Do more lazy-formatting in librustdoc 🦥

src/librustdoc/clean/cfg.rs

@@ -13,8 +13,8 @@ use rustc_session::parse::ParseSess;
 use rustc_span::Span;
 use rustc_span::symbol::{Symbol, sym};
 
+use crate::display::Joined as _;
 use crate::html::escape::Escape;
-use crate::joined::Joined as _;
 
 #[cfg(test)]
 mod tests;

src/librustdoc/joined.rs → src/librustdoc/display.rs

@@ -1,3 +1,5 @@
+//! Various utilities for working with [`fmt::Display`] implementations.
+
 use std::fmt::{self, Display, Formatter};
 
 pub(crate) trait Joined: IntoIterator {
@@ -27,3 +29,19 @@ where
         Ok(())
     }
 }
+
+pub(crate) trait MaybeDisplay {
+    /// For a given `Option<T: Display>`, returns a `Display` implementation that will display `t` if `Some(t)`, or nothing if `None`.
+    fn maybe_display(self) -> impl Display;
+}
+
+impl<T: Display> MaybeDisplay for Option<T> {
+    fn maybe_display(self) -> impl Display {
+        fmt::from_fn(move |f| {
+            if let Some(t) = self.as_ref() {
+                t.fmt(f)?;
+            }
+            Ok(())
+        })
+    }
+}

src/librustdoc/html/format.rs

@@ -30,11 +30,11 @@ use super::url_parts_builder::{UrlPartsBuilder, estimate_item_path_byte_length};
 use crate::clean::types::ExternalLocation;
 use crate::clean::utils::find_nearest_parent_module;
 use crate::clean::{self, ExternalCrate, PrimitiveType};
+use crate::display::Joined as _;
 use crate::formats::cache::Cache;
 use crate::formats::item_type::ItemType;
 use crate::html::escape::{Escape, EscapeBodyText};
 use crate::html::render::Context;
-use crate::joined::Joined as _;
 use crate::passes::collect_intra_doc_links::UrlFragment;
 
 pub(crate) fn write_str(s: &mut String, f: fmt::Arguments<'_>) {
@@ -709,19 +709,22 @@ fn resolved_path(
     if w.alternate() {
         write!(w, "{}{:#}", last.name, last.args.print(cx))?;
     } else {
-        let path = if use_absolute {
-            if let Ok((_, _, fqp)) = href(did, cx) {
-                format!(
-                    "{path}::{anchor}",
-                    path = join_with_double_colon(&fqp[..fqp.len() - 1]),
-                    anchor = anchor(did, *fqp.last().unwrap(), cx)
-                )
+        let path = fmt::from_fn(|f| {
+            if use_absolute {
+                if let Ok((_, _, fqp)) = href(did, cx) {
+                    write!(
+                        f,
+                        "{path}::{anchor}",
+                        path = join_with_double_colon(&fqp[..fqp.len() - 1]),
+                        anchor = anchor(did, *fqp.last().unwrap(), cx)
+                    )
+                } else {
+                    write!(f, "{}", last.name)
+                }
             } else {
-                last.name.to_string()
+                write!(f, "{}", anchor(did, last.name, cx))
             }
-        } else {
-            anchor(did, last.name, cx).to_string()
-        };
+        });
         write!(w, "{path}{args}", args = last.args.print(cx))?;
     }
     Ok(())
@@ -749,16 +752,20 @@ fn primitive_link_fragment(
         match m.primitive_locations.get(&prim) {
             Some(&def_id) if def_id.is_local() => {
                 let len = cx.current.len();
-                let path = if len == 0 {
-                    let cname_sym = ExternalCrate { crate_num: def_id.krate }.name(cx.tcx());
-                    format!("{cname_sym}/")
-                } else {
-                    "../".repeat(len - 1)
-                };
+                let path = fmt::from_fn(|f| {
+                    if len == 0 {
+                        let cname_sym = ExternalCrate { crate_num: def_id.krate }.name(cx.tcx());
+                        write!(f, "{cname_sym}/")?;
+                    } else {
+                        for _ in 0..(len - 1) {
+                            f.write_str("../")?;
+                        }
+                    }
+                    Ok(())
+                });
                 write!(
                     f,
-                    "<a class=\"primitive\" href=\"{}primitive.{}.html{fragment}\">",
-                    path,
+                    "<a class=\"primitive\" href=\"{path}primitive.{}.html{fragment}\">",
                     prim.as_sym()
                 )?;
                 needs_termination = true;

src/librustdoc/html/render/context.rs

@@ -1,8 +1,9 @@
 use std::cell::RefCell;
 use std::collections::BTreeMap;
+use std::fmt::{self, Write as _};
+use std::io;
 use std::path::{Path, PathBuf};
 use std::sync::mpsc::{Receiver, channel};
-use std::{fmt, io};
 
 use rinja::Template;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
@@ -265,21 +266,25 @@ impl<'tcx> Context<'tcx> {
                     // preventing an infinite redirection loop in the generated
                     // documentation.
 
-                    let mut path = String::new();
-                    for name in &names[..names.len() - 1] {
-                        path.push_str(name.as_str());
-                        path.push('/');
-                    }
-                    path.push_str(&item_path(ty, names.last().unwrap().as_str()));
+                    let path = fmt::from_fn(|f| {
+                        for name in &names[..names.len() - 1] {
+                            write!(f, "{name}/")?;
+                        }
+                        write!(f, "{}", item_path(ty, names.last().unwrap().as_str()))
+                    });
                     match self.shared.redirections {
                         Some(ref redirections) => {
                             let mut current_path = String::new();
                             for name in &self.current {
                                 current_path.push_str(name.as_str());
                                 current_path.push('/');
                             }
-                            current_path.push_str(&item_path(ty, names.last().unwrap().as_str()));
-                            redirections.borrow_mut().insert(current_path, path);
+                            let _ = write!(
+                                current_path,
+                                "{}",
+                                item_path(ty, names.last().unwrap().as_str())
+                            );
+                            redirections.borrow_mut().insert(current_path, path.to_string());
                         }
                         None => {
                             return layout::redirect(&format!(
@@ -854,9 +859,9 @@ impl<'tcx> FormatRenderer<'tcx> for Context<'tcx> {
         if !buf.is_empty() {
             let name = item.name.as_ref().unwrap();
             let item_type = item.type_();
-            let file_name = &item_path(item_type, name.as_str());
+            let file_name = item_path(item_type, name.as_str()).to_string();
             self.shared.ensure_dir(&self.dst)?;
-            let joint_dst = self.dst.join(file_name);
+            let joint_dst = self.dst.join(&file_name);
             self.shared.fs.write(joint_dst, buf)?;
 
             if !self.info.render_redirect_pages {
@@ -873,7 +878,7 @@ impl<'tcx> FormatRenderer<'tcx> for Context<'tcx> {
                         format!("{crate_name}/{file_name}"),
                     );
                 } else {
-                    let v = layout::redirect(file_name);
+                    let v = layout::redirect(&file_name);
                     let redir_dst = self.dst.join(redir_name);
                     self.shared.fs.write(redir_dst, v)?;
                 }

src/librustdoc/html/render/mod.rs

@@ -63,6 +63,7 @@ pub(crate) use self::context::*;
 pub(crate) use self::span_map::{LinkFromSrc, collect_spans_and_sources};
 pub(crate) use self::write_shared::*;
 use crate::clean::{self, ItemId, RenderedLink};
+use crate::display::{Joined as _, MaybeDisplay as _};
 use crate::error::Error;
 use crate::formats::Impl;
 use crate::formats::cache::Cache;
@@ -568,17 +569,27 @@ fn document_short<'a, 'cx: 'a>(
             let (mut summary_html, has_more_content) =
                 MarkdownSummaryLine(&s, &item.links(cx)).into_string_with_has_more_content();
 
-            if has_more_content {
-                let link = format!(" <a{}>Read more</a>", assoc_href_attr(item, link, cx));
+            let link = if has_more_content {
+                let link = fmt::from_fn(|f| {
+                    write!(
+                        f,
+                        " <a{}>Read more</a>",
+                        assoc_href_attr(item, link, cx).maybe_display()
+                    )
+                });
 
                 if let Some(idx) = summary_html.rfind("</p>") {
-                    summary_html.insert_str(idx, &link);
+                    summary_html.insert_str(idx, &link.to_string());
+                    None
                 } else {
-                    summary_html.push_str(&link);
+                    Some(link)
                 }
+            } else {
+                None
             }
+            .maybe_display();
 
-            write!(f, "<div class='docblock'>{summary_html}</div>")?;
+            write!(f, "<div class='docblock'>{summary_html}{link}</div>")?;
         }
         Ok(())
     })
@@ -788,13 +799,23 @@ pub(crate) fn render_impls(
 }
 
 /// Build a (possibly empty) `href` attribute (a key-value pair) for the given associated item.
-fn assoc_href_attr(it: &clean::Item, link: AssocItemLink<'_>, cx: &Context<'_>) -> String {
+fn assoc_href_attr<'a, 'tcx>(
+    it: &clean::Item,
+    link: AssocItemLink<'a>,
+    cx: &Context<'tcx>,
+) -> Option<impl fmt::Display + 'a + Captures<'tcx>> {
     let name = it.name.unwrap();
     let item_type = it.type_();
 
+    enum Href<'a> {
+        AnchorId(&'a str),
+        Anchor(ItemType),
+        Url(String, ItemType),
+    }
+
     let href = match link {
-        AssocItemLink::Anchor(Some(ref id)) => Some(format!("#{id}")),
-        AssocItemLink::Anchor(None) => Some(format!("#{item_type}.{name}")),
+        AssocItemLink::Anchor(Some(ref id)) => Href::AnchorId(id),
+        AssocItemLink::Anchor(None) => Href::Anchor(item_type),
         AssocItemLink::GotoSource(did, provided_methods) => {
             // We're creating a link from the implementation of an associated item to its
             // declaration in the trait declaration.
@@ -814,7 +835,7 @@ fn assoc_href_attr(it: &clean::Item, link: AssocItemLink<'_>, cx: &Context<'_>)
             };
 
             match href(did.expect_def_id(), cx) {
-                Ok((url, ..)) => Some(format!("{url}#{item_type}.{name}")),
+                Ok((url, ..)) => Href::Url(url, item_type),
                 // The link is broken since it points to an external crate that wasn't documented.
                 // Do not create any link in such case. This is better than falling back to a
                 // dummy anchor like `#{item_type}.{name}` representing the `id` of *this* impl item
@@ -826,15 +847,25 @@ fn assoc_href_attr(it: &clean::Item, link: AssocItemLink<'_>, cx: &Context<'_>)
                 // those two items are distinct!
                 // In this scenario, the actual `id` of this impl item would be
                 // `#{item_type}.{name}-{n}` for some number `n` (a disambiguator).
-                Err(HrefError::DocumentationNotBuilt) => None,
-                Err(_) => Some(format!("#{item_type}.{name}")),
+                Err(HrefError::DocumentationNotBuilt) => return None,
+                Err(_) => Href::Anchor(item_type),
             }
         }
     };
 
+    let href = fmt::from_fn(move |f| match &href {
+        Href::AnchorId(id) => write!(f, "#{id}"),
+        Href::Url(url, item_type) => {
+            write!(f, "{url}#{item_type}.{name}")
+        }
+        Href::Anchor(item_type) => {
+            write!(f, "#{item_type}.{name}")
+        }
+    });
+
     // If there is no `href` for the reason explained above, simply do not render it which is valid:
     // https://html.spec.whatwg.org/multipage/links.html#links-created-by-a-and-area-elements
-    href.map(|href| format!(" href=\"{href}\"")).unwrap_or_default()
+    Some(fmt::from_fn(move |f| write!(f, " href=\"{href}\"")))
 }
 
 #[derive(Debug)]
@@ -865,7 +896,7 @@ fn assoc_const(
             "{indent}{vis}const <a{href} class=\"constant\">{name}</a>{generics}: {ty}",
             indent = " ".repeat(indent),
             vis = visibility_print_with_space(it, cx),
-            href = assoc_href_attr(it, link, cx),
+            href = assoc_href_attr(it, link, cx).maybe_display(),
             name = it.name.as_ref().unwrap(),
             generics = generics.print(cx),
             ty = ty.print(cx),
@@ -905,7 +936,7 @@ fn assoc_type(
             "{indent}{vis}type <a{href} class=\"associatedtype\">{name}</a>{generics}",
             indent = " ".repeat(indent),
             vis = visibility_print_with_space(it, cx),
-            href = assoc_href_attr(it, link, cx),
+            href = assoc_href_attr(it, link, cx).maybe_display(),
             name = it.name.as_ref().unwrap(),
             generics = generics.print(cx),
         ),
@@ -948,7 +979,7 @@ fn assoc_method(
     let asyncness = header.asyncness.print_with_space();
     let safety = header.safety.print_with_space();
     let abi = print_abi_with_space(header.abi).to_string();
-    let href = assoc_href_attr(meth, link, cx);
+    let href = assoc_href_attr(meth, link, cx).maybe_display();
 
     // NOTE: `{:#}` does not print HTML formatting, `{}` does. So `g.print` can't be reused between the length calculation and `write!`.
     let generics_len = format!("{:#}", g.print(cx)).len();
@@ -962,7 +993,7 @@ fn assoc_method(
         + name.as_str().len()
         + generics_len;
 
-    let notable_traits = notable_traits_button(&d.output, cx);
+    let notable_traits = notable_traits_button(&d.output, cx).maybe_display();
 
     let (indent, indent_str, end_newline) = if parent == ItemType::Trait {
         header_len += 4;
@@ -990,7 +1021,6 @@ fn assoc_method(
             name = name,
             generics = g.print(cx),
             decl = d.full_print(header_len, indent, cx),
-            notable_traits = notable_traits.unwrap_or_default(),
             where_clause = print_where_clause(g, cx, indent, end_newline),
         ),
     );
@@ -1438,7 +1468,10 @@ fn should_render_item(item: &clean::Item, deref_mut_: bool, tcx: TyCtxt<'_>) ->
     }
 }
 
-pub(crate) fn notable_traits_button(ty: &clean::Type, cx: &Context<'_>) -> Option<String> {
+pub(crate) fn notable_traits_button<'a, 'tcx>(
+    ty: &'a clean::Type,
+    cx: &'a Context<'tcx>,
+) -> Option<impl fmt::Display + 'a + Captures<'tcx>> {
     let mut has_notable_trait = false;
 
     if ty.is_unit() {
@@ -1480,15 +1513,16 @@ pub(crate) fn notable_traits_button(ty: &clean::Type, cx: &Context<'_>) -> Optio
         }
     }
 
-    if has_notable_trait {
+    has_notable_trait.then(|| {
         cx.types_with_notable_traits.borrow_mut().insert(ty.clone());
-        Some(format!(
-            " <a href=\"#\" class=\"tooltip\" data-notable-ty=\"{ty}\">ⓘ</a>",
-            ty = Escape(&format!("{:#}", ty.print(cx))),
-        ))
-    } else {
-        None
-    }
+        fmt::from_fn(|f| {
+            write!(
+                f,
+                " <a href=\"#\" class=\"tooltip\" data-notable-ty=\"{ty}\">ⓘ</a>",
+                ty = Escape(&format!("{:#}", ty.print(cx))),
+            )
+        })
+    })
 }
 
 fn notable_traits_decl(ty: &clean::Type, cx: &Context<'_>) -> (String, String) {
@@ -2117,11 +2151,11 @@ pub(crate) fn render_impl_summary(
 ) {
     let inner_impl = i.inner_impl();
     let id = cx.derive_id(get_id_for_impl(cx.tcx(), i.impl_item.item_id));
-    let aliases = if aliases.is_empty() {
-        String::new()
-    } else {
-        format!(" data-aliases=\"{}\"", aliases.join(","))
-    };
+    let aliases = (!aliases.is_empty())
+        .then_some(fmt::from_fn(|f| {
+            write!(f, " data-aliases=\"{}\"", fmt::from_fn(|f| aliases.iter().joined(",", f)))
+        }))
+        .maybe_display();
     write_str(w, format_args!("<section id=\"{id}\" class=\"impl\"{aliases}>"));
     render_rightside(w, cx, &i.impl_item, RenderMode::Normal);
     write_str(