Remove associated_consts crate feature

Rewrite has_associated_consts probe to better represent internal usage
Update README

Cargo.toml

@@ -22,4 +22,3 @@ features = ["std"]
 default = ["std"]
 std = []
 i128 = []
-associated_consts = []

README.md

@@ -42,9 +42,8 @@ Implementations for `i128` and `u128` are only available with Rust 1.26 and
 later.  The build script automatically detects this, but you can make it
 mandatory by enabling the `i128` crate feature.
 
-Similarly, additional traits making use of associated constants are available
-with Rust 1.22 and later. Again, the build script automatically detects this,
-but you can make it mandatory by enabling the `associated_consts` crate feature.
+Traits making use of associated constants are available with Rust 1.22 and
+later. This is automatically detected by the build script.
 
 ## Releases
 

build.rs

@@ -9,14 +9,16 @@ fn main() {
         panic!("i128 support was not detected!");
     }
 
+    // Verbose probe to ensure we can blanket impl associated const traits.
+    // Early versions of associated consts prohibited this due to a restriction
+    // of const expressions to non-Drop types.
     if probe("fn main() {
-        struct TestDrop(); impl Drop for TestDrop { fn drop(&mut self) {} }
-        trait TestConst { const TEST: Self; }
-        impl TestConst for TestDrop { const TEST: TestDrop = TestDrop(); }
+        trait Const { const CONST: Self; }
+        impl<T: Const> Const for Option<T> {
+            const CONST: Option<T> = Some(T::CONST);
+        }
     }") {
         println!("cargo:rustc-cfg=has_associated_consts");
-    } else if env::var_os("CARGO_FEATURE_ASSOCIATED_CONSTS").is_some() {
-        panic!("associated constant support was not detected!");
     }
 }