Why use `match` in function no error but in main happened?

[Michael-F-Bryan]

When you call open(&args.path), where &args.path is a &String and open() accepts a &str and String can dereference to a str, the compiler will automatically dereference the &String for you. This is one of the few implicit type coersions in Rust, and is also referred to as "auto deref".

This thread on auto deref even mentions how the compiler won't automatically dereference when doing pattern matching (i.e. your match situation):

Automatic dereferencing

How would you characterize the set of built-in types for which dereferencing happens automatically? I see the list of Deref trait implementors at Deref in std::ops - Rust. It includes types like String and Vec. It also says this: impl<'_, T> Deref for &'_ T where T: ?Sized, impl<'_, T> Deref for &'_ mut T where T: ?Sized, I'm assuming that covers references to types like bool, char, str, tuples, and arrays. Are there any built-in types that do not implement the Deref trait?

The equality check in your if-statement isn't doing any special conversions.

It's just combining impl PartialEq<str> for String with the general impl<A, B> PartialEq<&B> for &A where B: PartialEq<A> to infer that &String: PartialEq<&str>, and therefore we can use == with a &String on the left-hand side and &str on the right.