Disallow shadowing const parameters

[FabianWolff]

This pull request fixes #85348. Trying to shadow a const parameter as follows:

fn foo<const N: i32>() {
    let N @ _ = 0;
}

currently causes an ICE. With my changes, I get:

error[E0530]: let bindings cannot shadow const parameters
 --> test.rs:2:9
  |
1 | fn foo<const N: i32>() {
  |              - the const parameter `N` is defined here
2 |     let N @ _ = 0;
  |         ^ cannot be named the same as a const parameter

error: aborting due to previous error

This is the same error you get when trying to shadow a constant:

const N: i32 = 0;
let N @ _ = 0;

error[E0530]: let bindings cannot shadow constants
 --> src/lib.rs:3:5
  |
2 | const N: i32 = 0;
  | ----------------- the constant `N` is defined here
3 | let N @ _ = 0;
  |     ^ cannot be named the same as a constant

error: aborting due to previous error

The reason for disallowing shadowing in both cases is described here (the comment there only talks about constants, but the same reasoning applies to const parameters).

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

@bors r+

[bors]

📌 Commit f749d88 has been approved by petrochenkov

[bors]

⌛ Testing commit f749d88 with merge 863653d8b770162da5f63592bc8cbbcbc88b5154...

[bors]

💥 Test timed out

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[petrochenkov]

@bors retry