Rollup merge of #74159 - lcnr:const-generic-ty-decl, r=varkor

forbid generic params in the type of const params

implements and closes #74152

fixes #74101, closes #71169, fixes #73491, closes #62878

@eddyb and I talked [on zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/type.20of.20const.20parameters/near/203405696) about this and we probably want to also forbid generic consts in the default
type of a parameter, e.g. `struct Foo<T, U = [u8; std::mem::size_of::<T>()]>`, this is currently still allowed
and I will probably fix that in a followup PR.

r? @varkor @eddyb

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

@@ -0,0 +1,15 @@
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+
+// Currently, const parameters cannot depend on other generic parameters,
+// as our current implementation can't really support this.
+//
+// We may want to lift this restriction in the future.
+
+pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
+//~^ ERROR: the type of const parameters must not depend on other generic parameters
+
+pub struct SelfDependent<const N: [u8; N]>;
+//~^ ERROR: the type of const parameters must not depend on other generic parameters
+
+fn main() {}

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

@@ -0,0 +1,24 @@
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/const-param-type-depends-on-const-param.rs:9:52
+   |
+LL | pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
+   |                                                    ^ the type must not depend on the parameter `N`
+
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/const-param-type-depends-on-const-param.rs:12:40
+   |
+LL | pub struct SelfDependent<const N: [u8; N]>;
+   |                                        ^ the type must not depend on the parameter `N`
+
+warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/const-param-type-depends-on-const-param.rs:1:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0770`.

src/test/ui/const-generics/const-param-type-depends-on-type-param-ungated.rs

@@ -1,6 +1,6 @@
 use std::marker::PhantomData;
 
 struct B<T, const N: T>(PhantomData<[T; N]>); //~ ERROR const generics are unstable
-//~^ ERROR `T` is not guaranteed to `#[derive(PartialEq, Eq)]`
+//~^ ERROR the type of const parameters must not depend on other generic parameters
 
 fn main() {}

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

@@ -1,3 +1,9 @@
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:22
+   |
+LL | struct B<T, const N: T>(PhantomData<[T; N]>);
+   |                      ^ the type must not depend on the parameter `T`
+
 error[E0658]: const generics are unstable
   --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:19
    |
@@ -7,15 +13,7 @@ LL | struct B<T, const N: T>(PhantomData<[T; N]>);
    = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
    = help: add `#![feature(const_generics)]` to the crate attributes to enable
 
-error[E0741]: `T` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be used as the type of a const parameter
-  --> $DIR/const-param-type-depends-on-type-param-ungated.rs:3:22
-   |
-LL | struct B<T, const N: T>(PhantomData<[T; N]>);
-   |                      ^ `T` may not derive both `PartialEq` and `Eq`
-   |
-   = note: it is not currently possible to use a type parameter as the type of a const parameter
-
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0658, E0741.
+Some errors have detailed explanations: E0658, E0770.
 For more information about an error, try `rustc --explain E0658`.

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

@@ -1,12 +1,13 @@
 #![feature(const_generics)]
 //~^ WARN the feature `const_generics` is incomplete
 
-// Currently, const parameters cannot depend on type parameters, because there is no way to
-// enforce the structural-match property on an arbitrary type parameter. This restriction
-// may be relaxed in the future. See https://github.com/rust-lang/rfcs/pull/2000 for more
-// details.
+// Currently, const parameters cannot depend on other generic parameters,
+// as our current implementation can't really support this.
+//
+// We may want to lift this restriction in the future.
 
 pub struct Dependent<T, const X: T>([(); X]);
-//~^ ERROR `T` is not guaranteed to `#[derive(PartialEq, Eq)]`
+//~^ ERROR: the type of const parameters must not depend on other generic parameters
+//~| ERROR: parameter `T` is never used
 
 fn main() {}

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

@@ -1,3 +1,9 @@
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:34
+   |
+LL | pub struct Dependent<T, const X: T>([(); X]);
+   |                                  ^ the type must not depend on the parameter `T`
+
 warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/const-param-type-depends-on-type-param.rs:1:12
    |
@@ -7,14 +13,15 @@ LL | #![feature(const_generics)]
    = note: `#[warn(incomplete_features)]` on by default
    = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
 
-error[E0741]: `T` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be used as the type of a const parameter
-  --> $DIR/const-param-type-depends-on-type-param.rs:9:34
+error[E0392]: parameter `T` is never used
+  --> $DIR/const-param-type-depends-on-type-param.rs:9:22
    |
 LL | pub struct Dependent<T, const X: T>([(); X]);
-   |                                  ^ `T` may not derive both `PartialEq` and `Eq`
+   |                      ^ unused parameter
    |
-   = note: it is not currently possible to use a type parameter as the type of a const parameter
+   = help: consider removing `T`, referring to it in a field, or using a marker such as `std::marker::PhantomData`
 
-error: aborting due to previous error; 1 warning emitted
+error: aborting due to 2 previous errors; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0741`.
+Some errors have detailed explanations: E0392, E0770.
+For more information about an error, try `rustc --explain E0392`.

src/test/ui/const-generics/issues/issue-71169.rs

@@ -0,0 +1,10 @@
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+fn foo<const LEN: usize, const DATA: [u8; LEN]>() {}
+//~^ ERROR the type of const parameters must not
+fn main() {
+    const DATA: [u8; 4] = *b"ABCD";
+    foo::<4, DATA>();
+    //~^ ERROR constant expression depends on
+}

src/test/ui/const-generics/issues/issue-71169.stderr

@@ -0,0 +1,17 @@
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/issue-71169.rs:4:43
+   |
+LL | fn foo<const LEN: usize, const DATA: [u8; LEN]>() {}
+   |                                           ^^^ the type must not depend on the parameter `LEN`
+
+error: constant expression depends on a generic parameter
+  --> $DIR/issue-71169.rs:8:14
+   |
+LL |     foo::<4, DATA>();
+   |              ^^^^
+   |
+   = note: this may fail depending on what value the parameter takes
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0770`.

src/test/ui/const-generics/issues/issue-71381.rs

@@ -12,6 +12,7 @@ unsafe extern "C" fn pass(args: PassArg) {
 impl Test {
     pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "C" fn(Args)>(&self) {
         //~^ ERROR: using function pointers as const generic parameters is forbidden
+        //~| ERROR: the type of const parameters must not depend on other generic parameters
         self.0 = Self::trampiline::<Args, IDX, FN> as _
     }
 
@@ -20,6 +21,7 @@ impl Test {
         const IDX: usize,
         const FN: unsafe extern "C" fn(Args),
         //~^ ERROR: using function pointers as const generic parameters is forbidden
+        //~| ERROR: the type of const parameters must not depend on other generic parameters
     >(
         args: Args,
     ) {

src/test/ui/const-generics/issues/issue-71381.stderr

@@ -1,3 +1,15 @@
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/issue-71381.rs:13:82
+   |
+LL |     pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "C" fn(Args)>(&self) {
+   |                                                                                  ^^^^ the type must not depend on the parameter `Args`
+
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/issue-71381.rs:22:40
+   |
+LL |         const FN: unsafe extern "C" fn(Args),
+   |                                        ^^^^ the type must not depend on the parameter `Args`
+
 error: using function pointers as const generic parameters is forbidden
   --> $DIR/issue-71381.rs:13:61
    |
@@ -5,10 +17,11 @@ LL |     pub fn call_me<Args: Sized, const IDX: usize, const FN: unsafe extern "
    |                                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: using function pointers as const generic parameters is forbidden
-  --> $DIR/issue-71381.rs:21:19
+  --> $DIR/issue-71381.rs:22:19
    |
 LL |         const FN: unsafe extern "C" fn(Args),
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0770`.

src/test/ui/const-generics/issues/issue-71611.rs

@@ -3,6 +3,7 @@
 
 fn func<A, const F: fn(inner: A)>(outer: A) {
     //~^ ERROR: using function pointers as const generic parameters is forbidden
+    //~| ERROR: the type of const parameters must not depend on other generic parameters
     F(outer);
 }
 

src/test/ui/const-generics/issues/issue-71611.stderr

@@ -1,8 +1,15 @@
+error[E0770]: the type of const parameters must not depend on other generic parameters
+  --> $DIR/issue-71611.rs:4:31
+   |
+LL | fn func<A, const F: fn(inner: A)>(outer: A) {
+   |                               ^ the type must not depend on the parameter `A`
+
 error: using function pointers as const generic parameters is forbidden
   --> $DIR/issue-71611.rs:4:21
    |
 LL | fn func<A, const F: fn(inner: A)>(outer: A) {
    |                     ^^^^^^^^^^^^
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0770`.

src/test/ui/const-generics/issues/issue-73491.rs

@@ -0,0 +1,9 @@
+// check-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+const LEN: usize = 1024;
+
+fn hoge<const IN: [u32; LEN]>() {}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-74101.rs

@@ -0,0 +1,9 @@
+// check-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+fn test<const N: [u8; 1 + 2]>() {}
+
+struct Foo<const N: [u8; 1 + 2]>;
+
+fn main() {}