Use delayed error handling for Encodable and Encoder infallible.

There are two impls of the `Encoder` trait: `opaque::Encoder` and
`opaque::FileEncoder`. The former encodes into memory and is infallible, the
latter writes to file and is fallible.

Currently, standard `Result`/`?`/`unwrap` error handling is used, but this is a
bit verbose and has non-trivial cost, which is annoying given how rare failures
are (especially in the infallible `opaque::Encoder` case).

This commit changes how `Encoder` fallibility is handled. All the `emit_*`
methods are now infallible. `opaque::Encoder` requires no great changes for
this. `opaque::FileEncoder` now implements a delayed error handling strategy.
If a failure occurs, it records this via the `res` field, and all subsequent
encoding operations are skipped if `res` indicates an error has occurred. Once
encoding is complete, the new `finish` method is called, which returns a
`Result`. In other words, there is now a single `Result`-producing method
instead of many of them.

This has very little effect on how any file errors are reported if
`opaque::FileEncoder` has any failures.

Much of this commit is boring mechanical changes, removing `Result` return
values and `?` or `unwrap` from expressions. The more interesting parts are as
follows.
- serialize.rs: The `Encoder` trait gains an `Ok` associated type. The
  `into_inner` method is changed into `finish`, which returns
  `Result<Vec<u8>, !>`.
- opaque.rs: The `FileEncoder` adopts the delayed error handling
  strategy. Its `Ok` type is a `usize`, returning the number of bytes
  written, replacing previous uses of `FileEncoder::position`.
- Various methods that take an encoder now consume it, rather than being
  passed a mutable reference, e.g. `serialize_query_result_cache`.

Author: nnethercote

compiler/rustc_ast/src/ast.rs

@@ -2473,9 +2473,7 @@ rustc_index::newtype_index! {
 }
 
 impl<S: Encoder> rustc_serialize::Encodable<S> for AttrId {
-    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
-        Ok(())
-    }
+    fn encode(&self, _s: &mut S) {}
 }
 
 impl<D: Decoder> rustc_serialize::Decodable<D> for AttrId {

compiler/rustc_ast/src/ptr.rs

@@ -121,8 +121,8 @@ impl<D: Decoder, T: 'static + Decodable<D>> Decodable<D> for P<T> {
 }
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<T> {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        (**self).encode(s)
+    fn encode(&self, s: &mut S) {
+        (**self).encode(s);
     }
 }
 
@@ -191,8 +191,8 @@ impl<'a, T> IntoIterator for &'a P<[T]> {
 }
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<[T]> {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        Encodable::encode(&**self, s)
+    fn encode(&self, s: &mut S) {
+        Encodable::encode(&**self, s);
     }
 }
 

compiler/rustc_ast/src/tokenstream.rs

@@ -142,9 +142,9 @@ impl fmt::Debug for LazyTokenStream {
 }
 
 impl<S: Encoder> Encodable<S> for LazyTokenStream {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+    fn encode(&self, s: &mut S) {
         // Used by AST json printing.
-        Encodable::encode(&self.create_token_stream(), s)
+        Encodable::encode(&self.create_token_stream(), s);
     }
 }
 

compiler/rustc_codegen_ssa/src/lib.rs

@@ -29,7 +29,7 @@ use rustc_middle::dep_graph::WorkProduct;
 use rustc_middle::middle::dependency_format::Dependencies;
 use rustc_middle::middle::exported_symbols::SymbolExportKind;
 use rustc_middle::ty::query::{ExternProviders, Providers};
-use rustc_serialize::{opaque, Decodable, Decoder, Encoder};
+use rustc_serialize::{opaque, Decodable, Decoder, Encodable, Encoder};
 use rustc_session::config::{CrateType, OutputFilenames, OutputType, RUST_CGU_EXT};
 use rustc_session::cstore::{self, CrateSource};
 use rustc_session::utils::NativeLibKind;
@@ -204,13 +204,13 @@ const RUSTC_VERSION: Option<&str> = option_env!("CFG_VERSION");
 impl CodegenResults {
     pub fn serialize_rlink(codegen_results: &CodegenResults) -> Vec<u8> {
         let mut encoder = opaque::Encoder::new();
-        encoder.emit_raw_bytes(RLINK_MAGIC).unwrap();
+        encoder.emit_raw_bytes(RLINK_MAGIC);
         // `emit_raw_bytes` is used to make sure that the version representation does not depend on
         // Encoder's inner representation of `u32`.
-        encoder.emit_raw_bytes(&RLINK_VERSION.to_be_bytes()).unwrap();
-        encoder.emit_str(RUSTC_VERSION.unwrap()).unwrap();
-        rustc_serialize::Encodable::encode(codegen_results, &mut encoder).unwrap();
-        encoder.into_inner()
+        encoder.emit_raw_bytes(&RLINK_VERSION.to_be_bytes());
+        encoder.emit_str(RUSTC_VERSION.unwrap());
+        Encodable::encode(codegen_results, &mut encoder);
+        encoder.finish().unwrap()
     }
 
     pub fn deserialize_rlink(data: Vec<u8>) -> Result<Self, String> {

compiler/rustc_data_structures/src/fingerprint.rs

@@ -144,9 +144,8 @@ impl_stable_hash_via_hash!(Fingerprint);
 
 impl<E: rustc_serialize::Encoder> Encodable<E> for Fingerprint {
     #[inline]
-    fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        s.emit_raw_bytes(&self.to_le_bytes())?;
-        Ok(())
+    fn encode(&self, s: &mut E) {
+        s.emit_raw_bytes(&self.to_le_bytes());
     }
 }
 
@@ -187,10 +186,10 @@ impl std::fmt::Display for PackedFingerprint {
 
 impl<E: rustc_serialize::Encoder> Encodable<E> for PackedFingerprint {
     #[inline]
-    fn encode(&self, s: &mut E) -> Result<(), E::Error> {
+    fn encode(&self, s: &mut E) {
         // Copy to avoid taking reference to packed field.
         let copy = self.0;
-        copy.encode(s)
+        copy.encode(s);
     }
 }
 

compiler/rustc_data_structures/src/svh.rs

@@ -49,8 +49,8 @@ impl fmt::Display for Svh {
 }
 
 impl<S: Encoder> Encodable<S> for Svh {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        s.emit_u64(self.as_u64().to_le())
+    fn encode(&self, s: &mut S) {
+        s.emit_u64(self.as_u64().to_le());
     }
 }
 

compiler/rustc_incremental/src/persist/file_format.rs

@@ -30,22 +30,20 @@ const HEADER_FORMAT_VERSION: u16 = 0;
 /// the Git commit hash.
 const RUSTC_VERSION: Option<&str> = option_env!("CFG_VERSION");
 
-pub(crate) fn write_file_header(stream: &mut FileEncoder, nightly_build: bool) -> FileEncodeResult {
-    stream.emit_raw_bytes(FILE_MAGIC)?;
-    stream.emit_raw_bytes(&[
-        (HEADER_FORMAT_VERSION >> 0) as u8,
-        (HEADER_FORMAT_VERSION >> 8) as u8,
-    ])?;
+pub(crate) fn write_file_header(stream: &mut FileEncoder, nightly_build: bool) {
+    stream.emit_raw_bytes(FILE_MAGIC);
+    stream
+        .emit_raw_bytes(&[(HEADER_FORMAT_VERSION >> 0) as u8, (HEADER_FORMAT_VERSION >> 8) as u8]);
 
     let rustc_version = rustc_version(nightly_build);
     assert_eq!(rustc_version.len(), (rustc_version.len() as u8) as usize);
-    stream.emit_raw_bytes(&[rustc_version.len() as u8])?;
-    stream.emit_raw_bytes(rustc_version.as_bytes())
+    stream.emit_raw_bytes(&[rustc_version.len() as u8]);
+    stream.emit_raw_bytes(rustc_version.as_bytes());
 }
 
 pub(crate) fn save_in<F>(sess: &Session, path_buf: PathBuf, name: &str, encode: F)
 where
-    F: FnOnce(&mut FileEncoder) -> FileEncodeResult,
+    F: FnOnce(FileEncoder) -> FileEncodeResult,
 {
     debug!("save: storing data in {}", path_buf.display());
 
@@ -80,28 +78,21 @@ where
         }
     };
 
-    if let Err(err) = write_file_header(&mut encoder, sess.is_nightly_build()) {
-        sess.err(&format!("failed to write {} header to `{}`: {}", name, path_buf.display(), err));
-        return;
-    }
-
-    if let Err(err) = encode(&mut encoder) {
-        sess.err(&format!("failed to write {} to `{}`: {}", name, path_buf.display(), err));
-        return;
-    }
+    write_file_header(&mut encoder, sess.is_nightly_build());
 
-    if let Err(err) = encoder.flush() {
-        sess.err(&format!("failed to flush {} to `{}`: {}", name, path_buf.display(), err));
-        return;
+    match encode(encoder) {
+        Ok(position) => {
+            sess.prof.artifact_size(
+                &name.replace(' ', "_"),
+                path_buf.file_name().unwrap().to_string_lossy(),
+                position as u64,
+            );
+            debug!("save: data written to disk successfully");
+        }
+        Err(err) => {
+            sess.err(&format!("failed to write {} to `{}`: {}", name, path_buf.display(), err));
+        }
     }
-
-    sess.prof.artifact_size(
-        &name.replace(' ', "_"),
-        path_buf.file_name().unwrap().to_string_lossy(),
-        encoder.position() as u64,
-    );
-
-    debug!("save: data written to disk successfully");
 }
 
 /// Reads the contents of a file with a file header as defined in this module.

compiler/rustc_incremental/src/persist/save.rs

@@ -3,7 +3,7 @@ use rustc_data_structures::sync::join;
 use rustc_middle::dep_graph::{DepGraph, SerializedDepGraph, WorkProduct, WorkProductId};
 use rustc_middle::ty::TyCtxt;
 use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
-use rustc_serialize::Encodable as RustcEncodable;
+use rustc_serialize::{Encodable as RustcEncodable, Encoder};
 use rustc_session::Session;
 use std::fs;
 
@@ -96,8 +96,9 @@ pub fn save_work_product_index(
     debug!("save_work_product_index()");
     dep_graph.assert_ignored();
     let path = work_products_path(sess);
-    file_format::save_in(sess, path, "work product index", |e| {
-        encode_work_product_index(&new_work_products, e)
+    file_format::save_in(sess, path, "work product index", |mut e| {
+        encode_work_product_index(&new_work_products, &mut e);
+        e.finish()
     });
 
     // We also need to clean out old work-products, as not all of them are
@@ -123,7 +124,7 @@ pub fn save_work_product_index(
 fn encode_work_product_index(
     work_products: &FxHashMap<WorkProductId, WorkProduct>,
     encoder: &mut FileEncoder,
-) -> FileEncodeResult {
+) {
     let serialized_products: Vec<_> = work_products
         .iter()
         .map(|(id, work_product)| SerializedWorkProduct {
@@ -135,7 +136,7 @@ fn encode_work_product_index(
     serialized_products.encode(encoder)
 }
 
-fn encode_query_cache(tcx: TyCtxt<'_>, encoder: &mut FileEncoder) -> FileEncodeResult {
+fn encode_query_cache(tcx: TyCtxt<'_>, encoder: FileEncoder) -> FileEncodeResult {
     tcx.sess.time("incr_comp_serialize_result_cache", || tcx.serialize_query_result_cache(encoder))
 }
 
@@ -170,24 +171,10 @@ pub fn build_dep_graph(
         }
     };
 
-    if let Err(err) = file_format::write_file_header(&mut encoder, sess.is_nightly_build()) {
-        sess.err(&format!(
-            "failed to write dependency graph header to `{}`: {}",
-            path_buf.display(),
-            err
-        ));
-        return None;
-    }
+    file_format::write_file_header(&mut encoder, sess.is_nightly_build());
 
     // First encode the commandline arguments hash
-    if let Err(err) = sess.opts.dep_tracking_hash(false).encode(&mut encoder) {
-        sess.err(&format!(
-            "failed to write dependency graph hash `{}`: {}",
-            path_buf.display(),
-            err
-        ));
-        return None;
-    }
+    sess.opts.dep_tracking_hash(false).encode(&mut encoder);
 
     Some(DepGraph::new(
         &sess.prof,

compiler/rustc_index/src/vec.rs

@@ -60,8 +60,8 @@ pub struct IndexVec<I: Idx, T> {
 unsafe impl<I: Idx, T> Send for IndexVec<I, T> where T: Send {}
 
 impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for IndexVec<I, T> {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        Encodable::encode(&self.raw, s)
+    fn encode(&self, s: &mut S) {
+        Encodable::encode(&self.raw, s);
     }
 }
 

compiler/rustc_macros/src/newtype.rs

@@ -137,8 +137,8 @@ impl Parse for Newtype {
                     }
                 }
                 impl<E: ::rustc_serialize::Encoder> ::rustc_serialize::Encodable<E> for #name {
-                    fn encode(&self, e: &mut E) -> Result<(), E::Error> {
-                        e.emit_u32(self.private)
+                    fn encode(&self, e: &mut E) {
+                        e.emit_u32(self.private);
                     }
                 }
             }

compiler/rustc_macros/src/serialize.rs

@@ -146,21 +146,17 @@ fn encodable_body(
                     .map(|binding| {
                         let bind_ident = &binding.binding;
                         let result = quote! {
-                            match ::rustc_serialize::Encodable::<#encoder_ty>::encode(
+                            ::rustc_serialize::Encodable::<#encoder_ty>::encode(
                                 #bind_ident,
                                 __encoder,
-                            ) {
-                                ::std::result::Result::Ok(()) => (),
-                                ::std::result::Result::Err(__err)
-                                    => return ::std::result::Result::Err(__err),
-                            }
+                            );
                         };
                         result
                     })
                     .collect::<TokenStream>()
             });
             quote! {
-                ::std::result::Result::Ok(match *self { #encode_inner })
+                match *self { #encode_inner }
             }
         }
         _ => {
@@ -172,14 +168,10 @@ fn encodable_body(
                     .map(|binding| {
                         let bind_ident = &binding.binding;
                         let result = quote! {
-                            match ::rustc_serialize::Encodable::<#encoder_ty>::encode(
+                            ::rustc_serialize::Encodable::<#encoder_ty>::encode(
                                 #bind_ident,
                                 __encoder,
-                            ) {
-                                ::std::result::Result::Ok(()) => (),
-                                ::std::result::Result::Err(__err)
-                                    => return ::std::result::Result::Err(__err),
-                            }
+                            );
                         };
                         result
                     })
@@ -190,7 +182,7 @@ fn encodable_body(
                         ::rustc_serialize::Encoder::emit_enum_variant(
                             __encoder,
                             #variant_idx,
-                            |__encoder| { ::std::result::Result::Ok({ #encode_fields }) }
+                            |__encoder| { #encode_fields }
                         )
                     }
                 } else {
@@ -223,7 +215,7 @@ fn encodable_body(
             fn encode(
                 &self,
                 __encoder: &mut #encoder_ty,
-            ) -> ::std::result::Result<(), <#encoder_ty as ::rustc_serialize::Encoder>::Error> {
+            ) {
                 #lints
                 #encode_body
             }

compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs

@@ -4,7 +4,7 @@ use crate::rmeta::MetadataBlob;
 use rustc_data_structures::owning_ref::OwningRef;
 use rustc_hir::def_path_hash_map::{Config as HashMapConfig, DefPathHashMap};
 use rustc_middle::parameterized_over_tcx;
-use rustc_serialize::{opaque, Decodable, Decoder, Encodable, Encoder};
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::def_id::{DefIndex, DefPathHash};
 
 pub(crate) enum DefPathHashMapRef<'tcx> {
@@ -29,12 +29,12 @@ impl DefPathHashMapRef<'_> {
 }
 
 impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for DefPathHashMapRef<'tcx> {
-    fn encode(&self, e: &mut EncodeContext<'a, 'tcx>) -> opaque::EncodeResult {
+    fn encode(&self, e: &mut EncodeContext<'a, 'tcx>) {
         match *self {
             DefPathHashMapRef::BorrowedFromTcx(def_path_hash_map) => {
                 let bytes = def_path_hash_map.raw_bytes();
-                e.emit_usize(bytes.len())?;
-                e.emit_raw_bytes(bytes)
+                e.emit_usize(bytes.len());
+                e.emit_raw_bytes(bytes);
             }
             DefPathHashMapRef::OwnedFromMetadata(_) => {
                 panic!("DefPathHashMap::OwnedFromMetadata variant only exists for deserialization")