Using godot-rust v0.11.2 with Godot v3.5.1, I created a property with StringHint::Dir as the property hint, but the directory icon is not displayed, it was just treated as a string property.

#[derive(NativeClass)]
#[inherit(Node)]
pub struct MyNode {
    #[property(hint = "dir_hint")]
    pub dir: GodotString,
}
pub fn dir_hint() -> StringHint {
    StringHint::Dir
}

This is probably because the version of gdnative-sys/godot_headers used by godot-rust v0.11.2 does not match the version used by Godot v3.5.1.
There is a difference in the typedef of godot_property_hint.
Godot v3.5.1 :
https://github.com/godotengine/godot-headers/blob/godot-3.5.1-stable/nativescript/godot_nativescript.h
godot-rust v0.11.2 :
https://github.com/godot-rust/gdnative/blob/0.11.2/gdnative-sys/godot_headers/nativescript/godot_nativescript.h