Improve ZST DST error message

Helps address #1867

src/util/macros.rs

@@ -638,13 +638,13 @@ macro_rules! maybe_const_trait_bounded_fn {
 /// The type that this expression evaluates to must be `Copy`, or else the
 /// non-panicking desugaring will fail to compile.
 macro_rules! const_panic {
-    ($fmt:literal) => {{
+    ($($arg:tt)+) => {{
         #[cfg(zerocopy_panic_in_const_and_vec_try_reserve)]
-        panic!($fmt);
+        panic!($($arg)+);
         #[cfg(not(zerocopy_panic_in_const_and_vec_try_reserve))]
-        const_panic!(@non_panic $fmt)
+        const_panic!(@non_panic $($arg)+)
     }};
-    (@non_panic $fmt:expr) => {{
+    (@non_panic $($_arg:tt)+) => {{
         // This will type check to whatever type is expected based on the call
         // site.
         let panic: [_; 0] = [];
@@ -669,7 +669,18 @@ macro_rules! const_assert {
                 let _: () = const_panic!(@non_panic concat!("assertion failed: ", stringify!($e)));
             }
         }
-    }}
+    }};
+    ($e:expr, $($args:tt)+) => {{
+        #[cfg(zerocopy_panic_in_const_and_vec_try_reserve)]
+        assert!($e, $($args)+);
+        #[cfg(not(zerocopy_panic_in_const_and_vec_try_reserve))]
+        {
+            let e = $e;
+            if !e {
+                let _: () = const_panic!(@non_panic concat!("assertion failed: ", stringify!($e), ": ", stringify!($arg)), $($args)*);
+            }
+        }
+    }};
 }
 
 /// Like `const_assert!`, but relative to `debug_assert!`.
@@ -709,28 +720,28 @@ macro_rules! const_unreachable {
 /// it cannot be evaluated in a runtime context. The condition is checked after
 /// monomorphization and, upon failure, emits a compile error.
 macro_rules! static_assert {
-    (Self $(: $(? $optbound:ident $(+)?)* $($bound:ident $(+)?)* )? => $condition:expr) => {{
+    (Self $(: $(? $optbound:ident $(+)?)* $($bound:ident $(+)?)* )? => $condition:expr $(, $args:tt)*) => {{
         trait StaticAssert {
             const ASSERT: bool;
         }
 
         impl<T $(: $(? $optbound +)* $($bound +)*)?> StaticAssert for T {
             const ASSERT: bool = {
-                const_assert!($condition);
+                const_assert!($condition $(, $args)*);
                 $condition
             };
         }
 
         const_assert!(<Self as StaticAssert>::ASSERT);
     }};
-    ($($tyvar:ident $(: $(? $optbound:ident $(+)?)* $($bound:ident $(+)?)* )?),* => $condition:expr) => {{
+    ($($tyvar:ident $(: $(? $optbound:ident $(+)?)* $($bound:ident $(+)?)* )?),* => $condition:expr $(, $args:tt)*) => {{
         trait StaticAssert {
             const ASSERT: bool;
         }
 
         impl<$($tyvar $(: $(? $optbound +)* $($bound +)*)?,)*> StaticAssert for ($($tyvar,)*) {
             const ASSERT: bool = {
-                const_assert!($condition);
+                const_assert!($condition $(, $args)*);
                 $condition
             };
         }
@@ -752,6 +763,6 @@ macro_rules! static_assert_dst_is_not_zst {
                 }
             };
             !dst_is_zst
-        });
+        }, "cannot call this method on a dynamically-sized type whose trailing slice element is zero-sized");
     }}
 }