impl downcasts

just accidentally discovered that I could write impl From<Box<dyn xxx>>

README.md

@@ -10,6 +10,8 @@
 
 This crate aims to provide a unified trait for parsers and writers of different audio file formats. This means that you can parse tags in mp3, flac, and m4a files with a single function: `Tag::default().read_from_path()` and get fields by directly calling `.album()`, `.artist()` on its result. Without this crate, you would otherwise need to learn different APIs in **id3**, **mp4ameta** etc. in order to parse metadata in different file formats.
 
+I'm relatively new to Rust and programming in general, and this is my first attempt to make a non-trivial crate. If you see anything that you think isn't right, just say it!
+
 ## Examples
 
 Examples can be found in the [manual](https://tianyishi2001.github.io/audiotags).

audiotags-dev-macro/src/lib.rs

@@ -1,3 +1,10 @@
+#[macro_export]
+macro_rules! downcast {
+    ($tag:expr, $type:ty) => {
+        $tag.into_any().downcast_ref::<$type>().unwrap().into()
+    };
+}
+
 #[macro_export]
 macro_rules! impl_audiotag_config {
     ($tag:ident) => {
@@ -58,5 +65,11 @@ macro_rules! impl_tag {
                 }
             }
         }
+
+        impl From<Box<dyn AudioTag>> for $inner {
+            fn from(inp: Box<dyn AudioTag>) -> Self {
+                downcast!(inp, $tag)
+            }
+        }
     };
 }

rtfm/04-downcast.Rmd

@@ -21,7 +21,7 @@ fn main() {
         .expect("Fail to read!");
     assert_eq!(id3tag_reload.title(), Some("title from metaflac::Tag"));
 
-    let mut id3tag_inner: id3::Tag = downcast!(id3tag_reload, Id3v2Tag);
+    let mut id3tag_inner: id3::Tag = id3tag_reload.into();
     let timestamp = id3::Timestamp {
         year: 2013,
         month: Some(2u8),

src/lib.rs

@@ -144,10 +144,3 @@ impl Tag {
         }
     }
 }
-
-#[macro_export]
-macro_rules! downcast {
-    ($tag:expr, $type:ty) => {
-        $tag.into_any().downcast_ref::<$type>().unwrap().into()
-    };
-}

tests/inner.rs

@@ -17,7 +17,7 @@ fn test_inner() {
         .expect("Fail to read!");
     assert_eq!(id3tag_reload.title(), Some("title from metaflac::Tag"));
 
-    let mut id3tag_inner: id3::Tag = downcast!(id3tag_reload, Id3v2Tag);
+    let mut id3tag_inner: id3::Tag = id3tag_reload.into();
     let timestamp = id3::Timestamp {
         year: 2013,
         month: Some(2u8),