[Rust] Look through typedefs to pass borrowed strings as &str (#1102)

wit-bindgen already knows to special-case passing strings by reference
and pass them as `&str` instead of naively passing them as `&String` and
requiring callers to have owned `Strings`. Implement this behavior for
type aliases of strings as well, so that in code like this:
```wit
type my-string = string;
foo: func(x: my-string);
```
the argument is `&str` instead of `&MyString` which is effectively
`&String`. And similar for lists.

This comes up in several functions in wasi-http; for example, in the
bindings for the `Fields::append` function, it enables this change:

```diff
@@ -5075,8 +5075,8 @@ pub mod wasi {
                 /// `field-value` are syntactically invalid.
                 pub fn append(
                     &self,
-                    name: &FieldName,
-                    value: &FieldValue,
+                    name: &str,
+                    value: &[u8],
                 ) -> Result<(), HeaderError> {
                     unsafe {
                         #[repr(align(1))]
```

where `FieldName` and `FieldValue` are defined as:
```wit
            pub type FieldKey = _rt::String;
            pub type FieldName = FieldKey;
            pub type FieldValue = _rt::Vec<u8>;
```

Author: sunfishcode

crates/rust/src/interface.rs

@@ -1113,6 +1113,30 @@ macro_rules! {macro_name} {{
     }
 
     fn print_ty(&mut self, ty: &Type, mode: TypeMode) {
+        // If we have a typedef of a string or a list, the typedef is an alias
+        // for `String` or `Vec<T>`. If this is a borrow, instead of borrowing
+        // them as `&String` or `&Vec<T>`, use `&str` or `&[T]` so that callers
+        // don't need to create owned copies.
+        if let Type::Id(id) = ty {
+            let id = dealias(self.resolve, *id);
+            let typedef = &self.resolve.types[id];
+            match &typedef.kind {
+                TypeDefKind::Type(Type::String) => {
+                    if let Some(lt) = mode.lifetime {
+                        self.print_borrowed_str(lt);
+                        return;
+                    }
+                }
+                TypeDefKind::List(element) => {
+                    if mode.lifetime.is_some() {
+                        self.print_list(element, mode);
+                        return;
+                    }
+                }
+                _ => {}
+            }
+        }
+
         match ty {
             Type::Id(t) => self.print_tyid(*t, mode),
             Type::Bool => self.push_str("bool"),

crates/rust/tests/codegen.rs

@@ -82,6 +82,58 @@ mod strings {
     }
 }
 
+/// Like `strings` but with a type alias.
+mod aliased_strings {
+    wit_bindgen::generate!({
+        inline: "
+            package my:strings;
+
+            world not-used-name {
+                import cat: interface {
+                    type my-string = string;
+                    foo: func(x: my-string);
+                    bar: func() -> my-string;
+                }
+            }
+        ",
+    });
+
+    #[allow(dead_code)]
+    fn test() {
+        // Test the argument is `&str`.
+        cat::foo("hello");
+
+        // Test the return type is `String`.
+        let _t: String = cat::bar();
+    }
+}
+
+/// Like `aliased_string` but with lists instead of strings.
+mod aliased_lists {
+    wit_bindgen::generate!({
+        inline: "
+            package my:strings;
+
+            world not-used-name {
+                import cat: interface {
+                    type my-list = list<u8>;
+                    foo: func(x: my-list);
+                    bar: func() -> my-list;
+                }
+            }
+        ",
+    });
+
+    #[allow(dead_code)]
+    fn test() {
+        // Test the argument is `&[u8]`.
+        cat::foo(b"hello");
+
+        // Test the return type is `Vec<u8>`.
+        let _t: Vec<u8> = cat::bar();
+    }
+}
+
 mod run_ctors_once_workaround {
     wit_bindgen::generate!({
         inline: "
@@ -96,7 +148,7 @@ mod run_ctors_once_workaround {
     });
 }
 
-/// Like `strings` but with raw_strings`.
+/// Like `strings` but with `raw_strings`.
 mod raw_strings {
     wit_bindgen::generate!({
         inline: "