Move finish out of the Encoder trait.

This simplifies things, but requires making `CacheEncoder` non-generic.

(This was previously merged as commit 4 in #94732 and then was reverted
in #97905 because it caused a perf regression.)

compiler/rustc_serialize/src/opaque.rs

@@ -24,6 +24,10 @@ impl MemEncoder {
     pub fn position(&self) -> usize {
         self.data.len()
     }
+
+    pub fn finish(self) -> Vec<u8> {
+        self.data
+    }
 }
 
 macro_rules! write_leb128 {
@@ -54,9 +58,6 @@ macro_rules! write_leb128 {
 const STR_SENTINEL: u8 = 0xC1;
 
 impl Encoder for MemEncoder {
-    type Ok = Vec<u8>;
-    type Err = !;
-
     #[inline]
     fn emit_usize(&mut self, v: usize) {
         write_leb128!(self, v, usize, write_usize_leb128)
@@ -150,10 +151,6 @@ impl Encoder for MemEncoder {
     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>;
@@ -389,6 +386,13 @@ 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 {
@@ -426,9 +430,6 @@ macro_rules! file_encoder_write_leb128 {
 }
 
 impl 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)
@@ -522,13 +523,6 @@ impl 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

@@ -18,13 +18,10 @@ 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 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.
+/// 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.
 pub trait Encoder {
-    type Ok;
-    type Err;
-
     // Primitive types:
     fn emit_usize(&mut self, v: usize);
     fn emit_u128(&mut self, v: u128);
@@ -64,9 +61,6 @@ 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.

compiler/rustc_serialize/tests/opaque.rs

@@ -2,7 +2,7 @@
 
 use rustc_macros::{Decodable, Encodable};
 use rustc_serialize::opaque::{MemDecoder, MemEncoder};
-use rustc_serialize::{Decodable, Encodable, Encoder as EncoderTrait};
+use rustc_serialize::{Decodable, Encodable};
 use std::fmt::Debug;
 
 #[derive(PartialEq, Clone, Debug, Encodable, Decodable)]
@@ -38,7 +38,7 @@ fn check_round_trip<
         Encodable::encode(value, &mut encoder);
     }
 
-    let data = encoder.finish().unwrap();
+    let data = encoder.finish();
     let mut decoder = MemDecoder::new(&data[..], 0);
 
     for value in values {