Rollup merge of rust-lang#79374 - mendess:const-param-expr-diagnostic, r=lcnr

Add note to use nightly when using expr in const generics

As recommended by `@Icnr` in rust-lang#73899 and in zulip, I've added a note saying that const expressions can be used in nightly.

```
error: generic parameters may not be used in const operations
  --> $DIR/issue-61935.rs:10:23
   |
 6 |         Self:FooImpl<{N==0}>
   |                       ^ cannot perform const operation using `N`
   |
   = help: const parameters may only be used as standalone arguments, i.e. `N`
   = note: use feature(const_generics) and feature(const_evaluatable_checked) to enable this

error: aborting due to previous error
```

I hope the note is well written :sweat_smile:

Author: jonas-schievink

compiler/rustc_resolve/src/diagnostics.rs

@@ -481,6 +481,7 @@ impl<'a> Resolver<'a> {
                         name
                     ));
                 }
+                err.help("use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions");
 
                 err
             }

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -329,7 +329,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
                             ),
                         )
                         .note("the only supported types are integers, `bool` and `char`")
-                        .note("more complex types are supported with `#[feature(const_generics)]`")
+                        .help("more complex types are supported with `#[feature(const_generics)]`")
                         .emit()
                 }
             };

src/test/ui/const-generics/array-size-in-generic-struct-param.min.stderr

@@ -5,6 +5,7 @@ LL | struct ArithArrayLen<const N: usize>([u32; 0 + N]);
    |                                                ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/array-size-in-generic-struct-param.rs:20:15
@@ -13,6 +14,7 @@ LL |     arr: [u8; CFG.arr_size],
    |               ^^^ cannot perform const operation using `CFG`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `CFG`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: `Config` is forbidden as the type of a const generic parameter
   --> $DIR/array-size-in-generic-struct-param.rs:18:21
@@ -21,7 +23,7 @@ LL | struct B<const CFG: Config> {
    |                     ^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: aborting due to 3 previous errors
 

src/test/ui/const-generics/const-arg-in-const-arg.min.stderr

@@ -5,6 +5,7 @@ LL |     let _: [u8; foo::<T>()];
    |                       ^ cannot perform const operation using `T`
    |
    = note: type parameters may not be used in const expressions
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:15:23
@@ -13,6 +14,7 @@ LL |     let _: [u8; bar::<N>()];
    |                       ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:25:23
@@ -21,6 +23,7 @@ LL |     let _ = [0; bar::<N>()];
    |                       ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:30:24
@@ -29,6 +32,7 @@ LL |     let _: Foo<{ foo::<T>() }>;
    |                        ^ cannot perform const operation using `T`
    |
    = note: type parameters may not be used in const expressions
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:31:24
@@ -37,6 +41,7 @@ LL |     let _: Foo<{ bar::<N>() }>;
    |                        ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:36:27
@@ -45,6 +50,7 @@ LL |     let _ = Foo::<{ foo::<T>() }>;
    |                           ^ cannot perform const operation using `T`
    |
    = note: type parameters may not be used in const expressions
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/const-arg-in-const-arg.rs:37:27
@@ -53,6 +59,7 @@ LL |     let _ = Foo::<{ bar::<N>() }>;
    |                           ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error[E0658]: a non-static lifetime is not allowed in a `const`
   --> $DIR/const-arg-in-const-arg.rs:16:23

src/test/ui/const-generics/const-argument-if-length.min.stderr

@@ -5,6 +5,7 @@ LL |     pad: [u8; is_zst::<T>()],
    |                        ^ cannot perform const operation using `T`
    |
    = note: type parameters may not be used in const expressions
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` 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:17:12

src/test/ui/const-generics/const-param-before-other-params.min.stderr

@@ -17,7 +17,7 @@ LL | fn bar<const X: (), 'a>(_: &'a ()) {
    |                 ^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: `()` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-before-other-params.rs:11:17
@@ -26,7 +26,7 @@ LL | fn foo<const X: (), T>(_: &T) {}
    |                 ^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: aborting due to 4 previous errors
 

src/test/ui/const-generics/const-param-elided-lifetime.min.stderr

@@ -35,7 +35,7 @@ LL | struct A<const N: &u8>;
    |                   ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: `&'static u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:16:15
@@ -44,7 +44,7 @@ LL | impl<const N: &u8> A<N> {
    |               ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: `&'static u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:24:15
@@ -53,7 +53,7 @@ LL | impl<const N: &u8> B for A<N> {}
    |               ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: `&'static u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:28:17
@@ -62,7 +62,7 @@ LL | fn bar<const N: &u8>() {}
    |                 ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: `&'static u8` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-elided-lifetime.rs:19:21
@@ -71,7 +71,7 @@ LL |     fn foo<const M: &u8>(&self) {}
    |                     ^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: aborting due to 10 previous errors
 

src/test/ui/const-generics/const-param-type-depends-on-const-param.min.stderr

@@ -17,7 +17,7 @@ LL | pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
    |                                               ^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: `[u8; _]` is forbidden as the type of a const generic parameter
   --> $DIR/const-param-type-depends-on-const-param.rs:16:35
@@ -26,7 +26,7 @@ LL | pub struct SelfDependent<const N: [u8; N]>;
    |                                   ^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`
-   = note: more complex types are supported with `#[feature(const_generics)]`
+   = help: more complex types are supported with `#[feature(const_generics)]`
 
 error: aborting due to 4 previous errors
 

src/test/ui/const-generics/const_evaluatable_checked/feature-gate-const_evaluatable_checked.min.stderr

@@ -5,6 +5,7 @@ LL | type Arr<const N: usize> = [u8; N - 1];
    |                                 ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: aborting due to previous error
 

src/test/ui/const-generics/const_evaluatable_checked/simple.min.stderr

@@ -5,6 +5,7 @@ LL | fn test<const N: usize>() -> [u8; N - 1] where [u8; N - 1]: Default {
    |                                                     ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
   --> $DIR/simple.rs:8:35
@@ -13,6 +14,7 @@ LL | fn test<const N: usize>() -> [u8; N - 1] where [u8; N - 1]: Default {
    |                                   ^ cannot perform const operation using `N`
    |
    = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: aborting due to 2 previous errors