clippy_lints/src/large_enum_variant.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 //! lint when there is a large size difference between variants on an enum
  11
  12 use crate::utils::{snippet_opt, span_lint_and_then};
  13 use rustc::hir::*;
  14 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  15 use rustc::ty::layout::LayoutOf;
  16 use rustc::{declare_tool_lint, lint_array};
  17 use rustc_errors::Applicability;
  18
  19 /// **What it does:** Checks for large size differences between variants on
  20 /// `enum`s.
  21 ///
  22 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
  23 /// large variant
  24 /// can penalize the memory layout of that enum.
  25 ///
  26 /// **Known problems:** None.
  27 ///
  28 /// **Example:**
  29 /// ```rust
  30 /// enum Test {
  31 ///     A(i32),
  32 ///     B([i32; 8000]),
  33 /// }
  34 /// ```
  35 declare_clippy_lint! {
  36     pub LARGE_ENUM_VARIANT,
  37     perf,
  38     "large size difference between variants on an enum"
  39 }
  40
  41 #[derive(Copy, Clone)]
  42 pub struct LargeEnumVariant {
  43     maximum_size_difference_allowed: u64,
  44 }
  45
  46 impl LargeEnumVariant {
  47     pub fn new(maximum_size_difference_allowed: u64) -> Self {
  48         Self {
  49             maximum_size_difference_allowed,
  50         }
  51     }
  52 }
  53
  54 impl LintPass for LargeEnumVariant {
  55     fn get_lints(&self) -> LintArray {
  56         lint_array!(LARGE_ENUM_VARIANT)
  57     }
  58 }
  59
  60 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
  61     fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item) {
  62         let did = cx.tcx.hir().local_def_id(item.id);
  63         if let ItemKind::Enum(ref def, _) = item.node {
  64             let ty = cx.tcx.type_of(did);
  65             let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
  66
  67             let mut smallest_variant: Option<(_, _)> = None;
  68             let mut largest_variant: Option<(_, _)> = None;
  69
  70             for (i, variant) in adt.variants.iter().enumerate() {
  71                 let size: u64 = variant
  72                     .fields
  73                     .iter()
  74                     .filter_map(|f| {
  75                         let ty = cx.tcx.type_of(f.did);
  76                         // don't count generics by filtering out everything
  77                         // that does not have a layout
  78                         cx.layout_of(ty).ok().map(|l| l.size.bytes())
  79                     })
  80                     .sum();
  81
  82                 let grouped = (size, (i, variant));
  83
  84                 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  85                 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  86             }
  87
  88             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  89                 let difference = largest.0 - smallest.0;
  90
  91                 if difference > self.maximum_size_difference_allowed {
  92                     let (i, variant) = largest.1;
  93
  94                     span_lint_and_then(
  95                         cx,
  96                         LARGE_ENUM_VARIANT,
  97                         def.variants[i].span,
  98                         "large size difference between variants",
  99                         |db| {
 100                             if variant.fields.len() == 1 {
 101                                 let span = match def.variants[i].node.data {
 102                                     VariantData::Struct(ref fields, _) | VariantData::Tuple(ref fields, _) => {
 103                                         fields[0].ty.span
 104                                     },
 105                                     VariantData::Unit(_) => unreachable!(),
 106                                 };
 107                                 if let Some(snip) = snippet_opt(cx, span) {
 108                                     db.span_suggestion_with_applicability(
 109                                         span,
 110                                         "consider boxing the large fields to reduce the total size of the \
 111                                          enum",
 112                                         format!("Box<{}>", snip),
 113                                         Applicability::MaybeIncorrect,
 114                                     );
 115                                     return;
 116                                 }
 117                             }
 118                             db.span_help(
 119                                 def.variants[i].span,
 120                                 "consider boxing the large fields to reduce the total size of the enum",
 121                             );
 122                         },
 123                     );
 124                 }
 125             }
 126         }
 127     }
 128 }
 129
 130 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 131 where
 132     F: Fn(&T, &T) -> bool,
 133 {
 134     if let Some(ref mut val) = *old {
 135         if f(val, &new) {
 136             *val = new;
 137         }
 138     } else {
 139         *old = Some(new);
 140     }
 141 }