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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::{approx_ty_size, is_copy, AdtVariantInfo},
};
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::{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 one
/// 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 the impact of generic types to the type layout by
/// assuming every type parameter is zero-sized. Depending on your use case,
/// this may lead to a false positive.
///
/// ### 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,
}
}
}
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.owner_id);
let Adt(adt, subst) = ty.kind() else {
panic!("already checked whether this is an enum")
};
if adt.variants().len() <= 1 {
return;
}
let variants_size = AdtVariantInfo::new(cx, *adt, subst);
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,
item.span,
"large size difference between variants",
|diag| {
diag.span_label(
item.span,
format!("the entire enum is at least {} bytes", approx_ty_size(cx, ty)),
);
diag.span_label(
def.variants[variants_size[0].ind].span,
format!("the largest variant contains at least {} bytes", variants_size[0].size),
);
diag.span_label(
def.variants[variants_size[1].ind].span,
if variants_size[1].fields_size.is_empty() {
"the second-largest variant carries no data at all".to_owned()
} else {
format!(
"the second-largest variant contains at least {} bytes",
variants_size[1].size
)
},
);
let fields = def.variants[variants_size[0].ind].data.fields();
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(|&(ind, size)| {
if difference > self.maximum_size_difference_allowed {
difference = difference.saturating_sub(size);
Some((
fields[ind].ty.span,
format!(
"Box<{}>",
snippet_with_applicability(
cx,
fields[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
}
|
