clippy_lints/src/mem_discriminant.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10
  11 use crate::rustc::hir::{Expr, ExprKind};
  12 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  13 use crate::rustc::{declare_tool_lint, lint_array};
  14 use crate::rustc_errors::Applicability;
  15 use crate::utils::{match_def_path, opt_def_id, paths, snippet, span_lint_and_then, walk_ptrs_ty_depth};
  16 use if_chain::if_chain;
  17
  18 use std::iter;
  19
  20 /// **What it does:** Checks for calls of `mem::discriminant()` on a non-enum type.
  21 ///
  22 /// **Why is this bad?** The value of `mem::discriminant()` on non-enum types
  23 /// is unspecified.
  24 ///
  25 /// **Known problems:** None.
  26 ///
  27 /// **Example:**
  28 /// ```rust
  29 /// mem::discriminant(&"hello");
  30 /// mem::discriminant(&&Some(2));
  31 /// ```
  32 declare_clippy_lint! {
  33     pub MEM_DISCRIMINANT_NON_ENUM,
  34     correctness,
  35     "calling mem::descriminant on non-enum type"
  36 }
  37
  38 pub struct MemDiscriminant;
  39
  40 impl LintPass for MemDiscriminant {
  41     fn get_lints(&self) -> LintArray {
  42         lint_array![MEM_DISCRIMINANT_NON_ENUM]
  43     }
  44 }
  45
  46 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemDiscriminant {
  47     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  48         if_chain! {
  49             if let ExprKind::Call(ref func, ref func_args) = expr.node;
  50             // is `mem::discriminant`
  51             if let ExprKind::Path(ref func_qpath) = func.node;
  52             if let Some(def_id) = opt_def_id(cx.tables.qpath_def(func_qpath, func.hir_id));
  53             if match_def_path(cx.tcx, def_id, &paths::MEM_DISCRIMINANT);
  54             // type is non-enum
  55             let ty_param = cx.tables.node_substs(func.hir_id).type_at(0);
  56             if !ty_param.is_enum();
  57
  58             then {
  59                 span_lint_and_then(
  60                     cx,
  61                     MEM_DISCRIMINANT_NON_ENUM,
  62                     expr.span,
  63                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  64                     |db| {
  65                         // if this is a reference to an enum, suggest dereferencing
  66                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  67                         if ptr_depth >= 1 && base_ty.is_enum() {
  68                             let param = &func_args[0];
  69
  70                             // cancel out '&'s first
  71                             let mut derefs_needed = ptr_depth;
  72                             let mut cur_expr = param;
  73                             while derefs_needed > 0  {
  74                                 if let ExprKind::AddrOf(_, ref inner_expr) = cur_expr.node {
  75                                     derefs_needed -= 1;
  76                                     cur_expr = inner_expr;
  77                                 } else {
  78                                     break;
  79                                 }
  80                             }
  81
  82                             let derefs: String = iter::repeat('*').take(derefs_needed).collect();
  83                             db.span_suggestion_with_applicability(
  84                                 param.span,
  85                                 "try dereferencing",
  86                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  87                                 Applicability::MachineApplicable,
  88                             );
  89                         }
  90                     },
  91                 )
  92             }
  93         }
  94     }
  95 }