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openedon Nov 9, 2018
Spawned off of #54528 (specificially this card in the project).
Consider the following code:
const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
fn main() { }With AST-borrowck, you get (play):
error: trait bounds other than `Sized` on const fn parameters are unstable
--> src/main.rs:1:32
|
1 | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
With NLL (here using migration a la 2018 edition), you get (play):
error: trait bounds other than `Sized` on const fn parameters are unstable
--> src/main.rs:1:32
|
1 | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning[E0515]: cannot return reference to temporary value
--> src/main.rs:1:63
|
1 | const fn no_dyn_trait_ret() -> &'static dyn std::fmt::Debug { &() }
| ^--
| ||
| |temporary value created here
| returns a reference to data owned by the current function
|
= warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
It represents potential unsoundness in your code.
This warning will become a hard error in the future.
error: aborting due to previous error
I have not managed to replicate the above borrow-check error on code that does not exhibit some other const fn error.
And the problem goes away if you opt into #![feature(const_fn)] (play).
So, this is really a very minor diagnostic issue.
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Area: const fn foo(..) {..}. Pure functions which can be applied at compile time.Area: Messages for errors, warnings, and lintsCategory: An issue proposing an enhancement or a PR with one.Call for participation: An issue has been fixed and does not reproduce, but no test has been added.Working towards the "diagnostic parity" goalRelevant to the compiler team, which will review and decide on the PR/issue.