clippy_lints/src/large_enum_variant.rs

   1 //! lint when there is a large size difference between variants on an enum
   2
   3 use crate::utils::{snippet_opt, span_lint_and_then};
   4 use rustc::hir::*;
   5 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
   6 use rustc::ty::layout::LayoutOf;
   7 use rustc::{declare_tool_lint, lint_array};
   8 use rustc_errors::Applicability;
   9
  10 /// **What it does:** Checks for large size differences between variants on
  11 /// `enum`s.
  12 ///
  13 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
  14 /// large variant
  15 /// can penalize the memory layout of that enum.
  16 ///
  17 /// **Known problems:** None.
  18 ///
  19 /// **Example:**
  20 /// ```rust
  21 /// enum Test {
  22 ///     A(i32),
  23 ///     B([i32; 8000]),
  24 /// }
  25 /// ```
  26 declare_clippy_lint! {
  27     pub LARGE_ENUM_VARIANT,
  28     perf,
  29     "large size difference between variants on an enum"
  30 }
  31
  32 #[derive(Copy, Clone)]
  33 pub struct LargeEnumVariant {
  34     maximum_size_difference_allowed: u64,
  35 }
  36
  37 impl LargeEnumVariant {
  38     pub fn new(maximum_size_difference_allowed: u64) -> Self {
  39         Self {
  40             maximum_size_difference_allowed,
  41         }
  42     }
  43 }
  44
  45 impl LintPass for LargeEnumVariant {
  46     fn get_lints(&self) -> LintArray {
  47         lint_array!(LARGE_ENUM_VARIANT)
  48     }
  49 }
  50
  51 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
  52     fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item) {
  53         let did = cx.tcx.hir().local_def_id(item.id);
  54         if let ItemKind::Enum(ref def, _) = item.node {
  55             let ty = cx.tcx.type_of(did);
  56             let adt = ty.ty_adt_def().expect("already checked whether this is an enum");
  57
  58             let mut smallest_variant: Option<(_, _)> = None;
  59             let mut largest_variant: Option<(_, _)> = None;
  60
  61             for (i, variant) in adt.variants.iter().enumerate() {
  62                 let size: u64 = variant
  63                     .fields
  64                     .iter()
  65                     .filter_map(|f| {
  66                         let ty = cx.tcx.type_of(f.did);
  67                         // don't count generics by filtering out everything
  68                         // that does not have a layout
  69                         cx.layout_of(ty).ok().map(|l| l.size.bytes())
  70                     })
  71                     .sum();
  72
  73                 let grouped = (size, (i, variant));
  74
  75                 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  76                 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  77             }
  78
  79             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  80                 let difference = largest.0 - smallest.0;
  81
  82                 if difference > self.maximum_size_difference_allowed {
  83                     let (i, variant) = largest.1;
  84
  85                     span_lint_and_then(
  86                         cx,
  87                         LARGE_ENUM_VARIANT,
  88                         def.variants[i].span,
  89                         "large size difference between variants",
  90                         |db| {
  91                             if variant.fields.len() == 1 {
  92                                 let span = match def.variants[i].node.data {
  93                                     VariantData::Struct(ref fields, _) | VariantData::Tuple(ref fields, _) => {
  94                                         fields[0].ty.span
  95                                     },
  96                                     VariantData::Unit(_) => unreachable!(),
  97                                 };
  98                                 if let Some(snip) = snippet_opt(cx, span) {
  99                                     db.span_suggestion_with_applicability(
 100                                         span,
 101                                         "consider boxing the large fields to reduce the total size of the \
 102                                          enum",
 103                                         format!("Box<{}>", snip),
 104                                         Applicability::MaybeIncorrect,
 105                                     );
 106                                     return;
 107                                 }
 108                             }
 109                             db.span_help(
 110                                 def.variants[i].span,
 111                                 "consider boxing the large fields to reduce the total size of the enum",
 112                             );
 113                         },
 114                     );
 115                 }
 116             }
 117         }
 118     }
 119 }
 120
 121 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 122 where
 123     F: Fn(&T, &T) -> bool,
 124 {
 125     if let Some(ref mut val) = *old {
 126         if f(val, &new) {
 127             *val = new;
 128         }
 129     } else {
 130         *old = Some(new);
 131     }
 132 }