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|
//! lint when there is a large size difference between variants on an enum
use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::snippet_with_applicability;
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_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.
///
/// ### Example
/// ```rust
/// // Bad
/// enum Test {
/// A(i32),
/// B([i32; 8000]),
/// }
///
/// // 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<'_>, 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");
if adt.variants.len() <= 1 {
return;
}
let mut variants_size: Vec<VariantInfo> = adt
.variants
.iter()
.enumerate()
.map(|(i, variant)| {
let mut fields_size = Vec::new();
let size: u64 = variant
.fields
.iter()
.enumerate()
.filter_map(|(i, 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| {
let size = l.size.bytes();
fields_size.push(FieldInfo { ind: i, size });
size
})
})
.sum();
VariantInfo {
ind: i,
size,
fields_size,
}
})
.collect();
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;
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);
},
);
}
}
}
}
|
