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//! lint when there is a large size difference between variants on an enum
use clippy_utils::source::snippet_with_applicability;
use clippy_utils::{diagnostics::span_lint_and_then, ty::is_copy};
use rustc_errors::Applicability;
use rustc_hir::{Item, ItemKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::layout::LayoutOf;
use rustc_middle::ty::{Adt, Ty};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::source_map::Span;
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.
///
/// For types that implement `Copy`, the suggestion to `Box` a variant's
/// data would require removing the trait impl. The types can of course
/// still be `Clone`, but that is worse ergonomically. Depending on the
/// use case it may be possible to store the large data in an auxiliary
/// structure (e.g. Arena or ECS).
///
/// The lint will ignore generic types if the layout depends on the
/// generics, even if the size difference will be large anyway.
///
/// ### Example
/// ```rust
/// enum Test {
/// A(i32),
/// B([i32; 8000]),
/// }
/// ```
///
/// Use instead:
/// ```rust
/// // Possibly better
/// enum Test2 {
/// A(i32),
/// B(Box<[i32; 8000]>),
/// }
/// ```
#[clippy::version = "pre 1.29.0"]
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,
}
}
}
struct FieldInfo {
ind: usize,
size: u64,
}
struct VariantInfo {
ind: usize,
size: u64,
fields_size: Vec<FieldInfo>,
}
impl_lint_pass!(LargeEnumVariant => [LARGE_ENUM_VARIANT]);
impl<'tcx> LateLintPass<'tcx> for LargeEnumVariant {
fn check_item(&mut self, cx: &LateContext<'tcx>, item: &Item<'tcx>) {
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");
if adt.variants().len() <= 1 {
return;
}
let mut variants_size: Vec<VariantInfo> = Vec::new();
for (i, variant) in adt.variants().iter().enumerate() {
let mut fields_size = Vec::new();
for (i, f) in variant.fields.iter().enumerate() {
let ty = cx.tcx.type_of(f.did);
// don't lint variants which have a field of generic type.
match cx.layout_of(ty) {
Ok(l) => {
let fsize = l.size.bytes();
fields_size.push(FieldInfo { ind: i, size: fsize });
},
Err(_) => {
return;
},
}
}
let size: u64 = fields_size.iter().map(|info| info.size).sum();
variants_size.push(VariantInfo {
ind: i,
size,
fields_size,
});
}
variants_size.sort_by(|a, b| (b.size.cmp(&a.size)));
let mut difference = variants_size[0].size - variants_size[1].size;
if difference > self.maximum_size_difference_allowed {
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[variants_size[0].ind].span,
"large size difference between variants",
|diag| {
diag.span_label(
def.variants[variants_size[0].ind].span,
&format!("this variant is {} bytes", variants_size[0].size),
);
diag.span_note(
def.variants[variants_size[1].ind].span,
&format!("and the second-largest variant is {} bytes:", variants_size[1].size),
);
let fields = def.variants[variants_size[0].ind].data.fields();
variants_size[0].fields_size.sort_by(|a, b| (a.size.cmp(&b.size)));
let mut applicability = Applicability::MaybeIncorrect;
if is_copy(cx, ty) || maybe_copy(cx, ty) {
diag.span_note(
item.ident.span,
"boxing a variant would require the type no longer be `Copy`",
);
} else {
let sugg: Vec<(Span, String)> = variants_size[0]
.fields_size
.iter()
.rev()
.map_while(|val| {
if difference > self.maximum_size_difference_allowed {
difference = difference.saturating_sub(val.size);
Some((
fields[val.ind].ty.span,
format!(
"Box<{}>",
snippet_with_applicability(
cx,
fields[val.ind].ty.span,
"..",
&mut applicability
)
.into_owned()
),
))
} else {
None
}
})
.collect();
if !sugg.is_empty() {
diag.multipart_suggestion(help_text, sugg, Applicability::MaybeIncorrect);
return;
}
}
diag.span_help(def.variants[variants_size[0].ind].span, help_text);
},
);
}
}
}
}
fn maybe_copy<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
if let Adt(_def, substs) = ty.kind()
&& substs.types().next().is_some()
&& let Some(copy_trait) = cx.tcx.lang_items().copy_trait()
{
return cx.tcx.non_blanket_impls_for_ty(copy_trait, ty).next().is_some();
}
false
}
|
