Auto merge of rust-lang#121101 - GnomedDev:dyn-small-c-string, r=<try>

Reduce monomorphisation bloat in small_c_string

This is a code path usually next to an FFI call, so taking the `dyn` slowdown for the 1159 llvm-line (fat lto, codegen-units 1, release build) drop in my testing program [t2fanrd](https://github.com/GnomedDev/t2fanrd) is worth it imo.

Author: bors

library/std/src/sys/pal/common/small_c_string.rs

@@ -17,20 +17,32 @@ const NUL_ERR: io::Error =
 #[inline]
 pub fn run_path_with_cstr<T, F>(path: &Path, f: F) -> io::Result<T>
 where
-    F: FnOnce(&CStr) -> io::Result<T>,
+    F: FnMut(&CStr) -> io::Result<T>,
 {
     run_with_cstr(path.as_os_str().as_encoded_bytes(), f)
 }
 
 #[inline]
-pub fn run_with_cstr<T, F>(bytes: &[u8], f: F) -> io::Result<T>
+pub fn run_with_cstr<T, F>(bytes: &[u8], mut f: F) -> io::Result<T>
 where
-    F: FnOnce(&CStr) -> io::Result<T>,
+    F: FnMut(&CStr) -> io::Result<T>,
 {
+    // Dispatch and dyn erase the closure type to prevent mono bloat.
+    // See https://github.com/rust-lang/rust/pull/121101.
     if bytes.len() >= MAX_STACK_ALLOCATION {
-        return run_with_cstr_allocating(bytes, f);
+        run_with_cstr_allocating(bytes, &mut f)
+    } else {
+        unsafe { run_with_cstr_stack(bytes, &mut f) }
     }
+}
 
+/// # Safety
+///
+/// `bytes` must have a length less than `MAX_STACK_ALLOCATION`.
+unsafe fn run_with_cstr_stack<T>(
+    bytes: &[u8],
+    f: &mut dyn FnMut(&CStr) -> io::Result<T>,
+) -> io::Result<T> {
     let mut buf = MaybeUninit::<[u8; MAX_STACK_ALLOCATION]>::uninit();
     let buf_ptr = buf.as_mut_ptr() as *mut u8;
 
@@ -47,10 +59,10 @@ where
 
 #[cold]
 #[inline(never)]
-fn run_with_cstr_allocating<T, F>(bytes: &[u8], f: F) -> io::Result<T>
-where
-    F: FnOnce(&CStr) -> io::Result<T>,
-{
+fn run_with_cstr_allocating<T>(
+    bytes: &[u8],
+    f: &mut dyn FnMut(&CStr) -> io::Result<T>,
+) -> io::Result<T> {
     match CString::new(bytes) {
         Ok(s) => f(&s),
         Err(_) => Err(NUL_ERR),

src/tools/miri/tests/fail/shims/fs/isolated_file.stderr

@@ -11,6 +11,7 @@ LL |         let fd = cvt_r(|| unsafe { open64(path.as_ptr(), flags, opts.mode a
    = note: inside `std::sys::pal::PLATFORM::cvt_r::<i32, {closure@std::sys::pal::PLATFORM::fs::File::open_c::{closure#0}}>` at RUSTLIB/std/src/sys/pal/PLATFORM/mod.rs:LL:CC
    = note: inside `std::sys::pal::PLATFORM::fs::File::open_c` at RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC
    = note: inside closure at RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC
+   = note: inside `std::sys::pal::PLATFORM::small_c_string::run_with_cstr_stack::<std::sys::pal::PLATFORM::fs::File>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
    = note: inside `std::sys::pal::PLATFORM::small_c_string::run_with_cstr::<std::sys::pal::PLATFORM::fs::File, {closure@std::sys::pal::PLATFORM::fs::File::open::{closure#0}}>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
    = note: inside `std::sys::pal::PLATFORM::small_c_string::run_path_with_cstr::<std::sys::pal::PLATFORM::fs::File, {closure@std::sys::pal::PLATFORM::fs::File::open::{closure#0}}>` at RUSTLIB/std/src/sys/pal/PLATFORM/small_c_string.rs:LL:CC
    = note: inside `std::sys::pal::PLATFORM::fs::File::open` at RUSTLIB/std/src/sys/pal/PLATFORM/fs.rs:LL:CC