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`.

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;
+    write_file_header(&mut encoder, sess.is_nightly_build());
+
+    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));
+        }
     }
-
-    if let Err(err) = encode(&mut encoder) {
-        sess.err(&format!("failed to write {} to `{}`: {}", name, path_buf.display(), err));
-        return;
-    }
-
-    if let Err(err) = encoder.flush() {
-        sess.err(&format!("failed to flush {} to `{}`: {}", name, path_buf.display(), err));
-        return;
-    }
-
-    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,