//! 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 clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::snippet_opt;
use rustc_errors::Applicability;
+use rustc_hir::{Item, ItemKind, VariantData};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::lint::in_external_macro;
+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.
+ /// large variant can penalize the memory layout of that enum.
///
- /// **Known problems:** None.
+ /// **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,
}
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<'a, 'tcx> LateLintPass<'a, 'tcx> for LargeEnumVariant {
- fn check_item(&mut self, cx: &LateContext<'_, '_>, item: &Item) {
- let did = cx.tcx.hir().local_def_id_from_hir_id(item.hir_id);
- if let ItemKind::Enum(ref def, _) = item.node {
- let ty = cx.tcx.type_of(did);
+impl<'tcx> LateLintPass<'tcx> for LargeEnumVariant {
+ fn check_item(&mut self, cx: &LateContext<'_>, item: &Item<'_>) {
+ if in_external_macro(cx.tcx.sess, item.span) {
+ return;
+ }
+ if let ItemKind::Enum(ref def, _) = item.kind {
+ let ty = cx.tcx.type_of(item.def_id);
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;
+ let mut second_variant: Option<(_, _)> = None;
for (i, variant) in adt.variants.iter().enumerate() {
let size: u64 = variant
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 grouped.0 >= largest_variant.map_or(0, |x| x.0) {
+ second_variant = largest_variant;
+ largest_variant = Some(grouped);
+ }
}
- if let (Some(smallest), Some(largest)) = (smallest_variant, largest_variant) {
- let difference = largest.0 - smallest.0;
+ 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",
- |db| {
+ |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].node.data {
- VariantData::Struct(ref fields, ..) | VariantData::Tuple(ref fields, ..) => {
+ let span = match def.variants[i].data {
+ VariantData::Struct(fields, ..) | VariantData::Tuple(fields, ..) => {
fields[0].ty.span
},
VariantData::Unit(..) => unreachable!(),
};
if let Some(snip) = snippet_opt(cx, span) {
- db.span_suggestion(
+ diag.span_suggestion(
span,
- "consider boxing the large fields to reduce the total size of the \
- enum",
+ help_text,
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",
- );
+ diag.span_help(def.variants[i].span, help_text);
},
);
}
}
}
}
-
-fn update_if<T, F>(old: &mut Option<T>, 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);
- }
-}
projects / rust.git / blobdiff