error: generic parameters may not be used in const operations
  --> $DIR/const-argument-if-length.rs:18:24
   |
LL |     pad: [u8; is_zst::<T>()],
   |                        ^ cannot perform const operation using `T`
   |
   = note: type parameters may not be used in const expressions
   = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions

error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> $DIR/const-argument-if-length.rs:16:12
   |
LL | pub struct AtLeastByte<T: ?Sized> {
   |                        - this type parameter needs to be `Sized`
LL |     value: T,
   |            ^ doesn't have a size known at compile-time
   |
   = note: only the last field of a struct may have a dynamically sized type
   = help: change the field's type to have a statically known size
help: consider removing the `?Sized` bound to make the type parameter `Sized`
   |
LL - pub struct AtLeastByte<T: ?Sized> {
LL + pub struct AtLeastByte<T> {
   |
help: borrowed types always have a statically known size
   |
LL |     value: &T,
   |            +
help: the `Box` type always has a statically known size and allocates its contents in the heap
   |
LL |     value: Box<T>,
   |            ++++ +

error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> $DIR/const-argument-if-length.rs:7:28
   |
LL | pub const fn is_zst<T: ?Sized>() -> usize {
   |                     - this type parameter needs to be `Sized`
LL |     if std::mem::size_of::<T>() == 0 {
   |                            ^ doesn't have a size known at compile-time
   |
note: required by an implicit `Sized` bound in `std::mem::size_of`
  --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
help: consider removing the `?Sized` bound to make the type parameter `Sized`
   |
LL - pub const fn is_zst<T: ?Sized>() -> usize {
LL + pub const fn is_zst<T>() -> usize {
   |

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0277`.