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Improve associated constant item CTFE timing section #1147

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Merged
merged 3 commits into from
Feb 8, 2022
Merged

Improve associated constant item CTFE timing section #1147

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cf06f4a
Don't implicitly talk about free constant CTFE timing
XrXr Jan 27, 2022
d0b33a2
Mention associated const CTFE interactions with generics
XrXr Feb 5, 2022
37e3c15
Use generics in associated const CTFE timing example
XrXr Feb 5, 2022
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26 changes: 19 additions & 7 deletions src/items/associated-items.md
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Original file line number Diff line number Diff line change
Expand Up @@ -293,22 +293,35 @@ type that the definition has to implement.
An *associated constant definition* defines a constant associated with a
type. It is written the same as a [constant item].

Unlike [free] constants, associated constant definitions undergo
[constant evaluation] only when referenced.
Associated constant definitions undergo [constant evaluation] only when
referenced. Further, definitions that include [generic parameters] are
evaluated after monomorphization.

```rust
```rust,compile_fail
struct Struct;
struct GenericStruct<const ID: i32>;
impl Struct {
const ID: i32 = 1;
// Definition not immediately evaluated
const PANIC: () = panic!("compile-time panic");
}
impl<const ID: i32> GenericStruct<ID> {
// Definition not immediately evaluated
const NON_ZERO: () = if ID == 0 {
panic!("contradiction")
};
}
fn main() {
assert_eq!(1, Struct::ID);
// Referencing Struct::PANIC causes compilation error
// let _ = Struct::PANIC;
let _ = Struct::PANIC;
// Fine, ID is not 0
let _ = GenericStruct::<1>::NON_ZERO;
// Compilation error from evaluating NON_ZERO with ID=0
let _ = GenericStruct::<0>::NON_ZERO;
}
```

Expand Down Expand Up @@ -381,5 +394,4 @@ fn main() {
[regular function parameters]: functions.md#attributes-on-function-parameters
[generic parameters]: generics.md
[where clauses]: generics.md#where-clauses
[free]: ../glossary.md#free-item
[constant evaluation]: ../const_eval.md