Compile-time Execution (Scoped Effect)
Description
The const keyword marks functions as "is allowed to be evaluated during
compilation". When used in scope position its meaning changes slightly to: "this
will be evaluated during compilation". There is no way to declare "must be
evaluated at compilation" functions, causing the meaning of "const" to be
context-dependent.
| declaration | usage | |
|---|---|---|
| keyword never applies | fn meow() {} | fn hello() { meow() } |
| keyword always applies | - | const CAT: () = {}; |
| keyword conditionally applies | const fn meow() {} | const fn hello() { meow() } |
Feature Status
The const feature is integrated in a lot of the stdlib and ecosystem already,
but it's notoriously missing any form of const-traits. Because a lot of Rust's
language features make use of traits, this means const contexts have no access
to iteration, Drop handlers, closures, and more.
Feature categorization
| Position | Syntax |
|---|---|
| Effect | const fn |
| Yield | N/A |
| Apply | automatic |
| Consume | const {}, const X: Ty = {} |
| Reification | N/A |
Positions Available
| Position | Available | Example |
|---|---|---|
| Manual trait impl | ❌ | N/A |
| Free functions | ✅ | const fn meow() {} |
| Inherent functions | ✅ | impl Cat { const fn meow() {} } |
| Trait methods | ⏳ | trait Cat { const fn meow() {} } |
| Trait declarations | ❌ | const trait Cat {} |
| Block scope | ✅ | fn meow() { const {} } |
| Argument qualifiers | ❌ | fn meow(cat: impl const Cat) {} |
| Data types | ❌ | const struct Cat {} |
| Drop | ❌ | impl const Drop for Cat {} |
| Closures | ❌ | const ǀǀ {} |
| Iterators | ❌ | for cat in cats {} |
Refinements
There are currently no refiments to the compile-time execution effect.