Rollup merge of #74487 - lcnr:const-in-ty-default, r=varkor

Forbid generic parameters in anon consts inside of type defaults

Emit a resolution error for `struct Foo<T, U = [u8; std::mem::size_of::<T>()]>`.
We are unable to support this with the way `ty::Generics` is currently used,
so let's just forbid it entirely for now.

Fixes some ICE on stable, e.g.
```rust
struct Foo<T, U = [u8; std::mem::size_of::<*mut T>()]>(T, U);
```

r? @varkor @eddyb

src/test/ui/const-generics/params-in-ct-in-ty-param-lazy-norm.rs

@@ -0,0 +1,11 @@
+#![feature(const_generics)] //~ WARN the feature `const_generics` is incomplete
+
+struct Foo<T, U = [u8; std::mem::size_of::<T>()]>(T, U);
+//~^ ERROR constant values inside of type parameter defaults
+
+// FIXME(const_generics:defaults): We still don't know how to we deal with type defaults.
+struct Bar<T = [u8; N], const N: usize>(T);
+//~^ ERROR constant values inside of type parameter defaults
+//~| ERROR type parameters with a default
+
+fn main() {}

src/test/ui/const-generics/params-in-ct-in-ty-param-lazy-norm.stderr

@@ -0,0 +1,31 @@
+error: type parameters with a default must be trailing
+  --> $DIR/params-in-ct-in-ty-param-lazy-norm.rs:7:12
+   |
+LL | struct Bar<T = [u8; N], const N: usize>(T);
+   |            ^
+   |
+   = note: using type defaults and const parameters in the same parameter list is currently not permitted
+
+error: constant values inside of type parameter defaults must not depend on generic parameters
+  --> $DIR/params-in-ct-in-ty-param-lazy-norm.rs:3:44
+   |
+LL | struct Foo<T, U = [u8; std::mem::size_of::<T>()]>(T, U);
+   |                                            ^ the anonymous constant must not depend on the parameter `T`
+
+error: constant values inside of type parameter defaults must not depend on generic parameters
+  --> $DIR/params-in-ct-in-ty-param-lazy-norm.rs:7:21
+   |
+LL | struct Bar<T = [u8; N], const N: usize>(T);
+   |                     ^ the anonymous constant 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/params-in-ct-in-ty-param-lazy-norm.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 3 previous errors; 1 warning emitted
+

src/test/ui/generic/param-in-ct-in-ty-param-default.rs

@@ -0,0 +1,4 @@
+struct Foo<T, U = [u8; std::mem::size_of::<T>()]>(T, U);
+//~^ ERROR constant values inside of type parameter defaults
+
+fn main() {}

src/test/ui/generic/param-in-ct-in-ty-param-default.stderr

@@ -0,0 +1,8 @@
+error: constant values inside of type parameter defaults must not depend on generic parameters
+  --> $DIR/param-in-ct-in-ty-param-default.rs:1:44
+   |
+LL | struct Foo<T, U = [u8; std::mem::size_of::<T>()]>(T, U);
+   |                                            ^ the anonymous constant must not depend on the parameter `T`
+
+error: aborting due to previous error
+