How to get context for associated const errors?

When there's an error in an associated constant, the default error message doesn't contain enough context for me to find the actual problem. Is there any way to get rustc to say what the values of the type parameters were when the error occurred?

As a somewhat contrived example, consider the code below. The errors are quite legitimate, because page4 has been declared with a header type too big for the page, but there's no indication in the error as to the root cause, which is that an incompatible type parameter was specified.

use std::mem;
use std::marker::PhantomData;

const PAGE_SIZE:usize = 4096;

struct Page<Head, Rec> {
    bytes: [u8;PAGE_SIZE],

impl<Head,Rec> Page<Head,Rec> {
    const PAYLOAD_SIZE:usize = PAGE_SIZE - mem::size_of::<Head>();
    const CAPACITY:usize = Self::PAYLOAD_SIZE / mem::size_of::<Rec>();
    fn get_capacity(&self)->usize { Self::CAPACITY }
    fn new()-> Self { Page{ bytes: [0u8; PAGE_SIZE], head:PhantomData, rec:PhantomData }}

fn main() {
    let page1:Page<usize,usize> = Page::new();
    println!("Page 1 capacity: {:?}", page1.get_capacity());

    let page2:Page<usize,[usize;200]> = Page::new();
    println!("Page 2 capacity: {:?}", page2.get_capacity());
    let page3:Page<usize,[usize;800]> = Page::new();
    println!("Page 3 capacity: {:?}", page3.get_capacity());
    let page4:Page<[usize;600], usize> = Page::new();
    println!("Page 4 capacity: {:?}", page4.get_capacity());



   Compiling playground v0.0.1 (/playground)
error: any use of this value will cause an error
  --> src/
14 |     const PAYLOAD_SIZE:usize = PAGE_SIZE - mem::size_of::<Head>();
   |     ---------------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
   |                                |
   |                                attempt to subtract with overflow
   = note: `#[deny(const_err)]` on by default

error: any use of this value will cause an error
  --> src/
15 |     const CAPACITY:usize = Self::PAYLOAD_SIZE / mem::size_of::<Rec>();
   |     -----------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
   |                            |
   |                            referenced constant has errors

error: aborting due to 2 previous errors

error: could not compile `playground`.

To learn more, run the command again with --verbose.

I don't have a solution for this, but your writeup looks like it could be a pretty great bug report in rust-lang/rust. Since no one else seems to have a workaround for this, would you be alright with submitting this as a bug?

Someone should come along and tag it as "error reporting" or something like that - there are a lot of bugsin the rust repo (most fixed/closed) which track bad error messages like this.

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