Auto merge of #136748 - yotamofek:pr/rustdoc-remove-buffer, r=<try>

Nuke `Buffer` abstraction from `librustdoc` 💣

In #136656  I found out that the `for_html` field in the `Buffer` struct was never read, and pondered if `Buffer` had any utility at all. `@GuillaumeGomez`  said he agrees that it can be just removed. So this PR is me removing it. So, r? `@GuillaumeGomez`

But...
this got *a lot* bigger than I had planned.
A lot of functions had a `&mut Buffer` arg, but instead of replacing it with a `buf: impl fmt::Write` arg, I decided to change those functions to return an opaque `impl fmt::Display` instead.

If this PR turns out to be contentious I can try to make a PR that just removes the `Buffer` struct and tries to make less invasive changes, but personally I do like some of the cleanups that this PR allows. Let's see what others think! I think it'll be better to review this without whitespace (If this gets positive reactions, I'll need to rebase and maybe try to separate this into logical commits, but not sure if that's very practical)

While most of the PR is "cosmetic", I did make some small changes, mostly trying to make some of the formatting lazier, and do less allocations. So a perf run will be nice :)

### Pros and cons of returning `impl fmt::Display` instead of taking a `impl fmt::Write`

#### Cons:
- Named lifetimes: function signatures got a lot more verbose because the RPIT opaque type needs to be explicitly bound by the lifetimes of the refs in the arguments
- Having to use `fmt::from_fn` causes another level of indentation
- Immutable closures, can't move out of non-`Copy` items (wasn't much of a problem in practice)

#### Pros:
- Less arguments, no un-Rusty "out" argument
- Nicer composability, allows the returned type to be directly used in format strings

### Interchangeability

A function receiving a `impl fmt::Write` can be turned into a function returning a `impl fmt::Display` by using `fmt::from_fn`.

Author: bors

src/librustdoc/html/format.rs

@@ -7,7 +7,6 @@
 //! some of them support an alternate format that emits text, but that should
 //! not be used external to this module.
 
-use std::borrow::Cow;
 use std::cmp::Ordering;
 use std::fmt::{self, Display, Write};
 use std::iter::{self, once};
@@ -37,115 +36,6 @@ use crate::html::render::Context;
 use crate::joined::Joined as _;
 use crate::passes::collect_intra_doc_links::UrlFragment;
 
-pub(crate) trait Print {
-    fn print(self, buffer: &mut Buffer);
-}
-
-impl<F> Print for F
-where
-    F: FnOnce(&mut Buffer),
-{
-    fn print(self, buffer: &mut Buffer) {
-        (self)(buffer)
-    }
-}
-
-impl Print for String {
-    fn print(self, buffer: &mut Buffer) {
-        buffer.write_str(&self);
-    }
-}
-
-impl Print for &'_ str {
-    fn print(self, buffer: &mut Buffer) {
-        buffer.write_str(self);
-    }
-}
-
-#[derive(Debug, Clone)]
-pub(crate) struct Buffer {
-    for_html: bool,
-    buffer: String,
-}
-
-impl core::fmt::Write for Buffer {
-    #[inline]
-    fn write_str(&mut self, s: &str) -> fmt::Result {
-        self.buffer.write_str(s)
-    }
-
-    #[inline]
-    fn write_char(&mut self, c: char) -> fmt::Result {
-        self.buffer.write_char(c)
-    }
-
-    #[inline]
-    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> fmt::Result {
-        self.buffer.write_fmt(args)
-    }
-}
-
-impl Buffer {
-    pub(crate) fn empty_from(v: &Buffer) -> Buffer {
-        Buffer { for_html: v.for_html, buffer: String::new() }
-    }
-
-    pub(crate) fn html() -> Buffer {
-        Buffer { for_html: true, buffer: String::new() }
-    }
-
-    pub(crate) fn new() -> Buffer {
-        Buffer { for_html: false, buffer: String::new() }
-    }
-
-    pub(crate) fn is_empty(&self) -> bool {
-        self.buffer.is_empty()
-    }
-
-    pub(crate) fn into_inner(self) -> String {
-        self.buffer
-    }
-
-    pub(crate) fn push(&mut self, c: char) {
-        self.buffer.push(c);
-    }
-
-    pub(crate) fn push_str(&mut self, s: &str) {
-        self.buffer.push_str(s);
-    }
-
-    pub(crate) fn push_buffer(&mut self, other: Buffer) {
-        self.buffer.push_str(&other.buffer);
-    }
-
-    // Intended for consumption by write! and writeln! (std::fmt) but without
-    // the fmt::Result return type imposed by fmt::Write (and avoiding the trait
-    // import).
-    pub(crate) fn write_str(&mut self, s: &str) {
-        self.buffer.push_str(s);
-    }
-
-    // Intended for consumption by write! and writeln! (std::fmt) but without
-    // the fmt::Result return type imposed by fmt::Write (and avoiding the trait
-    // import).
-    pub(crate) fn write_fmt(&mut self, v: fmt::Arguments<'_>) {
-        self.buffer.write_fmt(v).unwrap();
-    }
-
-    pub(crate) fn to_display<T: Print>(mut self, t: T) -> String {
-        t.print(&mut self);
-        self.into_inner()
-    }
-
-    pub(crate) fn reserve(&mut self, additional: usize) {
-        self.buffer.reserve(additional)
-    }
-
-    pub(crate) fn len(&self) -> usize {
-        self.buffer.len()
-    }
-}
-
 pub(crate) fn print_generic_bounds<'a, 'tcx: 'a>(
     bounds: &'a [clean::GenericBound],
     cx: &'a Context<'tcx>,
@@ -772,27 +662,29 @@ pub(crate) fn link_tooltip(did: DefId, fragment: &Option<UrlFragment>, cx: &Cont
     else {
         return String::new();
     };
-    let mut buf = Buffer::new();
     let fqp = if *shortty == ItemType::Primitive {
         // primitives are documented in a crate, but not actually part of it
         &fqp[fqp.len() - 1..]
     } else {
         fqp
     };
-    if let &Some(UrlFragment::Item(id)) = fragment {
-        write!(buf, "{} ", cx.tcx().def_descr(id));
-        for component in fqp {
-            write!(buf, "{component}::");
-        }
-        write!(buf, "{}", cx.tcx().item_name(id));
-    } else if !fqp.is_empty() {
-        let mut fqp_it = fqp.iter();
-        write!(buf, "{shortty} {}", fqp_it.next().unwrap());
-        for component in fqp_it {
-            write!(buf, "::{component}");
+    fmt::from_fn(|f| {
+        if let &Some(UrlFragment::Item(id)) = fragment {
+            write!(f, "{} ", cx.tcx().def_descr(id))?;
+            for component in fqp {
+                write!(f, "{component}::")?;
+            }
+            write!(f, "{}", cx.tcx().item_name(id))?;
+        } else if !fqp.is_empty() {
+            let mut fqp_it = fqp.iter();
+            write!(f, "{shortty} {}", fqp_it.next().unwrap())?;
+            for component in fqp_it {
+                write!(f, "::{component}")?;
+            }
         }
-    }
-    buf.into_inner()
+        Ok(())
+    })
+    .to_string()
 }
 
 /// Used to render a [`clean::Path`].
@@ -954,7 +846,7 @@ pub(crate) fn anchor<'a: 'cx, 'cx>(
                 text = EscapeBodyText(text.as_str()),
             )
         } else {
-            f.write_str(text.as_str())
+            write!(f, "{text}")
         }
     })
 }
@@ -1533,53 +1425,51 @@ impl clean::FnDecl {
 }
 
 pub(crate) fn visibility_print_with_space<'a, 'tcx: 'a>(
-    item: &clean::Item,
+    item: &'a clean::Item,
     cx: &'a Context<'tcx>,
 ) -> impl Display + 'a + Captures<'tcx> {
-    use std::fmt::Write as _;
-    let vis: Cow<'static, str> = match item.visibility(cx.tcx()) {
-        None => "".into(),
-        Some(ty::Visibility::Public) => "pub ".into(),
-        Some(ty::Visibility::Restricted(vis_did)) => {
-            // FIXME(camelid): This may not work correctly if `item_did` is a module.
-            //                 However, rustdoc currently never displays a module's
-            //                 visibility, so it shouldn't matter.
-            let parent_module = find_nearest_parent_module(cx.tcx(), item.item_id.expect_def_id());
-
-            if vis_did.is_crate_root() {
-                "pub(crate) ".into()
-            } else if parent_module == Some(vis_did) {
-                // `pub(in foo)` where `foo` is the parent module
-                // is the same as no visibility modifier
-                "".into()
-            } else if parent_module.and_then(|parent| find_nearest_parent_module(cx.tcx(), parent))
-                == Some(vis_did)
-            {
-                "pub(super) ".into()
-            } else {
-                let path = cx.tcx().def_path(vis_did);
-                debug!("path={path:?}");
-                // modified from `resolved_path()` to work with `DefPathData`
-                let last_name = path.data.last().unwrap().data.get_opt_name().unwrap();
-                let anchor = anchor(vis_did, last_name, cx);
-
-                let mut s = "pub(in ".to_owned();
-                for seg in &path.data[..path.data.len() - 1] {
-                    let _ = write!(s, "{}::", seg.data.get_opt_name().unwrap());
-                }
-                let _ = write!(s, "{anchor}) ");
-                s.into()
-            }
-        }
-    };
-
     let is_doc_hidden = item.is_doc_hidden();
     fmt::from_fn(move |f| {
         if is_doc_hidden {
             f.write_str("#[doc(hidden)] ")?;
         }
 
-        f.write_str(&vis)
+        match item.visibility(cx.tcx()) {
+            None => Ok(()),
+            Some(ty::Visibility::Public) => f.write_str("pub "),
+            Some(ty::Visibility::Restricted(vis_did)) => {
+                // FIXME(camelid): This may not work correctly if `item_did` is a module.
+                //                 However, rustdoc currently never displays a module's
+                //                 visibility, so it shouldn't matter.
+                let parent_module =
+                    find_nearest_parent_module(cx.tcx(), item.item_id.expect_def_id());
+
+                if vis_did.is_crate_root() {
+                    f.write_str("pub(crate) ")
+                } else if parent_module == Some(vis_did) {
+                    // `pub(in foo)` where `foo` is the parent module
+                    // is the same as no visibility modifier
+                    Ok(())
+                } else if parent_module
+                    .and_then(|parent| find_nearest_parent_module(cx.tcx(), parent))
+                    == Some(vis_did)
+                {
+                    f.write_str("pub(super) ")
+                } else {
+                    let path = cx.tcx().def_path(vis_did);
+                    debug!("path={path:?}");
+                    // modified from `resolved_path()` to work with `DefPathData`
+                    let last_name = path.data.last().unwrap().data.get_opt_name().unwrap();
+                    let anchor = anchor(vis_did, last_name, cx);
+
+                    f.write_str("pub(in ")?;
+                    for seg in &path.data[..path.data.len() - 1] {
+                        write!(f, "{}::", seg.data.get_opt_name().unwrap())?;
+                    }
+                    write!(f, "{anchor}) ")
+                }
+            }
+        }
     })
 }