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Note difference in CTFE timing between associated and free constants #1120

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Jan 24, 2022
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Note difference in CTFE timing between associated and free constants #1120

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Note difference in CTFE timing between associated and free constants
XrXr Jan 20, 2022
be49428
Move example closer to explaination
XrXr Jan 24, 2022
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23 changes: 23 additions & 0 deletions src/items/associated-items.md
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Expand Up @@ -293,6 +293,9 @@ 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.
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This seems odd, to make a guarantee by stating its absence on associated consts. Do we say explicitly anywhere that free constants are always evaluated, even when they are not used?

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Do we say explicitly anywhere that free constants are always evaluated, even when they are not used?

I went through Constant Evaluation and Constant items and could not find it explicitly stated. Some possible ways forward:

  1. reword this line to say "Associated constant definitions undergo constant evaluation only when referenced.", removing the reference to free constants
  2. mention that unused free constants always undergo CTFE in Constant items and reword this sentence to link to it.

Do you have any preferences or recommendations?

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2 sounds like something we will eventually want to do, but will probably require T-lang FCP to ensure that we are willing to commit to this as a guarantee.


### Associated Constants Examples

A basic example:
Expand Down Expand Up @@ -335,6 +338,24 @@ fn main() {
}
```

[Constant evaluation] timing:
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When I immediately read this, I didn't know what it meant by "timing".

Instead of placing this down in the "examples" section, can this code block be moved up to the paragraph that you added above? Generally in the reference, when there is an example demonstrating some concept, we try to write that example immediately after the paragraph that introduces that concept.

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@XrXr XrXr Jan 24, 2022 •
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Thanks, that makes sense. Moved in be49428


```rust
struct Struct;

impl Struct {
const ID: i32 = 1;
// Definition not immediately evaluated
const PANIC: () = panic!("compile-time panic");
}

fn main() {
assert_eq!(1, Struct::ID);
// Referencing Struct::PANIC causes compilation error
// let _ = Struct::PANIC;
}
```

[_ConstantItem_]: constant-items.md
[_Function_]: functions.md
[_MacroInvocationSemi_]: ../macros.md#macro-invocation
Expand Down Expand Up @@ -362,3 +383,5 @@ 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