Mention possibility of linting bad const default values

Ment

Author: estebank

text/0000-default-field-values.md

@@ -710,32 +710,36 @@ all the following rules apply when type-checking:
 2. The expression `def` must coerce to the type `ty`.
 
 3. Generic parameters of the current items are accessible
-```rust
-struct Bar<const A: usize> {
-    field: usize = A,
-}
-```
+   ```rust
+   struct Bar<const A: usize> {
+       field: usize = A,
+   }
+   ```
 
 4. Default const expressions are *not* evaluated at definition time, only
    during instantiation. This means that the following will not fail to compile:
-```rust
-struct Bar {
-    field1: usize = panic!(),
-    field2: usize = 42,
-}
+   ```rust
+   struct Bar {
+       field1: usize = panic!(),
+       field2: usize = 42,
+   }
 
-let _ = Bar { field1: 0, .. };
-```
+   let _ = Bar { field1: 0, .. };
+   ```
+   Having said that, it can be possible to proactivelly attempt to evaluate the
+   default values and emit a lint in a case where the expression is assured to always
+   fail (which would only be possible for expressions that do not reference `const`
+   parameters).
 
 5. The `struct`'s parameters are properly propagated, meaning the following is
    possible:
-```rust
-struct Bar<T> {
-    field: Vec<T> = Vec::new(),
-}
+   ```rust
+   struct Bar<T> {
+       field: Vec<T> = Vec::new(),
+   }
 
-let _ = Bar::<i32> { .. };
-```
+   let _ = Bar::<i32> { .. };
+   ```
 
 When lints check attributes such as `#[allow(lint_name)]` are placed on a
 `RecordField`, it also applies to `def` if it exists.