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
  11 //! lint when there is a large size difference between variants on an enum
  12
  13 use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
  14 use crate::rustc::{declare_tool_lint, lint_array};
  15 use crate::rustc::hir::*;
  16 use crate::utils::{snippet_opt, span_lint_and_then};
  17 use crate::rustc::ty::layout::LayoutOf;
  18 use crate::rustc_errors::Applicability;
  19
  20 /// **What it does:** Checks for large size differences between variants on
  21 /// `enum`s.
  22 ///
  23 /// **Why is this bad?** Enum size is bounded by the largest variant. Having a
  24 /// large variant
  25 /// can penalize the memory layout of that enum.
  26 ///
  27 /// **Known problems:** None.
  28 ///
  29 /// **Example:**
  30 /// ```rust
  31 /// enum Test {
  32 ///    A(i32),
  33 ///    B([i32; 8000]),
  34 /// }
  35 /// ```
  36 declare_clippy_lint! {
  37     pub LARGE_ENUM_VARIANT,
  38     perf,
  39     "large size difference between variants on an enum"
  40 }
  41
  42 #[derive(Copy, Clone)]
  43 pub struct LargeEnumVariant {
  44     maximum_size_difference_allowed: u64,
  45 }
  46
  47 impl LargeEnumVariant {
  48     pub fn new(maximum_size_difference_allowed: u64) -> Self {
  49         Self {
  50             maximum_size_difference_allowed,
  51         }
  52     }
  53 }
  54
  55 impl LintPass for LargeEnumVariant {
  56     fn get_lints(&self) -> LintArray {
  57         lint_array!(LARGE_ENUM_VARIANT)
  58     }
  59 }
  60
  61 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
  62     fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item) {
  63         let did = cx.tcx.hir.local_def_id(item.id);
  64         if let ItemKind::Enum(ref def, _) = item.node {
  65             let ty = cx.tcx.type_of(did);
  66             let adt = ty.ty_adt_def()
  67                 .expect("already checked whether this is an enum");
  68
  69             let mut smallest_variant: Option<(_, _)> = None;
  70             let mut largest_variant: Option<(_, _)> = None;
  71
  72             for (i, variant) in adt.variants.iter().enumerate() {
  73                 let size: u64 = variant
  74                     .fields
  75                     .iter()
  76                     .filter_map(|f| {
  77                         let ty = cx.tcx.type_of(f.did);
  78                         // don't count generics by filtering out everything
  79                         // that does not have a layout
  80                         cx.layout_of(ty).ok().map(|l| l.size.bytes())
  81                     })
  82                     .sum();
  83
  84                 let grouped = (size, (i, variant));
  85
  86                 update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0);
  87                 update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0);
  88             }
  89
  90             if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
  91                 let difference = largest.0 - smallest.0;
  92
  93                 if difference > self.maximum_size_difference_allowed {
  94                     let (i, variant) = largest.1;
  95
  96                     span_lint_and_then(
  97                         cx,
  98                         LARGE_ENUM_VARIANT,
  99                         def.variants[i].span,
 100                         "large size difference between variants",
 101                         |db| {
 102                             if variant.fields.len() == 1 {
 103                                 let span = match def.variants[i].node.data {
 104                                     VariantData::Struct(ref fields, _) | VariantData::Tuple(ref fields, _) => {
 105                                         fields[0].ty.span
 106                                     },
 107                                     VariantData::Unit(_) => unreachable!(),
 108                                 };
 109                                 if let Some(snip) = snippet_opt(cx, span) {
 110                                     db.span_suggestion_with_applicability(
 111                                         span,
 112                                         "consider boxing the large fields to reduce the total size of the \
 113                                          enum",
 114                                         format!("Box<{}>", snip),
 115                                         Applicability::MaybeIncorrect,
 116                                     );
 117                                     return;
 118                                 }
 119                             }
 120                             db.span_help(
 121                                 def.variants[i].span,
 122                                 "consider boxing the large fields to reduce the total size of the enum",
 123                             );
 124                         },
 125                     );
 126                 }
 127             }
 128         }
 129     }
 130 }
 131
 132 fn update_if<T, F>(old: &mut Option<T>, new: T, f: F)
 133 where
 134     F: Fn(&T, &T) -> bool,
 135 {
 136     if let Some(ref mut val) = *old {
 137         if f(val, &new) {
 138             *val = new;
 139         }
 140     } else {
 141         *old = Some(new);
 142     }
 143 }