).collect();
  79                             db.span_suggestion(
  80                                 param.span,
  81                                 "try dereferencing",
  82                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  83                                 Applicability::MachineApplicable,
  84                             );
  85                         }
  86                     },
  87                 )
  88             }
  89         }
  90     }
  91 }