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Sep 29, 2025
Merged

const functions: separate rule about users and rule about what is allowed in such functions #2013

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7bbf535
separate uses from what is allowed in const fns themselves
tshepang Sep 25, 2025
431465a
unwrap
tshepang Sep 25, 2025
b71f831
Combine the two const function sections
ehuss Sep 29, 2025
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37 changes: 26 additions & 11 deletions src/const_eval.md
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Expand Up @@ -258,27 +258,39 @@ generics.
r[const-eval.const-fn]
## Const functions

r[const-eval.const-fn.general]
A _const fn_ is a function that one is permitted to call from a const context.
r[const-eval.const-fn.intro]
A _const function_ is a function that can be called from a const context. It is defined with the `const` qualifier, and also includes [tuple struct] and [tuple enum variant] constructors.

r[const-eval.const-fn.usage]
Declaring a function
`const` has no effect on any existing uses, it only restricts the types that arguments and the
return type may use, and restricts the function body to constant expressions.
> [!EXAMPLE]
> ```rust
> const fn square(x: i32) -> i32 { x * x }
>
> const VALUE: i32 = square(12);
> ```

r[const-eval.const-fn.const-context]
When called from a const context, the function is interpreted by the
compiler at compile time. The interpretation happens in the
environment of the compilation target and not the host. So `usize` is
`32` bits if you are compiling against a `32` bit system, irrelevant
of whether you are building on a `64` bit or a `32` bit system.
When called from a const context, a const function is interpreted by the compiler at compile time. The interpretation happens in the environment of the compilation target and not the host. So `usize` is `32` bits if you are compiling against a `32` bit system, irrelevant of whether you are building on a `64` bit or a `32` bit system.

r[const-eval.const-fn.outside-context]
When a const function is called from outside a const context, it behaves the same as if it did not have the `const` qualifier.

r[const-eval.const-fn.body-restriction]
The body of a const function may only use [constant expressions].

r[const-eval.const-fn.async]
Const functions are not allowed to be [async].

r[const-eval.const-fn.type-restrictions]
The types of a const function's parameters and return type are restricted to those that are compatible with a const context.
<!-- TODO: Define the type restrictions. -->

[arithmetic]: expressions/operator-expr.md#arithmetic-and-logical-binary-operators
[array expressions]: expressions/array-expr.md
[array indexing]: expressions/array-expr.md#array-and-slice-indexing-expressions
[array indexing]: expressions/array-expr.md#array-and-slice-indexing-expressions
[array type length expressions]: types/array.md
[assignment expressions]: expressions/operator-expr.md#assignment-expressions
[async]: items/functions.md#async-functions
[compound assignment expressions]: expressions/operator-expr.md#compound-assignment-expressions
[block expressions]: expressions/block-expr.md
[borrow]: expressions/operator-expr.md#borrow-operators
Expand All @@ -289,6 +301,7 @@ of whether you are building on a `64` bit or a `32` bit system.
[const functions]: items/functions.md#const-functions
[const generic argument]: items/generics.md#const-generics
[const generic parameters]: items/generics.md#const-generics
[constant expressions]: #constant-expressions
[constants]: items/constant-items.md
[Const parameters]: items/generics.md
[dereference expression]: expressions/operator-expr.md#the-dereference-operator
Expand Down Expand Up @@ -321,5 +334,7 @@ of whether you are building on a `64` bit or a `32` bit system.
[statics]: items/static-items.md
[struct]: expressions/struct-expr.md
[temporary lifetime extension]: destructors.scope.lifetime-extension
[tuple enum variant]: items/enumerations.md
[tuple expressions]: expressions/tuple-expr.md
[tuple struct]: items/structs.md
[while]: expressions/loop-expr.md#predicate-loops
12 changes: 1 addition & 11 deletions src/items/functions.md
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Expand Up @@ -279,14 +279,7 @@ For other considerations and limitations regarding unwinding across FFI boundari
r[items.fn.const]
## Const functions

r[items.fn.const.intro]
Functions qualified with the `const` keyword are [const functions], as are [tuple struct] and [tuple enum variant] constructors. _Const functions_ can be called from within [const contexts].

r[items.fn.const.extern]
Const functions may use the [`extern`] function qualifier.

r[items.fn.const.exclusivity]
Const functions are not allowed to be [async](#async-functions).
See [const functions] for the definition of const functions.

r[items.fn.async]
## Async functions
Expand Down Expand Up @@ -467,9 +460,6 @@ fn foo_oof(#[some_inert_attribute] arg: u8) {

[const contexts]: ../const_eval.md#const-context
[const functions]: ../const_eval.md#const-functions
[tuple struct]: structs.md
[tuple enum variant]: enumerations.md
[`extern`]: #extern-function-qualifier
[external block]: external-blocks.md
[path]: ../paths.md
[block]: ../expressions/block-expr.md
Expand Down