Move __getrandom_custom definition into a const block

.github/workflows/tests.yml

@@ -268,7 +268,7 @@ jobs:
     strategy:
       matrix:
         target: [
-          x86_64-fuchsia,
+          x86_64-unknown-fuchsia,
           x86_64-unknown-redox,
           x86_64-fortanix-unknown-sgx,
         ]

src/custom.rs

@@ -76,16 +76,22 @@ use core::{mem::MaybeUninit, num::NonZeroU32};
 #[cfg_attr(docsrs, doc(cfg(feature = "custom")))]
 macro_rules! register_custom_getrandom {
     ($path:path) => {
-        // We use an extern "C" function to get the guarantees of a stable ABI.
-        #[no_mangle]
-        extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
-            let f: fn(&mut [u8]) -> Result<(), $crate::Error> = $path;
-            let slice = unsafe { ::core::slice::from_raw_parts_mut(dest, len) };
-            match f(slice) {
-                Ok(()) => 0,
-                Err(e) => e.code().get(),
+        // TODO(MSRV 1.37): change to unnamed block
+        const __getrandom_internal: () = {
+            // We use an extern "C" function to get the guarantees of a stable ABI.
+            #[no_mangle]
+            unsafe extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
+                // Make sure the passed function has the type of getrandom::getrandom
+                type F = fn(&mut [u8]) -> ::core::result::Result<(), $crate::Error>;
+                let _: F = $crate::getrandom;
+                let f: F = $path;
+                let slice = ::core::slice::from_raw_parts_mut(dest, len);
+                match f(slice) {
+                    Ok(()) => 0,
+                    Err(e) => e.code().get(),
+                }
             }
-        }
+        };
     };
 }