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 use crate::rustc::hir::{Expr, ExprKind};
  11 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  12 use crate::rustc::{declare_tool_lint, lint_array};
  13 use crate::rustc_errors::Applicability;
  14 use crate::utils::{match_def_path, opt_def_id, paths, snippet, span_lint_and_then, walk_ptrs_ty_depth};
  15 use if_chain::if_chain;
  16
  17 use std::iter;
  18
  19 /// **What it does:** Checks for calls of `mem::discriminant()` on a non-enum type.
  20 ///
  21 /// **Why is this bad?** The value of `mem::discriminant()` on non-enum types
  22 /// is unspecified.
  23 ///
  24 /// **Known problems:** None.
  25 ///
  26 /// **Example:**
  27 /// ```rust
  28 /// mem::discriminant(&"hello");
  29 /// mem::discriminant(&&Some(2));
  30 /// ```
  31 declare_clippy_lint! {
  32     pub MEM_DISCRIMINANT_NON_ENUM,
  33     correctness,
  34     "calling mem::descriminant on non-enum type"
  35 }
  36
  37 pub struct MemDiscriminant;
  38
  39 impl LintPass for MemDiscriminant {
  40     fn get_lints(&self) -> LintArray {
  41         lint_array![MEM_DISCRIMINANT_NON_ENUM]
  42     }
  43 }
  44
  45 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MemDiscriminant {
  46     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  47         if_chain! {
  48             if let ExprKind::Call(ref func, ref func_args) = expr.node;
  49             // is `mem::discriminant`
  50             if let ExprKind::Path(ref func_qpath) = func.node;
  51             if let Some(def_id) = opt_def_id(cx.tables.qpath_def(func_qpath, func.hir_id));
  52             if match_def_path(cx.tcx, def_id, &paths::MEM_DISCRIMINANT);
  53             // type is non-enum
  54             let ty_param = cx.tables.node_substs(func.hir_id).type_at(0);
  55             if !ty_param.is_enum();
  56
  57             then {
  58                 span_lint_and_then(
  59                     cx,
  60                     MEM_DISCRIMINANT_NON_ENUM,
  61                     expr.span,
  62                     &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
  63                     |db| {
  64                         // if this is a reference to an enum, suggest dereferencing
  65                         let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
  66                         if ptr_depth >= 1 && base_ty.is_enum() {
  67                             let param = &func_args[0];
  68
  69                             // cancel out '&'s first
  70                             let mut derefs_needed = ptr_depth;
  71                             let mut cur_expr = param;
  72                             while derefs_needed > 0  {
  73                                 if let ExprKind::AddrOf(_, ref inner_expr) = cur_expr.node {
  74                                     derefs_needed -= 1;
  75                                     cur_expr = inner_expr;
  76                                 } else {
  77                                     break;
  78                                 }
  79                             }
  80
  81                             let derefs: String = iter::repeat('*').take(derefs_needed).collect();
  82                             db.span_suggestion_with_applicability(
  83                                 param.span,
  84                                 "try dereferencing",
  85                                 format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
  86                                 Applicability::MachineApplicable,
  87                             );
  88                         }
  89                     },
  90                 )
  91             }
  92         }
  93     }
  94 }