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");
  }}
 }