Auto merge of #130803 - cuviper:file-buffered, r=joshtriplett

Add `File` constructors that return files wrapped with a buffer

In addition to the light convenience, these are intended to raise visibility that buffering is something you should consider when opening a file, since unbuffered I/O is a common performance footgun to Rust newcomers.

ACP: https://github.com/rust-lang/libs-team/issues/446
Tracking Issue: #130804

src/librustdoc/html/render/write_shared.rs

@@ -16,7 +16,7 @@
 use std::cell::RefCell;
 use std::ffi::OsString;
 use std::fs::File;
-use std::io::{self, BufWriter, Write as _};
+use std::io::{self, Write as _};
 use std::iter::once;
 use std::marker::PhantomData;
 use std::path::{Component, Path, PathBuf};
@@ -1020,8 +1020,7 @@ fn write_rendered_cci<T: CciPart, F>(
         for part in parts {
             template.append(part);
         }
-        let file = try_err!(File::create(&path), &path);
-        let mut file = BufWriter::new(file);
+        let mut file = try_err!(File::create_buffered(&path), &path);
         try_err!(write!(file, "{template}"), &path);
         try_err!(file.flush(), &path);
     }

src/librustdoc/json/mod.rs

@@ -286,7 +286,7 @@ fn after_krate(&mut self) -> Result<(), Error> {
 
             self.serialize_and_write(
                 output_crate,
-                BufWriter::new(try_err!(File::create(&p), p)),
+                try_err!(File::create_buffered(&p), p),
                 &p.display().to_string(),
             )
         } else {

src/librustdoc/lib.rs

@@ -5,6 +5,7 @@
 #![feature(rustc_private)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(impl_trait_in_assoc_type)]
 #![feature(iter_intersperse)]