rust/tests/ui/const-generics/const-argument-if-length.full.stderr
Esteban Küber 6b24fdf811 Provide structured suggestion for unconstrained generic constant
```
error: unconstrained generic constant
  --> $DIR/const-argument-if-length.rs:18:10
   |
LL |     pad: [u8; is_zst::<T>()],
   |          ^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
   |                                   ++++++++++++++++++++++++++
```

Detect when the constant expression isn't `usize` and suggest casting:

```
error: unconstrained generic constant
 --> f300.rs:6:10
  |
6 |     bb::<{!N}>();
  |          ^^^^
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs:3539:36
  |
help: try adding a `where` bound
  |
5 | fn b<const N: bool>() where [(); {!N} as usize]: {
  |                       ++++++++++++++++++++++++++
```

Fix #122395.
2024-03-21 00:03:59 +00:00

55 lines
1.9 KiB
Plaintext

error: unconstrained generic constant
--> $DIR/const-argument-if-length.rs:18:10
|
LL | pad: [u8; is_zst::<T>()],
| ^^^^^^^^^^^^^^^^^^^
|
help: try adding a `where` bound
|
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
| ++++++++++++++++++++++++++
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`.