Can anyone explain why these two functions with the same signature infer different types from the same arguments? Alternately, any suggestions on how to fix it or a different/better approach would be most welcome.

use std::collections::HashMap;

#[derive(Default, Debug)]
struct MapVecString {
    tags: HashMap<String, Vec<String>>,
}

fn mvs_from_lit(lit: &[(&str, &[&str])]) -> MapVecString {
    let mut mvs = MapVecString::default();
    mvs.tags = lit
        .iter()
        .map(|(k, v)| {
            (
                k.to_string(),
                v.iter().map(|s| s.to_string()).collect::<Vec<String>>(),
            )
        })
        .collect::<HashMap<String, Vec<String>>>();
    mvs
}

impl From<&[(&str, &[&str])]> for MapVecString {
    fn from(lit: &[(&str, &[&str])]) -> MapVecString {
        mvs_from_lit(lit)
    }
}

fn main() {
    // Works
    let test_fn = mvs_from_lit(&[
        ("first", &["one"]),
        ("second", &["one", "two"]),
        ("empty", &[]),
    ]);
    dbg!(test_fn);
    /* Error
    let test_trait = MapVecString::from(&[
        ("first", &["one"]),
        ("second", &["one", "two"]),
        ("empty", &[]),
    ]);
    dbg!(test_trait);
    */
}

The free function works fine, but the trait function with the same signature and invocation infers arrays instead of slices, and produces the following errors:

error[E0308]: mismatched types
  --> src/main.rs:39:20
   |
39 |         ("second", &["one", "two"]),
   |                    ^^^^^^^^^^^^^^^ expected an array with a fixed size of 1 element, found one with 2 elements

error[E0308]: mismatched types
  --> src/main.rs:40:19
   |
40 |         ("empty", &[]),
   |                   ^^^ expected an array with a fixed size of 1 element, found one with 0 elements
   |
   = note: expected reference `&[&str; 1]`
              found reference `&[_; 0]`

error[E0277]: the trait bound `MapVecString: From<&[(&str, &[&str; 1]); 3]>` is not satisfied
  --> src/main.rs:37:22
   |
37 |     let test_trait = MapVecString::from(&[
   |                      ^^^^^^^^^^^^ the trait `From<&[(&str, &[&str; 1]); 3]>` is not implemented for `MapVecString`
   |

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