Auto merge of rust-lang#137425 - yotamofek:pr/rustdoc/return-impl-display-redux, r=GuillaumeGomez

`librustdoc`: return `impl fmt::Display` in more places instead of writing to strings

Continuation of rust-lang#136784 , another attempt at landing the larger parts of rust-lang#136748 .
I'd like to, gradually, make all of the building blocks for rendering docs in `librustdoc` return `impl fmt::Display` instead of returning `Strings`, or receiving a `&mut String` (or `&mut impl fmt::Write`). Another smaller end goal is to be able to get rid of [`write_str`](https://github.com/rust-lang/rust/blob/8dac72bb1d12b2649acd0c190e41524f83da5683/src/librustdoc/html/format.rs#L40-L42).
This PR is a large step in that direction.

Most of the changes are quite mechanical, and split up into separate commits for easier reviewing (hopefully). I took `print_item` and then started by converting all the functions it called (and their dependencies), and the last commit does the conversion for `print_item` itself. Ignoring whitespace should make reviewing a bit easier.

And most importantly, perf run shows pretty good results locally, hopefully CI will also show green 😁

r? `@GuillaumeGomez` , if you feel like it.

Author: bors

src/librustdoc/html/format.rs

@@ -30,7 +30,7 @@ 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::display::{Joined as _, MaybeDisplay as _};
 use crate::formats::cache::Cache;
 use crate::formats::item_type::ItemType;
 use crate::html::escape::{Escape, EscapeBodyText};
@@ -178,12 +178,12 @@ pub(crate) fn print_where_clause<'a, 'tcx: 'a>(
     cx: &'a Context<'tcx>,
     indent: usize,
     ending: Ending,
-) -> impl Display + 'a + Captures<'tcx> {
-    fmt::from_fn(move |f| {
-        if gens.where_predicates.is_empty() {
-            return Ok(());
-        }
+) -> Option<impl Display + 'a + Captures<'tcx>> {
+    if gens.where_predicates.is_empty() {
+        return None;
+    }
 
+    Some(fmt::from_fn(move |f| {
         let where_preds = fmt::from_fn(|f| {
             gens.where_predicates
                 .iter()
@@ -246,7 +246,7 @@ pub(crate) fn print_where_clause<'a, 'tcx: 'a>(
             }
         };
         write!(f, "{clause}")
-    })
+    }))
 }
 
 impl clean::Lifetime {
@@ -1179,7 +1179,7 @@ impl clean::Impl {
                 self.print_type(&self.for_, f, use_absolute, cx)?;
             }
 
-            print_where_clause(&self.generics, cx, 0, Ending::Newline).fmt(f)
+            print_where_clause(&self.generics, cx, 0, Ending::Newline).maybe_display().fmt(f)
         })
     }
     fn print_type<'a, 'tcx: 'a>(

src/librustdoc/html/render/context.rs

@@ -237,8 +237,7 @@ impl<'tcx> Context<'tcx> {
         };
 
         if !render_redirect_pages {
-            let mut page_buffer = String::new();
-            print_item(self, it, &mut page_buffer);
+            let content = print_item(self, it);
             let page = layout::Page {
                 css_class: tyname_s,
                 root_path: &self.root_path(),
@@ -254,7 +253,7 @@ impl<'tcx> Context<'tcx> {
                 BufDisplay(|buf: &mut String| {
                     print_sidebar(self, it, buf);
                 }),
-                page_buffer,
+                content,
                 &self.shared.style_files,
             )
         } else {