use TryFrom instead of From for downcasting because it could fail

audiotags-dev-macro/src/lib.rs

@@ -1,10 +1,3 @@
-#[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) => {
@@ -66,9 +59,25 @@ macro_rules! impl_tag {
             }
         }
 
-        impl From<Box<dyn AudioTag>> for $inner {
-            fn from(inp: Box<dyn AudioTag>) -> Self {
-                downcast!(inp, $tag)
+        // downcasting
+
+        // impl std::convert::TryFrom<Box<dyn AudioTag>> for &$tag {
+        //     type Error = crate::Error;
+        //     fn try_from(inp: Box<dyn AudioTag>) -> crate::Result<Self> {
+        //         inp.into_any()
+        //             .downcast_ref::<$tag>()
+        //             .ok_or(crate::Error::DowncastError)
+        //     }
+        // }
+
+        impl std::convert::TryFrom<Box<dyn AudioTag>> for $inner {
+            type Error = crate::Error;
+            fn try_from(inp: Box<dyn AudioTag>) -> crate::Result<Self> {
+                let t: &$tag = inp
+                    .into_any()
+                    .downcast_ref::<$tag>()
+                    .ok_or(crate::Error::DowncastError)?;
+                Ok(t.into())
             }
         }
     };

rtfm/04-downcast.Rmd

@@ -21,7 +21,9 @@ fn main() {
         .expect("Fail to read!");
     assert_eq!(id3tag_reload.title(), Some("title from metaflac::Tag"));
 
-    let mut id3tag_inner: id3::Tag = id3tag_reload.into();
+    let mut id3tag_inner: id3::Tag = id3tag_reload.try_into().unwrap();
+    // this would fail if `id3tag_reload` isn't really a id3 tag.
+
     let timestamp = id3::Timestamp {
         year: 2013,
         month: Some(2u8),

src/error.rs

@@ -1,6 +1,9 @@
 /// Error types that could occur in this library.
 #[derive(thiserror::Error, Debug)]
 pub enum Error {
+    #[error("")]
+    DowncastError,
+
     /// Fail to guess the metadata format based on the file extension.
     #[error("Fail to guess the metadata format based on the file extension.")]
     UnknownFileExtension(String),

src/lib.rs

@@ -53,7 +53,7 @@ use std::convert::From;
 use std::fs::File;
 use std::path::Path;
 
-use std::convert::{TryFrom, TryInto};
+pub use std::convert::{TryFrom, TryInto};
 
 #[derive(Clone, Copy, Debug)]
 pub enum TagType {

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 = id3tag_reload.into();
+    let mut id3tag_inner: id3::Tag = id3tag_reload.try_into().unwrap();
     let timestamp = id3::Timestamp {
         year: 2013,
         month: Some(2u8),