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Clean up E0741 error explanation #74977

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Aug 1, 2020
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Clean up E0741 error explanation #74977

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12 changes: 8 additions & 4 deletions src/librustc_error_codes/error_codes/E0741.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
Only structural-match types (that is, types that derive `PartialEq` and `Eq`)
may be used as the types of const generic parameters.
A non-structural-match type was used as the type of a const generic parameter.

Erroneous code example:

```compile_fail,E0741
#![feature(const_generics)]
Expand All @@ -9,12 +10,15 @@ struct A;
struct B<const X: A>; // error!
```

To fix this example, we derive `PartialEq` and `Eq`.
Only structural-match types (that is, types that derive `PartialEq` and `Eq`)
may be used as the types of const generic parameters.
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@pickfire pickfire Aug 1, 2020 •
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Suggested change
may be used as the types of const generic parameters.
may be used as the type of const generic parameters.

Is using type here more correct?

Or maybe we can change the wording a bit.

Const generic parameters must be structural-match types (types that derive PartialEq and Eq).

Not sure if must be is the right wording. Or maybe must be structural matchable (...)?

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I *think* the current wording is correct. It's actually hard to phrase it haha.


To fix the previous code example, we derive `PartialEq` and `Eq`:

```
#![feature(const_generics)]

#[derive(PartialEq, Eq)]
#[derive(PartialEq, Eq)] // We derive both traits here.
struct A;

struct B<const X: A>; // ok!
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