| //! lint when there is a large size difference between variants on an enum |
| |
| use crate::utils::{snippet_opt, span_lint_and_then}; |
| use rustc_errors::Applicability; |
| use rustc_hir::{Item, ItemKind, VariantData}; |
| use rustc_lint::{LateContext, LateLintPass}; |
| use rustc_session::{declare_tool_lint, impl_lint_pass}; |
| use rustc_target::abi::LayoutOf; |
| |
| declare_clippy_lint! { |
| /// **What it does:** Checks for large size differences between variants on |
| /// `enum`s. |
| /// |
| /// **Why is this bad?** Enum size is bounded by the largest variant. Having a |
| /// large variant can penalize the memory layout of that enum. |
| /// |
| /// **Known problems:** This lint obviously cannot take the distribution of |
| /// variants in your running program into account. It is possible that the |
| /// smaller variants make up less than 1% of all instances, in which case |
| /// the overhead is negligible and the boxing is counter-productive. Always |
| /// measure the change this lint suggests. |
| /// |
| /// **Example:** |
| /// |
| /// ```rust |
| /// // Bad |
| /// enum Test { |
| /// A(i32), |
| /// B([i32; 8000]), |
| /// } |
| /// |
| /// // Possibly better |
| /// enum Test2 { |
| /// A(i32), |
| /// B(Box<[i32; 8000]>), |
| /// } |
| /// ``` |
| pub LARGE_ENUM_VARIANT, |
| perf, |
| "large size difference between variants on an enum" |
| } |
| |
| #[derive(Copy, Clone)] |
| pub struct LargeEnumVariant { |
| maximum_size_difference_allowed: u64, |
| } |
| |
| impl LargeEnumVariant { |
| #[must_use] |
| pub fn new(maximum_size_difference_allowed: u64) -> Self { |
| Self { |
| maximum_size_difference_allowed, |
| } |
| } |
| } |
| |
| impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]); |
| |
| impl<'tcx> LateLintPass<'tcx> for LargeEnumVariant { |
| fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) { |
| let did = cx.tcx.hir().local_def_id(item.hir_id); |
| if let ItemKind::Enum(ref def, _) = item.kind { |
| let ty = cx.tcx.type_of(did); |
| let adt = ty.ty_adt_def().expect("already checked whether this is an enum"); |
| |
| let mut largest_variant: Option<(_, _)> = None; |
| let mut second_variant: Option<(_, _)> = None; |
| |
| for (i, variant) in adt.variants.iter().enumerate() { |
| let size: u64 = variant |
| .fields |
| .iter() |
| .filter_map(|f| { |
| let ty = cx.tcx.type_of(f.did); |
| // don't count generics by filtering out everything |
| // that does not have a layout |
| cx.layout_of(ty).ok().map(|l| l.size.bytes()) |
| }) |
| .sum(); |
| |
| let grouped = (size, (i, variant)); |
| |
| if grouped.0 >= largest_variant.map_or(0, |x| x.0) { |
| second_variant = largest_variant; |
| largest_variant = Some(grouped); |
| } |
| } |
| |
| if let (Some(largest), Some(second)) = (largest_variant, second_variant) { |
| let difference = largest.0 - second.0; |
| |
| if difference > self.maximum_size_difference_allowed { |
| let (i, variant) = largest.1; |
| |
| let help_text = "consider boxing the large fields to reduce the total size of the enum"; |
| span_lint_and_then( |
| cx, |
| LARGE_ENUM_VARIANT, |
| def.variants[i].span, |
| "large size difference between variants", |
| |diag| { |
| diag.span_label( |
| def.variants[(largest.1).0].span, |
| &format!("this variant is {} bytes", largest.0), |
| ); |
| diag.span_note( |
| def.variants[(second.1).0].span, |
| &format!("and the second-largest variant is {} bytes:", second.0), |
| ); |
| if variant.fields.len() == 1 { |
| let span = match def.variants[i].data { |
| VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => { |
| fields[0].ty.span |
| }, |
| VariantData::Unit(..) => unreachable!(), |
| }; |
| if let Some(snip) = snippet_opt(cx, span) { |
| diag.span_suggestion( |
| span, |
| help_text, |
| format!("Box<{}>", snip), |
| Applicability::MaybeIncorrect, |
| ); |
| return; |
| } |
| } |
| diag.span_help(def.variants[i].span, help_text); |
| }, |
| ); |
| } |
| } |
| } |
| } |
| } |