rust-clippy/clippy_lints/src/large_enum_variant.rs

//! lint when there is a large size difference between variants on an enum

use rustc::lint::*;
use rustc::hir::*;
use utils::{span_lint_and_then, snippet_opt, type_size};
use rustc::ty::TypeFoldable;

/// **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]),
/// }
/// ```
declare_lint! {
    pub LARGE_ENUM_VARIANT,
    Warn,
    "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 {
        LargeEnumVariant { maximum_size_difference_allowed: maximum_size_difference_allowed }
    }
}

impl LintPass for LargeEnumVariant {
    fn get_lints(&self) -> LintArray {
        lint_array!(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.id);
        if let ItemEnum(ref def, _) = item.node {
            let ty = cx.tcx.item_type(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()
                    .map(|f| {
                        let ty = cx.tcx.item_type(f.did);
                        if ty.needs_subst() {
                            0 // we can't reason about generics, so we treat them as zero sized
                        } else {
                            type_size(cx, ty).expect("size should be computable for concrete type")
                        }
                    })
                    .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].node.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));
                                return;
                            }
                        }
                        db.span_help(def.variants[i].span,
                                     "consider boxing the large fields to reduce the total size of the enum");
                    });
                }
            }

        }
    }
}

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);
    }
}