Revert dc08bc51f2.

compiler/rustc_serialize/src/opaque.rs

@@ -24,10 +24,6 @@ impl Encoder {
     pub fn position(&self) -> usize {
         self.data.len()
     }
-
-    pub fn finish(self) -> Vec<u8> {
-        self.data
-    }
 }
 
 macro_rules! write_leb128 {
@@ -58,6 +54,9 @@ macro_rules! write_leb128 {
 const STR_SENTINEL: u8 = 0xC1;
 
 impl serialize::Encoder for Encoder {
+    type Ok = Vec<u8>;
+    type Err = !;
+
     #[inline]
     fn emit_usize(&mut self, v: usize) {
         write_leb128!(self, v, usize, write_usize_leb128)
@@ -151,6 +150,10 @@ impl serialize::Encoder for Encoder {
     fn emit_raw_bytes(&mut self, s: &[u8]) {
         self.data.extend_from_slice(s);
     }
+
+    fn finish(self) -> Result<Self::Ok, Self::Err> {
+        Ok(self.data)
+    }
 }
 
 pub type FileEncodeResult = Result<usize, io::Error>;
@@ -386,13 +389,6 @@ impl FileEncoder {
             }
         }
     }
-
-    pub fn finish(mut self) -> Result<usize, io::Error> {
-        self.flush();
-
-        let res = std::mem::replace(&mut self.res, Ok(()));
-        res.map(|()| self.position())
-    }
 }
 
 impl Drop for FileEncoder {
@@ -430,6 +426,9 @@ macro_rules! file_encoder_write_leb128 {
 }
 
 impl serialize::Encoder for FileEncoder {
+    type Ok = usize;
+    type Err = io::Error;
+
     #[inline]
     fn emit_usize(&mut self, v: usize) {
         file_encoder_write_leb128!(self, v, usize, write_usize_leb128)
@@ -523,6 +522,13 @@ impl serialize::Encoder for FileEncoder {
     fn emit_raw_bytes(&mut self, s: &[u8]) {
         self.write_all(s);
     }
+
+    fn finish(mut self) -> Result<usize, io::Error> {
+        self.flush();
+
+        let res = std::mem::replace(&mut self.res, Ok(()));
+        res.map(|()| self.position())
+    }
 }
 
 // -----------------------------------------------------------------------------

compiler/rustc_serialize/src/serialize.rs

@@ -18,10 +18,13 @@ use std::sync::Arc;
 /// is pervasive and has non-trivial cost. Instead, impls of this trait must
 /// implement a delayed error handling strategy. If a failure occurs, they
 /// should record this internally, and all subsequent encoding operations can
-/// be processed or ignored, whichever is appropriate. Then they should provide
-/// a `finish` method that finishes up encoding. If the encoder is fallible,
-/// `finish` should return a `Result` that indicates success or failure.
+/// be processed or ignored, whichever is appropriate. Then when `finish()` is
+/// called, an error result should be returned to indicate the failure. If no
+/// failures occurred, then `finish()` should return a success result.
 pub trait Encoder {
+    type Ok;
+    type Err;
+
     // Primitive types:
     fn emit_usize(&mut self, v: usize);
     fn emit_u128(&mut self, v: u128);
@@ -61,6 +64,9 @@ pub trait Encoder {
     fn emit_fieldless_enum_variant<const ID: usize>(&mut self) {
         self.emit_usize(ID)
     }
+
+    // Consume the encoder, getting the result.
+    fn finish(self) -> Result<Self::Ok, Self::Err>;
 }
 
 // Note: all the methods in this trait are infallible, which may be surprising.