Require rustc 1.51+ for IndexMap::from(array) and `IndexSet::from(a…

…rray)`

rust-lang/rust#74878 and
rust-lang/rust#65798 were both stabilized in
1.51.

build.rs

@@ -4,5 +4,6 @@ fn main() {
         Some(_) => autocfg::emit("has_std"),
         None => autocfg::new().emit_sysroot_crate("std"),
     }
+    autocfg::new().emit_rustc_version(1, 51);
     autocfg::rerun_path("build.rs");
 }

src/map.rs

@@ -1342,7 +1342,7 @@ where
     }
 }
 
-#[cfg(has_std)]
+#[cfg(all(has_std, rustc_1_51))]
 impl<K, V, const N: usize> From<[(K, V); N]> for IndexMap<K, V, RandomState>
 where
     K: Hash + Eq,
@@ -1857,6 +1857,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(all(has_std, rustc_1_51))]
     fn from_array() {
         let map = IndexMap::from([(1, 2), (3, 4)]);
         let mut expected = IndexMap::new();

src/set.rs

@@ -840,7 +840,7 @@ where
     }
 }
 
-#[cfg(has_std)]
+#[cfg(all(has_std, rustc_1_51))]
 impl<T, const N: usize> From<[T; N]> for IndexSet<T, RandomState>
 where
     T: Eq + Hash,
@@ -1728,6 +1728,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(all(has_std, rustc_1_51))]
     fn from_array() {
         let set1 = IndexSet::from([1, 2, 3, 4]);
         let set2: IndexSet<_> = [1, 2, 3, 4].into();