src/librustdoc/html/render/type_layout.rs

use askama::Template;

use rustc_data_structures::captures::Captures;
use rustc_hir::def_id::DefId;
use rustc_middle::span_bug;
use rustc_middle::ty::layout::LayoutError;
use rustc_middle::ty::Adt;
use rustc_span::symbol::Symbol;
use rustc_target::abi::{Primitive, TagEncoding, Variants};

use std::fmt;

use crate::html::format::display_fn;
use crate::html::render::Context;

#[derive(Template)]
#[template(path = "type_layout.html")]
struct TypeLayout<'cx> {
    variants: Vec<(Symbol, TypeLayoutSize)>,
    type_layout_size: Result<TypeLayoutSize, &'cx LayoutError<'cx>>,
}

#[derive(Template)]
#[template(path = "type_layout_size.html")]
struct TypeLayoutSize {
    is_unsized: bool,
    is_uninhabited: bool,
    size: u64,
}

pub(crate) fn document_type_layout<'a, 'cx: 'a>(
    cx: &'a Context<'cx>,
    ty_def_id: DefId,
) -> impl fmt::Display + 'a + Captures<'cx> {
    display_fn(move |f| {
        if !cx.shared.show_type_layout {
            return Ok(());
        }

        let tcx = cx.tcx();
        let param_env = tcx.param_env(ty_def_id);
        let ty = tcx.type_of(ty_def_id).instantiate_identity();
        let type_layout = tcx.layout_of(param_env.and(ty));

        let variants =
            if let Ok(type_layout) = type_layout &&
                let Variants::Multiple { variants, tag, tag_encoding, .. } =
                    type_layout.layout.variants() &&
                !variants.is_empty()
            {
                let tag_size =
                    if let TagEncoding::Niche { .. } = tag_encoding {
                        0
                    } else if let Primitive::Int(i, _) = tag.primitive() {
                        i.size().bytes()
                    } else {
                        span_bug!(tcx.def_span(ty_def_id), "tag is neither niche nor int")
                    };
                variants
                    .iter_enumerated()
                    .map(|(variant_idx, variant_layout)| {
                        let Adt(adt, _) = type_layout.ty.kind() else {
                            span_bug!(tcx.def_span(ty_def_id), "not an adt")
                        };
                        let name = adt.variant(variant_idx).name;
                        let is_unsized = variant_layout.abi.is_unsized();
                        let is_uninhabited = variant_layout.abi.is_uninhabited();
                        let size = variant_layout.size.bytes() - tag_size;
                        let type_layout_size = TypeLayoutSize { is_unsized, is_uninhabited, size };
                        (name, type_layout_size)
                    })
                    .collect()
            } else {
                Vec::new()
            };

        let type_layout_size = tcx.layout_of(param_env.and(ty)).map(|layout| {
            let is_unsized = layout.abi.is_unsized();
            let is_uninhabited = layout.abi.is_uninhabited();
            let size = layout.size.bytes();
            TypeLayoutSize { is_unsized, is_uninhabited, size }
        });

        Ok(TypeLayout { variants, type_layout_size }.render_into(f).unwrap())
    })
}