Bug nesting const generics

[jorendorff]

The test case below generates broken C code.

// Propagating const arguments through generics that use generics.

#[repr(C)]
struct Inner<N: usize> {
    bytes: [u8; N],
}

#[repr(C)]
struct Outer<N: usize> {
    inner: Inner<N>, // don't declare two different structs named `Inner_N`
}

#[no_mangle]
pub extern "C" fn one() -> Outer<1> {
    Outer { inner: Inner { bytes: [0] } }
}

#[no_mangle]
pub extern "C" fn two() -> Outer<2> {
    Outer { inner: Inner { bytes: [0, 0] } }
}

cargo run -- --lang c --style tag $FILE produces this:

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

struct Inner_N {
  uint8_t bytes[N];
};

struct Outer_1 {
  struct Inner_N inner;
};

struct Outer_2 {
  struct Inner_N inner;
};

struct Outer_1 one(void);

struct Outer_2 two(void);

The corresponding thing works fine with type parameters, so there must be something I failed to hook up in #759, or maybe we aren't modeling the value namespace at all. Filing the bug because I don't have time to look right now. I should have time in the next few days, though.