Add a microbenchmark for fstat. (#836)

See [this LWN article] for context. rustix uses the right syscall here.

[this LWN article]https://lwn.net/Articles/944214/

benches/mod.rs

@@ -4,22 +4,23 @@
 //! the "fs", "time", and "process" cargo features.
 //!
 //! ```sh
-//! RUSTFLAGS=--cfg=criterion cargo bench --features=fs,time,process
+//! RUSTFLAGS=--cfg=criterion cargo bench --features=fs,time,process,stdio
 //! ```
 
 #[cfg(any(
     not(criterion),
     not(feature = "fs"),
     not(feature = "process"),
     not(feature = "time"),
+    not(feature = "stdio"),
     windows,
     target_os = "emscripten",
     target_os = "redox",
     target_os = "wasi",
 ))]
 fn main() {
     unimplemented!(
-        "Add --cfg=criterion to RUSTFLAGS and enable the \"fs\", \"time\", and \"process\" cargo \
+        "Add --cfg=criterion to RUSTFLAGS and enable the \"fs\", \"time\", \"process\", and \"stdio\" cargo \
          features."
     )
 }
@@ -29,6 +30,7 @@ fn main() {
     not(feature = "fs"),
     not(feature = "process"),
     not(feature = "time"),
+    not(feature = "stdio"),
     windows,
     target_os = "emscripten",
     target_os = "redox",
@@ -41,6 +43,7 @@ use criterion::{criterion_group, criterion_main};
     not(feature = "fs"),
     not(feature = "process"),
     not(feature = "time"),
+    not(feature = "stdio"),
     windows,
     target_os = "emscripten",
     target_os = "redox",
@@ -107,6 +110,27 @@ mod suite {
         });
     }
 
+    pub(super) fn simple_fstat(c: &mut Criterion) {
+        use rustix::fs::fstat;
+
+        c.bench_function("simple fstat", |b| {
+            b.iter(|| {
+                fstat(rustix::stdio::stdin()).unwrap();
+            })
+        });
+    }
+
+    pub(super) fn simple_fstat_libc(c: &mut Criterion) {
+        c.bench_function("simple fstat libc", |b| {
+            b.iter(|| {
+                let mut s = std::mem::MaybeUninit::<libc::stat>::uninit();
+                unsafe {
+                    assert_eq!(libc::fstat(libc::STDIN_FILENO, s.as_mut_ptr(),), 0);
+                }
+            })
+        });
+    }
+
     #[cfg(not(target_os = "wasi"))]
     pub(super) fn simple_clock_gettime(c: &mut Criterion) {
         use rustix::time::{clock_gettime, ClockId};
@@ -160,6 +184,7 @@ mod suite {
     not(feature = "fs"),
     not(feature = "process"),
     not(feature = "time"),
+    not(feature = "stdio"),
     windows,
     target_os = "emscripten",
     target_os = "redox",
@@ -171,6 +196,8 @@ criterion_group!(
     suite::simple_statat_libc,
     suite::simple_statat_libc_cstr,
     suite::simple_statat_cstr,
+    suite::simple_fstat,
+    suite::simple_fstat_libc,
     suite::simple_clock_gettime,
     suite::simple_clock_gettime_libc,
     suite::simple_getpid,
@@ -181,6 +208,7 @@ criterion_group!(
     not(feature = "fs"),
     not(feature = "process"),
     not(feature = "time"),
+    not(feature = "stdio"),
     windows,
     target_os = "emscripten",
     target_os = "redox",