Skip to content

Rust fails to coerce to a fn pointer when passing an array as an argument to a generic function. #136420

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom
Open
theemathas opened this issue Feb 2, 2025 · 2 comments · May be fixed by #140283
Open

Rust fails to coerce to a fn pointer when passing an array as an argument to a generic function. #136420

theemathas opened this issue Feb 2, 2025 · 2 comments · May be fixed by #140283
Assignees
@adwinwhite
Labels
A-coercions Area: implicit and explicit `expr as Type` coercions A-inference Area: Type inference C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.

Comments

@theemathas
Copy link
Contributor

theemathas commented Feb 2, 2025 •
edited by rustbot
Loading

I tried this code:

fn foo() {}
fn bar() {}

fn main() {
    let _a = if true { foo } else { bar };
    let _b = vec![foo, bar];
    let _c = [foo, bar];
    d(if true { foo } else { bar });
    e(vec![foo, bar]);
    f([foo, bar]);
}

fn d<T>(_: T) {}
fn e<T>(_: Vec<T>) {}
fn f<T>(_: [T; 2]) {}

I expected the six lines to either all compile or all error. Instead, only the f([foo, bar]) line fails, with the following error:

error[E0308]: mismatched types
  --> src/main.rs:10:7
   |
10 |     f([foo, bar]);
   |     - ^^^^^^^^^^ expected `[fn() {foo}; 2]`, found `[fn(); 2]`
   |     |
   |     arguments to this function are incorrect
   |
   = note: expected array `[fn() {foo}; 2]`
              found array `[fn(); 2]`
note: function defined here
  --> src/main.rs:15:4
   |
15 | fn f<T>(_: [T; 2]) {}
   |    ^    ---------

For more information about this error, try `rustc --explain E0308`.

For some reason, it's failing to coerce the function items into function pointers, but only when an array is being passed to a generic function.

Discovered by @xero-lib

Meta

Issue reproducible on the playground with stable rust 1.84.1, and nightly rust 1.86.0-nightly (2025-02-01 8239a37f9c0951a037cf)

@rustbot labels +A-inference +A-coercions
@theemathas theemathas added the C-bug Category: This is a bug. label Feb 2, 2025
@rustbot rustbot added needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. A-coercions Area: implicit and explicit `expr as Type` coercions A-inference Area: Type inference labels Feb 2, 2025
@jieyouxu jieyouxu added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue. needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Feb 4, 2025
@Noratrieb Noratrieb removed the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Mar 1, 2025
@adwinwhite
Copy link
Contributor

adwinwhite commented Apr 14, 2025 •
edited
Loading

The cause is that f's type parameter is resolved during the coercion of the first element, rather than based on the final type of the array. Refer to fcx.coerce in CoerceMany::coerce_inner.

@adwinwhite
Copy link
Contributor

@rustbot claim

@adwinwhite adwinwhite linked a pull request Apr 25, 2025 that will close this issue
Open
@adwinwhite adwinwhite mentioned this issue May 9, 2025
Open
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Assignees

@adwinwhite adwinwhite

Labels
A-coercions Area: implicit and explicit `expr as Type` coercions A-inference Area: Type inference C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.
Projects
None yet
Milestone
No milestone
Development

Successfully merging a pull request may close this issue.

Fix accidental type inference in array coercion adwinwhite/rust
5 participants
@theemathas @jieyouxu @rustbot @Noratrieb @adwinwhite