Auto merge of rust-lang#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: rust-lang/libs-team#446
Tracking Issue: rust-lang#130804

Author: bors

Diff for: compiler/rustc_borrowck/src/facts.rs

@@ -1,7 +1,7 @@
 use std::error::Error;
 use std::fmt::Debug;
 use std::fs::{self, File};
-use std::io::{BufWriter, Write};
+use std::io::Write;
 use std::path::Path;
 
 use polonius_engine::{AllFacts as PoloniusFacts, Atom};
@@ -127,7 +127,7 @@ impl<'w> FactWriter<'w> {
         T: FactRow,
     {
         let file = &self.dir.join(file_name);
-        let mut file = BufWriter::new(File::create(file)?);
+        let mut file = File::create_buffered(file)?;
         for row in rows {
             row.write(&mut file, self.location_table)?;
         }

Diff for: compiler/rustc_borrowck/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
+#![feature(file_buffered)]
 #![feature(let_chains)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]

Diff for: compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -808,8 +808,7 @@ struct ThinLTOKeysMap {
 impl ThinLTOKeysMap {
     fn save_to_file(&self, path: &Path) -> io::Result<()> {
         use std::io::Write;
-        let file = File::create(path)?;
-        let mut writer = io::BufWriter::new(file);
+        let mut writer = File::create_buffered(path)?;
         // The entries are loaded back into a hash map in `load_from_file()`, so
         // the order in which we write them to file here does not matter.
         for (module, key) in &self.keys {
@@ -821,8 +820,8 @@ impl ThinLTOKeysMap {
     fn load_from_file(path: &Path) -> io::Result<Self> {
         use std::io::BufRead;
         let mut keys = BTreeMap::default();
-        let file = File::open(path)?;
-        for line in io::BufReader::new(file).lines() {
+        let file = File::open_buffered(path)?;
+        for line in file.lines() {
             let line = line?;
             let mut split = line.split(' ');
             let module = split.next().unwrap();

Diff for: compiler/rustc_codegen_llvm/src/lib.rs

@@ -11,6 +11,7 @@
 #![feature(assert_matches)]
 #![feature(exact_size_is_empty)]
 #![feature(extern_types)]
+#![feature(file_buffered)]
 #![feature(hash_raw_entry)]
 #![feature(impl_trait_in_assoc_type)]
 #![feature(iter_intersperse)]

Diff for: compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1,9 +1,8 @@
 use std::ffi::{OsStr, OsString};
 use std::fs::{self, File};
 use std::io::prelude::*;
-use std::io::{self, BufWriter};
 use std::path::{Path, PathBuf};
-use std::{env, iter, mem, str};
+use std::{env, io, iter, mem, str};
 
 use cc::windows_registry;
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
@@ -754,7 +753,7 @@ impl<'a> Linker for GccLinker<'a> {
         if self.sess.target.is_like_osx {
             // Write a plain, newline-separated list of symbols
             let res: io::Result<()> = try {
-                let mut f = BufWriter::new(File::create(&path)?);
+                let mut f = File::create_buffered(&path)?;
                 for sym in symbols {
                     debug!("  _{sym}");
                     writeln!(f, "_{sym}")?;
@@ -765,7 +764,7 @@ impl<'a> Linker for GccLinker<'a> {
             }
         } else if is_windows {
             let res: io::Result<()> = try {
-                let mut f = BufWriter::new(File::create(&path)?);
+                let mut f = File::create_buffered(&path)?;
 
                 // .def file similar to MSVC one but without LIBRARY section
                 // because LD doesn't like when it's empty
@@ -781,7 +780,7 @@ impl<'a> Linker for GccLinker<'a> {
         } else {
             // Write an LD version script
             let res: io::Result<()> = try {
-                let mut f = BufWriter::new(File::create(&path)?);
+                let mut f = File::create_buffered(&path)?;
                 writeln!(f, "{{")?;
                 if !symbols.is_empty() {
                     writeln!(f, "  global:")?;
@@ -1059,7 +1058,7 @@ impl<'a> Linker for MsvcLinker<'a> {
 
         let path = tmpdir.join("lib.def");
         let res: io::Result<()> = try {
-            let mut f = BufWriter::new(File::create(&path)?);
+            let mut f = File::create_buffered(&path)?;
 
             // Start off with the standard module name header and then go
             // straight to exports.
@@ -1648,7 +1647,7 @@ impl<'a> Linker for AixLinker<'a> {
     fn export_symbols(&mut self, tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
         let path = tmpdir.join("list.exp");
         let res: io::Result<()> = try {
-            let mut f = BufWriter::new(File::create(&path)?);
+            let mut f = File::create_buffered(&path)?;
             // FIXME: use llvm-nm to generate export list.
             for symbol in symbols {
                 debug!("  _{symbol}");
@@ -1961,7 +1960,7 @@ impl<'a> Linker for BpfLinker<'a> {
     fn export_symbols(&mut self, tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
         let path = tmpdir.join("symbols");
         let res: io::Result<()> = try {
-            let mut f = BufWriter::new(File::create(&path)?);
+            let mut f = File::create_buffered(&path)?;
             for sym in symbols {
                 writeln!(f, "{sym}")?;
             }

Diff for: compiler/rustc_codegen_ssa/src/lib.rs

@@ -6,6 +6,7 @@
 #![doc(rust_logo)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(negative_impls)]

Diff for: compiler/rustc_data_structures/src/lib.rs

@@ -23,6 +23,7 @@
 #![feature(cfg_match)]
 #![feature(core_intrinsics)]
 #![feature(extend_one)]
+#![feature(file_buffered)]
 #![feature(hash_raw_entry)]
 #![feature(macro_metavar_expr)]
 #![feature(map_try_insert)]

Diff for: compiler/rustc_data_structures/src/obligation_forest/graphviz.rs

@@ -1,6 +1,5 @@
 use std::env::var_os;
 use std::fs::File;
-use std::io::BufWriter;
 use std::path::Path;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
@@ -33,7 +32,7 @@ impl<O: ForestObligation> ObligationForest<O> {
 
         let file_path = dir.as_ref().join(format!("{counter:010}_{description}.gv"));
 
-        let mut gv_file = BufWriter::new(File::create(file_path).unwrap());
+        let mut gv_file = File::create_buffered(file_path).unwrap();
 
         dot::render(&self, &mut gv_file).unwrap();
     }

Diff for: compiler/rustc_incremental/src/assert_dep_graph.rs

@@ -35,7 +35,7 @@
 
 use std::env;
 use std::fs::{self, File};
-use std::io::{BufWriter, Write};
+use std::io::Write;
 
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_data_structures::graph::implementation::{Direction, INCOMING, NodeIndex, OUTGOING};
@@ -245,7 +245,7 @@ fn dump_graph(query: &DepGraphQuery) {
     {
         // dump a .txt file with just the edges:
         let txt_path = format!("{path}.txt");
-        let mut file = BufWriter::new(File::create(&txt_path).unwrap());
+        let mut file = File::create_buffered(&txt_path).unwrap();
         for (source, target) in &edges {
             write!(file, "{source:?} -> {target:?}\n").unwrap();
         }

Diff for: compiler/rustc_incremental/src/lib.rs

@@ -5,6 +5,7 @@
 #![deny(missing_docs)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
+#![feature(file_buffered)]
 #![feature(rustdoc_internals)]
 #![warn(unreachable_pub)]
 // tidy-alphabetical-end

Diff for: compiler/rustc_interface/src/lib.rs

@@ -1,5 +1,6 @@
 // tidy-alphabetical-start
 #![feature(decl_macro)]
+#![feature(file_buffered)]
 #![feature(let_chains)]
 #![feature(try_blocks)]
 #![warn(unreachable_pub)]

Diff for: compiler/rustc_interface/src/passes.rs

@@ -519,7 +519,7 @@ fn write_out_deps(tcx: TyCtxt<'_>, outputs: &OutputFilenames, out_filenames: &[P
                 write_deps_to_file(&mut file)?;
             }
             OutFileName::Real(ref path) => {
-                let mut file = BufWriter::new(fs::File::create(path)?);
+                let mut file = fs::File::create_buffered(path)?;
                 write_deps_to_file(&mut file)?;
             }
         }

Diff for: compiler/rustc_metadata/src/fs.rs

@@ -128,8 +128,7 @@ pub fn non_durable_rename(src: &Path, dst: &Path) -> std::io::Result<()> {
 }
 
 pub fn copy_to_stdout(from: &Path) -> io::Result<()> {
-    let file = fs::File::open(from)?;
-    let mut reader = io::BufReader::new(file);
+    let mut reader = fs::File::open_buffered(from)?;
     let mut stdout = io::stdout();
     io::copy(&mut reader, &mut stdout)?;
     Ok(())

Diff for: compiler/rustc_metadata/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(decl_macro)]
 #![feature(error_iter)]
 #![feature(extract_if)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(iter_from_coroutine)]
 #![feature(let_chains)]

Diff for: compiler/rustc_middle/src/lib.rs

@@ -45,6 +45,7 @@
 #![feature(discriminant_kind)]
 #![feature(extern_types)]
 #![feature(extract_if)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(intra_doc_pointers)]
 #![feature(iter_from_coroutine)]

Diff for: compiler/rustc_middle/src/mir/pretty.rs

@@ -277,9 +277,9 @@ pub fn create_dump_file<'tcx>(
             )
         })?;
     }
-    Ok(io::BufWriter::new(fs::File::create(&file_path).map_err(|e| {
+    Ok(fs::File::create_buffered(&file_path).map_err(|e| {
         io::Error::new(e.kind(), format!("IO error creating MIR dump file: {file_path:?}; {e}"))
-    })?))
+    })?)
 }
 
 ///////////////////////////////////////////////////////////////////////////

Diff for: compiler/rustc_mir_dataflow/src/framework/engine.rs

@@ -266,7 +266,7 @@ where
     A::Domain: DebugWithContext<A>,
 {
     use std::fs;
-    use std::io::{self, Write};
+    use std::io::Write;
 
     let def_id = body.source.def_id();
     let Ok(attrs) = RustcMirAttrs::parse(tcx, def_id) else {
@@ -281,8 +281,7 @@ where
                 if let Some(parent) = path.parent() {
                     fs::create_dir_all(parent)?;
                 }
-                let f = fs::File::create(&path)?;
-                io::BufWriter::new(f)
+                fs::File::create_buffered(&path)?
             }
 
             None if dump_enabled(tcx, A::NAME, def_id) => {

Diff for: compiler/rustc_mir_dataflow/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(exact_size_is_empty)]
+#![feature(file_buffered)]
 #![feature(let_chains)]
 #![feature(try_blocks)]
 #![warn(unreachable_pub)]

Diff for: compiler/rustc_mir_transform/src/dump_mir.rs

@@ -24,7 +24,7 @@ pub fn emit_mir(tcx: TyCtxt<'_>) -> io::Result<()> {
             write_mir_pretty(tcx, None, &mut f)?;
         }
         OutFileName::Real(path) => {
-            let mut f = io::BufWriter::new(File::create(&path)?);
+            let mut f = File::create_buffered(&path)?;
             write_mir_pretty(tcx, None, &mut f)?;
             if tcx.sess.opts.json_artifact_notifications {
                 tcx.dcx().emit_artifact_notification(&path, "mir");

Diff for: compiler/rustc_mir_transform/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(box_patterns)]
 #![feature(const_type_name)]
 #![feature(cow_is_borrowed)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(impl_trait_in_assoc_type)]
 #![feature(let_chains)]

Diff for: compiler/rustc_monomorphize/src/lib.rs

@@ -1,5 +1,6 @@
 // tidy-alphabetical-start
 #![feature(array_windows)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
 #![warn(unreachable_pub)]

Diff for: compiler/rustc_monomorphize/src/partitioning.rs

@@ -95,7 +95,7 @@
 use std::cmp;
 use std::collections::hash_map::Entry;
 use std::fs::{self, File};
-use std::io::{BufWriter, Write};
+use std::io::Write;
 use std::path::{Path, PathBuf};
 
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
@@ -1243,8 +1243,7 @@ fn dump_mono_items_stats<'tcx>(
     let ext = format.extension();
     let filename = format!("{crate_name}.mono_items.{ext}");
     let output_path = output_directory.join(&filename);
-    let file = File::create(&output_path)?;
-    let mut file = BufWriter::new(file);
+    let mut file = File::create_buffered(&output_path)?;
 
     // Gather instantiated mono items grouped by def_id
     let mut items_per_def_id: FxIndexMap<_, Vec<_>> = Default::default();

Diff for: library/std/src/fs.rs

@@ -375,6 +375,44 @@ impl File {
         OpenOptions::new().read(true).open(path.as_ref())
     }
 
+    /// Attempts to open a file in read-only mode with buffering.
+    ///
+    /// See the [`OpenOptions::open`] method, the [`BufReader`][io::BufReader] type,
+    /// and the [`BufRead`][io::BufRead] trait for more details.
+    ///
+    /// If you only need to read the entire file contents,
+    /// consider [`std::fs::read()`][self::read] or
+    /// [`std::fs::read_to_string()`][self::read_to_string] instead.
+    ///
+    /// # Errors
+    ///
+    /// This function will return an error if `path` does not already exist.
+    /// Other errors may also be returned according to [`OpenOptions::open`].
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(file_buffered)]
+    /// use std::fs::File;
+    /// use std::io::BufRead;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut f = File::open_buffered("foo.txt")?;
+    ///     assert!(f.capacity() > 0);
+    ///     for (line, i) in f.lines().zip(1..) {
+    ///         println!("{i:6}: {}", line?);
+    ///     }
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "file_buffered", issue = "130804")]
+    pub fn open_buffered<P: AsRef<Path>>(path: P) -> io::Result<io::BufReader<File>> {
+        // Allocate the buffer *first* so we don't affect the filesystem otherwise.
+        let buffer = io::BufReader::<Self>::try_new_buffer()?;
+        let file = File::open(path)?;
+        Ok(io::BufReader::with_buffer(file, buffer))
+    }
+
     /// Opens a file in write-only mode.
     ///
     /// This function will create a file if it does not exist,
@@ -404,6 +442,45 @@ impl File {
         OpenOptions::new().write(true).create(true).truncate(true).open(path.as_ref())
     }
 
+    /// Opens a file in write-only mode with buffering.
+    ///
+    /// This function will create a file if it does not exist,
+    /// and will truncate it if it does.
+    ///
+    /// Depending on the platform, this function may fail if the
+    /// full directory path does not exist.
+    ///
+    /// See the [`OpenOptions::open`] method and the
+    /// [`BufWriter`][io::BufWriter] type for more details.
+    ///
+    /// See also [`std::fs::write()`][self::write] for a simple function to
+    /// create a file with some given data.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(file_buffered)]
+    /// use std::fs::File;
+    /// use std::io::Write;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut f = File::create_buffered("foo.txt")?;
+    ///     assert!(f.capacity() > 0);
+    ///     for i in 0..100 {
+    ///         writeln!(&mut f, "{i}")?;
+    ///     }
+    ///     f.flush()?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "file_buffered", issue = "130804")]
+    pub fn create_buffered<P: AsRef<Path>>(path: P) -> io::Result<io::BufWriter<File>> {
+        // Allocate the buffer *first* so we don't affect the filesystem otherwise.
+        let buffer = io::BufWriter::<Self>::try_new_buffer()?;
+        let file = File::create(path)?;
+        Ok(io::BufWriter::with_buffer(file, buffer))
+    }
+
     /// Creates a new file in read-write mode; error if the file exists.
     ///
     /// This function will create a file if it does not exist, or return an error if it does. This