allow calling assert_type{,_eq}! at top-level & const ctx

This commit allows using `assert_type_eq!` and `assert_type!` macros
at top level.

This is done by using `const _` instead of `let _`. `const`s could not be
uninitialized, so PhantomData is also used.

Note: this requires rustc version 1.37.0 or higher.

.travis.yml

@@ -3,7 +3,7 @@ rust:
   - stable
   - beta
   - nightly
-  - 1.22.0
+  - 1.37.0
 cache: cargo
 os:
   - linux

CHANGELOG.md

@@ -2,11 +2,13 @@
 
 This project follows semantic versioning.
 
-The MSRV (Minimum Supported Rust Version) is 1.22.0, and typenum is tested against this Rust
+The MSRV (Minimum Supported Rust Version) is 1.37.0, and typenum is tested against this Rust
 version. Much of typenum should work on as low a version as 1.20.0, but that is not guaranteed.
 
 ### Unreleased
 
+- [changed] Allowed calling `assert_type_eq` and `assert_type` at top level
+
 ### 1.12.0 (2020-04-13)
 - [added] Feature `force_unix_path_separator` to support building without Cargo.
 - [added] Greatest common divisor operator `Gcd` with alias `Gcf`.

src/lib.rs

@@ -140,14 +140,14 @@ impl Ord for Equal {
 #[macro_export]
 macro_rules! assert_type_eq {
     ($a:ty, $b:ty) => {
-        let _: <$a as $crate::Same<$b>>::Output;
+        const _: core::marker::PhantomData<<$a as $crate::Same<$b>>::Output> = core::marker::PhantomData;
     };
 }
 
 /// Asserts that a type is `True`, aka `B1`.
 #[macro_export]
 macro_rules! assert_type {
     ($a:ty) => {
-        let _: <$a as $crate::Same<True>>::Output;
+        const _: core::marker::PhantomData<<$a as $crate::Same<True>>::Output> = core::marker::PhantomData;
     };
 }