error[E0308]: mismatched types --> $DIR/issue-15783.rs:8:19 | LL | let msg = foo(x); | --- ^ expected slice `[&str]`, found array `[&str; 1]` | | | arguments to this function are incorrect | = note: expected enum `Option<&[&str]>` found enum `Option<&[&str; 1]>` note: function defined here --> $DIR/issue-15783.rs:1:8 | LL | pub fn foo(params: Option<&[&str]>) -> usize { | ^^^ ----------------------- error: aborting due to previous error For more information about this error, try `rustc --explain E0308`.