a0215d8e46
Fundamentally, we have *three* disjoint categories of functions: 1. const-stable functions 2. private/unstable functions that are meant to be callable from const-stable functions 3. functions that can make use of unstable const features This PR implements the following system: - `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions. - `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category. - `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls. Also, several holes in recursive const stability checking are being closed. There's still one potential hole that is hard to avoid, which is when MIR building automatically inserts calls to a particular function in stable functions -- which happens in the panic machinery. Those need to *not* be `rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be sure they follow recursive const stability. But that's a fairly rare and special case so IMO it's fine. The net effect of this is that a `#[unstable]` or unmarked function can be constified simply by marking it as `const fn`, and it will then be const-callable from stable `const fn` and subject to recursive const stability requirements. If it is publicly reachable (which implies it cannot be unmarked), it will be const-unstable under the same feature gate. Only if the function ever becomes `#[stable]` does it need a `#[rustc_const_unstable]` or `#[rustc_const_stable]` marker to decide if this should also imply const-stability. Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to use unstable const lang features (including intrinsics), or (b) `#[stable]` functions that are not yet intended to be const-stable. Adding `#[rustc_const_stable]` is only needed for functions that are actually meant to be directly callable from stable const code. `#[rustc_const_stable_indirect]` is used to mark intrinsics as const-callable and for `#[rustc_const_unstable]` functions that are actually called from other, exposed-on-stable `const fn`. No other attributes are required.
54 lines
2.2 KiB
Plaintext
54 lines
2.2 KiB
Plaintext
error: const function that might be (indirectly) exposed to stable cannot use `#[feature(foo)]`
|
|
--> $DIR/dont_promote_unstable_const_fn.rs:14:25
|
|
|
|
|
LL | const fn bar() -> u32 { foo() }
|
|
| ^^^^^
|
|
|
|
|
= help: mark the callee as `#[rustc_const_stable_indirect]` if it does not itself require any unsafe features
|
|
help: if the caller is not (yet) meant to be exposed to stable, add `#[rustc_const_unstable]` (this is what you probably want to do)
|
|
|
|
|
LL + #[rustc_const_unstable(feature = "...", issue = "...")]
|
|
LL | const fn bar() -> u32 { foo() }
|
|
|
|
|
help: otherwise, as a last resort `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks (this requires team approval)
|
|
|
|
|
LL + #[rustc_allow_const_fn_unstable(foo)]
|
|
LL | const fn bar() -> u32 { foo() }
|
|
|
|
|
|
|
error[E0716]: temporary value dropped while borrowed
|
|
--> $DIR/dont_promote_unstable_const_fn.rs:17:28
|
|
|
|
|
LL | let _: &'static u32 = &foo();
|
|
| ------------ ^^^^^ creates a temporary value which is freed while still in use
|
|
| |
|
|
| type annotation requires that borrow lasts for `'static`
|
|
LL | }
|
|
| - temporary value is freed at the end of this statement
|
|
|
|
error[E0716]: temporary value dropped while borrowed
|
|
--> $DIR/dont_promote_unstable_const_fn.rs:21:28
|
|
|
|
|
LL | let _: &'static u32 = &meh();
|
|
| ------------ ^^^^^ creates a temporary value which is freed while still in use
|
|
| |
|
|
| type annotation requires that borrow lasts for `'static`
|
|
...
|
|
LL | }
|
|
| - temporary value is freed at the end of this statement
|
|
|
|
error[E0716]: temporary value dropped while borrowed
|
|
--> $DIR/dont_promote_unstable_const_fn.rs:22:26
|
|
|
|
|
LL | let x: &'static _ = &std::time::Duration::from_millis(42).subsec_millis();
|
|
| ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
|
|
| |
|
|
| type annotation requires that borrow lasts for `'static`
|
|
LL |
|
|
LL | }
|
|
| - temporary value is freed at the end of this statement
|
|
|
|
error: aborting due to 4 previous errors
|
|
|
|
For more information about this error, try `rustc --explain E0716`.
|