Move __getrandom_custom definition into a const block (#344)

This supersedes #341, and makes the following changes
  - All the code for implementing `__getrandom_custom` is now in an
    **named** `const` block (unnamed consts require Rust 1.37)
    - I found this approch [here](https://internals.rust-lang.org/t/anonymous-modules/15441)
    - Nothing inside the block can be referenced outside of it
  - `__getrandom_custom` is marked `unsafe`
    - It can't be accessed externally, but is "logically" unsafe as it
      dereferences raw pointers
  - The type of the function is moved to a typedef, so we can check that
    the defined type matches that of `getrandom:getrandom`.
  - Use `::core::result::Result` instead of `Result`
    - Similar to use use of `from_raw_parts_mut` this prevents
      compilation errors if `Result` is redefined.

Signed-off-by: Joe Richey <joerichey@google.com>

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(),
+                }
             }
-        }
+        };
     };
 }