Rollup merge of rust-lang#77825 - ethanboxx:min_const_generics_diagno…

…stic, r=lcnr

`min_const_generics` diagnostics improvements

As disscussed in [zulip/project-const-generics/non-trivial anonymous constant](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/non-trivial.20anonymous.20constants).

This is my first PR on the compiler.

@lcnr is mentoring me on this PR.

Related to rust-lang#60551.

compiler/rustc_resolve/src/diagnostics.rs

@@ -469,24 +469,17 @@ impl<'a> Resolver<'a> {
             ResolutionError::ParamInNonTrivialAnonConst { name, is_type } => {
                 let mut err = self.session.struct_span_err(
                     span,
-                    "generic parameters must not be used inside of non-trivial constant values",
-                );
-                err.span_label(
-                    span,
-                    &format!(
-                        "non-trivial anonymous constants must not depend on the parameter `{}`",
-                        name
-                    ),
+                    "generic parameters may not be used in const operations",
                 );
+                err.span_label(span, &format!("cannot perform const operation using `{}`", name));
 
                 if is_type {
-                    err.note("type parameters are currently not permitted in anonymous constants");
+                    err.note("type parameters may not be used in const expressions");
                 } else {
-                    err.help(
-                        &format!("it is currently only allowed to use either `{0}` or `{{ {0} }}` as generic constants",
-                                 name
-                        )
-                    );
+                    err.help(&format!(
+                        "const parameters may only be used as standalone arguments, i.e. `{}`",
+                        name
+                    ));
                 }
 
                 err

compiler/rustc_resolve/src/lib.rs

@@ -218,7 +218,7 @@ enum ResolutionError<'a> {
     ParamInTyOfConstParam(Symbol),
     /// constant values inside of type parameter defaults must not depend on generic parameters.
     ParamInAnonConstInTyDefault(Symbol),
-    /// generic parameters must not be used inside of non-trivial constant values.
+    /// generic parameters must not be used inside const evaluations.
     ///
     /// This error is only emitted when using `min_const_generics`.
     ParamInNonTrivialAnonConst { name: Symbol, is_type: bool },

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

@@ -1,18 +1,18 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/array-size-in-generic-struct-param.rs:9:48
    |
 LL | struct ArithArrayLen<const N: usize>([u32; 0 + N]);
-   |                                                ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |                                                ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/array-size-in-generic-struct-param.rs:20:15
    |
 LL |     arr: [u8; CFG.arr_size],
-   |               ^^^ non-trivial anonymous constants must not depend on the parameter `CFG`
+   |               ^^^ cannot perform const operation using `CFG`
    |
-   = help: it is currently only allowed to use either `CFG` or `{ CFG }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `CFG`
 
 error: `Config` is forbidden as the type of a const generic parameter
   --> $DIR/array-size-in-generic-struct-param.rs:18:21

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

@@ -8,7 +8,7 @@
 #[allow(dead_code)]
 struct ArithArrayLen<const N: usize>([u32; 0 + N]);
 //[full]~^ ERROR constant expression depends on a generic parameter
-//[min]~^^ ERROR generic parameters must not be used inside of non-trivial constant values
+//[min]~^^ ERROR generic parameters may not be used in const operations
 
 #[derive(PartialEq, Eq)]
 struct Config {
@@ -19,7 +19,7 @@ struct B<const CFG: Config> {
     //[min]~^ ERROR `Config` is forbidden
     arr: [u8; CFG.arr_size],
     //[full]~^ ERROR constant expression depends on a generic parameter
-    //[min]~^^ ERROR generic parameters must not be used inside of non-trivial
+    //[min]~^^ ERROR generic parameters may not be used in const operations
 }
 
 const C: Config = Config { arr_size: 5 };

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

@@ -1,10 +1,10 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/const-argument-if-length.rs:19:24
    |
 LL |     pad: [u8; is_zst::<T>()],
-   |                        ^ non-trivial anonymous constants must not depend on the parameter `T`
+   |                        ^ cannot perform const operation using `T`
    |
-   = note: type parameters are currently not permitted in anonymous constants
+   = note: type parameters may not be used in 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-argument-if-length.rs

@@ -17,7 +17,7 @@ pub struct AtLeastByte<T: ?Sized> {
     value: T,
     //~^ ERROR the size for values of type `T` cannot be known at compilation time
     pad: [u8; is_zst::<T>()],
-    //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+    //[min]~^ ERROR generic parameters may not be used in const operations
     //[full]~^^ ERROR evaluation of constant value failed
 }
 

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

@@ -1,10 +1,10 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/feature-gate-const_evaluatable_checked.rs:6:33
    |
 LL | type Arr<const N: usize> = [u8; N - 1];
-   |                                 ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |                                 ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: aborting due to previous error
 

src/test/ui/const-generics/const_evaluatable_checked/feature-gate-const_evaluatable_checked.rs

@@ -4,7 +4,7 @@
 #![cfg_attr(min, feature(min_const_generics))]
 
 type Arr<const N: usize> = [u8; N - 1];
-//[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+//[min]~^ ERROR generic parameters may not be used in const operations
 
 fn test<const N: usize>() -> Arr<N> where Arr<N>: Default {
     //[full]~^ ERROR constant expression depends

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

@@ -1,18 +1,18 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/simple.rs:8:53
    |
 LL | fn test<const N: usize>() -> [u8; N - 1] where [u8; N - 1]: Default {
-   |                                                     ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |                                                     ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/simple.rs:8:35
    |
 LL | fn test<const N: usize>() -> [u8; N - 1] where [u8; N - 1]: Default {
-   |                                   ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |                                   ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: aborting due to 2 previous errors
 

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

@@ -1,10 +1,10 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/simple_fail.rs:7:33
    |
 LL | type Arr<const N: usize> = [u8; N - 1];
-   |                                 ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |                                 ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: aborting due to previous error
 

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.rs

@@ -5,7 +5,7 @@
 #![allow(incomplete_features)]
 
 type Arr<const N: usize> = [u8; N - 1]; //[full]~ ERROR evaluation of constant
-//[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+//[min]~^ ERROR generic parameters may not be used in const operations
 
 fn test<const N: usize>() -> Arr<N> where Arr<N>: Sized {
     todo!()

src/test/ui/const-generics/generic-function-call-in-array-length.min.stderr

@@ -1,18 +1,18 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/generic-function-call-in-array-length.rs:9:39
    |
 LL | fn bar<const N: usize>() -> [u32; foo(N)] {
-   |                                       ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |                                       ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/generic-function-call-in-array-length.rs:12:13
    |
 LL |     [0; foo(N)]
-   |             ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |             ^ cannot perform const operation using `N`
    |
-   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/generic-function-call-in-array-length.rs

@@ -7,10 +7,10 @@
 const fn foo(n: usize) -> usize { n * 2 }
 
 fn bar<const N: usize>() -> [u32; foo(N)] {
-    //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+    //[min]~^ ERROR generic parameters may not be used in const operations
     //[full]~^^ ERROR constant expression depends on a generic parameter
     [0; foo(N)]
-    //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+    //[min]~^ ERROR generic parameters may not be used in const operations
 }
 
 fn main() {}

src/test/ui/const-generics/generic-sum-in-array-length.min.stderr

@@ -1,18 +1,18 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/generic-sum-in-array-length.rs:7:53
    |
 LL | fn foo<const A: usize, const B: usize>(bar: [usize; A + B]) {}
-   |                                                     ^ non-trivial anonymous constants must not depend on the parameter `A`
+   |                                                     ^ cannot perform const operation using `A`
    |
-   = help: it is currently only allowed to use either `A` or `{ A }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `A`
 
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/generic-sum-in-array-length.rs:7:57
    |
 LL | fn foo<const A: usize, const B: usize>(bar: [usize; A + B]) {}
-   |                                                         ^ non-trivial anonymous constants must not depend on the parameter `B`
+   |                                                         ^ cannot perform const operation using `B`
    |
-   = help: it is currently only allowed to use either `B` or `{ B }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `B`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/generic-sum-in-array-length.rs

@@ -5,8 +5,8 @@
 #![cfg_attr(min, feature(min_const_generics))]
 
 fn foo<const A: usize, const B: usize>(bar: [usize; A + B]) {}
-//[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
-//[min]~| ERROR generic parameters must not be used inside of non-trivial constant values
+//[min]~^ ERROR generic parameters may not be used in const operations
+//[min]~| ERROR generic parameters may not be used in const operations
 //[full]~^^^ ERROR constant expression depends on a generic parameter
 
 fn main() {}

src/test/ui/const-generics/intrinsics-type_name-as-const-argument.min.stderr

@@ -1,10 +1,10 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/intrinsics-type_name-as-const-argument.rs:15:44
    |
 LL |     T: Trait<{std::intrinsics::type_name::<T>()}>
-   |                                            ^ non-trivial anonymous constants must not depend on the parameter `T`
+   |                                            ^ cannot perform const operation using `T`
    |
-   = note: type parameters are currently not permitted in anonymous constants
+   = note: type parameters may not be used in const expressions
 
 error: `&'static str` is forbidden as the type of a const generic parameter
   --> $DIR/intrinsics-type_name-as-const-argument.rs:10:22

src/test/ui/const-generics/intrinsics-type_name-as-const-argument.rs

@@ -13,7 +13,7 @@ trait Trait<const S: &'static str> {}
 struct Bug<T>
 where
     T: Trait<{std::intrinsics::type_name::<T>()}>
-    //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+    //[min]~^ ERROR generic parameters may not be used in const operations
     //[full]~^^ ERROR constant expression depends on a generic parameter
 {
     t: T

src/test/ui/const-generics/issue-61522-array-len-succ.min.stderr

@@ -1,18 +1,18 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/issue-61522-array-len-succ.rs:7:45
    |
 LL | pub struct MyArray<const COUNT: usize>([u8; COUNT + 1]);
-   |                                             ^^^^^ non-trivial anonymous constants must not depend on the parameter `COUNT`
+   |                                             ^^^^^ cannot perform const operation using `COUNT`
    |
-   = help: it is currently only allowed to use either `COUNT` or `{ COUNT }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `COUNT`
 
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/issue-61522-array-len-succ.rs:12:30
    |
 LL |     fn inner(&self) -> &[u8; COUNT + 1] {
-   |                              ^^^^^ non-trivial anonymous constants must not depend on the parameter `COUNT`
+   |                              ^^^^^ cannot perform const operation using `COUNT`
    |
-   = help: it is currently only allowed to use either `COUNT` or `{ COUNT }` as generic constants
+   = help: const parameters may only be used as standalone arguments, i.e. `COUNT`
 
 error: aborting due to 2 previous errors
 

src/test/ui/const-generics/issue-61522-array-len-succ.rs

@@ -6,12 +6,12 @@
 
 pub struct MyArray<const COUNT: usize>([u8; COUNT + 1]);
 //[full]~^ ERROR constant expression depends on a generic parameter
-//[min]~^^ ERROR generic parameters must not be used
+//[min]~^^ ERROR generic parameters may not be used
 
 impl<const COUNT: usize> MyArray<COUNT> {
     fn inner(&self) -> &[u8; COUNT + 1] {
         //[full]~^ ERROR constant expression depends on a generic parameter
-        //[min]~^^ ERROR generic parameters must not be used
+        //[min]~^^ ERROR generic parameters may not be used
         &self.0
     }
 }

src/test/ui/const-generics/issue-67375.min.stderr

@@ -1,10 +1,10 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/issue-67375.rs:9:25
    |
 LL |     inner: [(); { [|_: &T| {}; 0].len() }],
-   |                         ^ non-trivial anonymous constants must not depend on the parameter `T`
+   |                         ^ cannot perform const operation using `T`
    |
-   = note: type parameters are currently not permitted in anonymous constants
+   = note: type parameters may not be used in const expressions
 
 error[E0392]: parameter `T` is never used
   --> $DIR/issue-67375.rs:7:12

src/test/ui/const-generics/issue-67375.rs

@@ -7,7 +7,7 @@
 struct Bug<T> {
     //~^ ERROR parameter `T` is never used
     inner: [(); { [|_: &T| {}; 0].len() }],
-    //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+    //[min]~^ ERROR generic parameters may not be used in const operations
     //[full]~^^ WARN cannot use constants which depend on generic parameters in types
     //[full]~^^^ WARN this was previously accepted by the compiler
 }

src/test/ui/const-generics/issue-67945-1.min.stderr

@@ -1,18 +1,18 @@
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/issue-67945-1.rs:14:16
    |
 LL |         let x: S = MaybeUninit::uninit();
-   |                ^ non-trivial anonymous constants must not depend on the parameter `S`
+   |                ^ cannot perform const operation using `S`
    |
-   = note: type parameters are currently not permitted in anonymous constants
+   = note: type parameters may not be used in const expressions
 
-error: generic parameters must not be used inside of non-trivial constant values
+error: generic parameters may not be used in const operations
   --> $DIR/issue-67945-1.rs:17:45
    |
 LL |         let b = &*(&x as *const _ as *const S);
-   |                                             ^ non-trivial anonymous constants must not depend on the parameter `S`
+   |                                             ^ cannot perform const operation using `S`
    |
-   = note: type parameters are currently not permitted in anonymous constants
+   = note: type parameters may not be used in const expressions
 
 error[E0392]: parameter `S` is never used
   --> $DIR/issue-67945-1.rs:11:12

src/test/ui/const-generics/issue-67945-1.rs

@@ -12,10 +12,10 @@ struct Bug<S> {
     //~^ ERROR parameter `S` is never used
     A: [(); {
         let x: S = MaybeUninit::uninit();
-        //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+        //[min]~^ ERROR generic parameters may not be used in const operations
         //[full]~^^ ERROR mismatched types
         let b = &*(&x as *const _ as *const S);
-        //[min]~^ ERROR generic parameters must not be used inside of non-trivial constant values
+        //[min]~^ ERROR generic parameters may not be used in const operations
         0
     }],
 }