Use concrete type to fix a potential soundness issue

Fixes #19

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "lonlat_bng"
 description = "Convert longitude and latitude coordinates to BNG coordinates, and vice versa"
-version = "0.6.32"
+version = "0.6.33"
 authors = ["Stephan Hügel <urschrei@gmail.com>"]
 license = "MIT"
 keywords = ["OSGB36", "BNG", "Geo", "ETRS89", "OSTN02", "OSTN15"]
@@ -17,7 +17,7 @@ criterion = "0.4.0"
 cbindgen = "0.24.2"
 
 [dependencies]
-libc = "0.2.139"
+libc = "0.2.147"
 rand = "0.8.5"
 ostn15_phf = { git = "https://github.com/urschrei/ostn15_phf.git" }
 rayon = "1.7.0"

src/ffi.rs

@@ -61,9 +61,9 @@ pub extern "C" fn drop_float_array(lons: Array, lats: Array) {
     let _: &mut [f64] = lats.into();
 }
 
-// Build an Array from &mut[T], so it can be leaked across the FFI boundary
-impl<'a, T> From<&'a mut [T]> for Array {
-    fn from(sl: &mut [T]) -> Self {
+// Build an Array so it can be leaked across the FFI boundary
+impl<'a> From<&'a mut [f64]> for Array {
+    fn from(sl: &mut [f64]) -> Self {
         Array {
             data: sl.as_mut_ptr().cast::<libc::c_void>(),
             len: sl.len() as libc::size_t,