Avoid depending on panic in const fn, to fix compilaton on Rust <= 1.56.

build.rs

@@ -6,6 +6,11 @@ fn main() {
     // which, outside of `std`, are only available on nightly.
     use_feature_or_nothing("rustc_attrs");
 
+    // Rust 1.56 and earlier don't support panic in const fn.
+    if has_panic_in_const_fn() {
+        use_feature("panic_in_const_fn")
+    }
+
     // Don't rerun this on changes other than build.rs, as we only depend on
     // the rustc version.
     println!("cargo:rerun-if-changed=build.rs");
@@ -40,3 +45,23 @@ fn has_feature(feature: &str) -> bool {
 
     child.wait().unwrap().success()
 }
+
+/// Test whether the rustc at `var("RUSTC")` supports panic in `const fn`.
+fn has_panic_in_const_fn() -> bool {
+    let out_dir = var("OUT_DIR").unwrap();
+    let rustc = var("RUSTC").unwrap();
+
+    let mut child = std::process::Command::new(rustc)
+        .arg("--crate-type=rlib") // Don't require `main`.
+        .arg("--emit=metadata") // Do as little as possible but still parse.
+        .arg("--out-dir")
+        .arg(out_dir) // Put the output somewhere inconsequential.
+        .arg("-") // Read from stdin.
+        .stdin(std::process::Stdio::piped()) // Stdin is a pipe.
+        .spawn()
+        .unwrap();
+
+    writeln!(child.stdin.take().unwrap(), "const fn foo() {{ panic!() }}").unwrap();
+
+    child.wait().unwrap().success()
+}

src/types.rs

@@ -444,7 +444,9 @@ impl BorrowedFd<'_> {
     /// the returned `BorrowedFd`, and it must not have the value `-1`.
     #[inline]
     pub const unsafe fn borrow_raw(fd: RawFd) -> Self {
+        #[cfg(panic_in_const_fn)]
         debug_assert!(fd != -1_i32 as RawFd);
+
         Self {
             fd,
             _phantom: PhantomData,