//! lint when there is a large size difference between variants on an enum use crate::utils::{snippet_opt, span_lint_and_then}; use rustc::hir::*; use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass}; use rustc::ty::layout::LayoutOf; use rustc::{declare_tool_lint, impl_lint_pass}; use rustc_errors::Applicability; 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:** None. /// /// **Example:** /// ```rust /// enum Test { /// A(i32), /// B([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 { pub fn new(maximum_size_difference_allowed: u64) -> Self { Self { maximum_size_difference_allowed, } } } impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]); impl<'a, 'tcx> LateLintPass<'a, '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 smallest_variant: Option<(_, _)> = None; let mut largest_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)); update_if(&mut smallest_variant, grouped, |a, b| b.0 <= a.0); update_if(&mut largest_variant, grouped, |a, b| b.0 >= a.0); } if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) { let difference = largest.0 - smallest.0; if difference > self.maximum_size_difference_allowed { let (i, variant) = largest.1; span_lint_and_then( cx, LARGE_ENUM_VARIANT, def.variants[i].span, "large size difference between variants", |db| { 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) { db.span_suggestion( span, "consider boxing the large fields to reduce the total size of the \ enum", format!("Box<{}>", snip), Applicability::MaybeIncorrect, ); return; } } db.span_help( def.variants[i].span, "consider boxing the large fields to reduce the total size of the enum", ); }, ); } } } } } fn update_if(old: &mut Option, new: T, f: F) where F: Fn(&T, &T) -> bool, { if let Some(ref mut val) = *old { if f(val, &new) { *val = new; } } else { *old = Some(new); } }