Auto merge of #93066 - nnethercote:infallible-decoder, r=bjorn3

Make `Decodable` and `Decoder` infallible.

`Decoder` has two impls:
- opaque: this impl is already partly infallible, i.e. in some places it
  currently panics on failure (e.g. if the input is too short, or on a
  bad `Result` discriminant), and in some places it returns an error
  (e.g. on a bad `Option` discriminant). The number of places where
  either happens is surprisingly small, just because the binary
  representation has very little redundancy and a lot of input reading
  can occur even on malformed data.
- json: this impl is fully fallible, but it's only used (a) for the
  `.rlink` file production, and there's a `FIXME` comment suggesting it
  should change to a binary format, and (b) in a few tests in
  non-fundamental ways. Indeed #85993 is open to remove it entirely.

And the top-level places in the compiler that call into decoding just
abort on error anyway. So the fallibility is providing little value, and
getting rid of it leads to some non-trivial performance improvements.

Much of this PR is pretty boring and mechanical. Some notes about
a few interesting parts:
- The commit removes `Decoder::{Error,error}`.
- `InternIteratorElement::intern_with`: the impl for `T` now has the same
  optimization for small counts that the impl for `Result<T, E>` has,
  because it's now much hotter.
- Decodable impls for SmallVec, LinkedList, VecDeque now all use
  `collect`, which is nice; the one for `Vec` uses unsafe code, because
  that gave better perf on some benchmarks.

r? `@bjorn3`

compiler/rustc_ast/src/ast.rs

@@ -2418,8 +2418,9 @@ impl<S: Encoder> rustc_serialize::Encodable<S> for AttrId {
 }
 
 impl<D: Decoder> rustc_serialize::Decodable<D> for AttrId {
-    fn decode(d: &mut D) -> Result<AttrId, D::Error> {
-        d.read_nil().map(|_| crate::attr::mk_attr_id())
+    fn decode(d: &mut D) -> AttrId {
+        d.read_unit();
+        crate::attr::mk_attr_id()
     }
 }
 

compiler/rustc_ast/src/ptr.rs

@@ -115,8 +115,8 @@ impl<T> fmt::Pointer for P<T> {
 }
 
 impl<D: Decoder, T: 'static + Decodable<D>> Decodable<D> for P<T> {
-    fn decode(d: &mut D) -> Result<P<T>, D::Error> {
-        Decodable::decode(d).map(P)
+    fn decode(d: &mut D) -> P<T> {
+        P(Decodable::decode(d))
     }
 }
 
@@ -204,8 +204,8 @@ impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<[T]> {
 }
 
 impl<D: Decoder, T: Decodable<D>> Decodable<D> for P<[T]> {
-    fn decode(d: &mut D) -> Result<P<[T]>, D::Error> {
-        Ok(P::from_vec(Decodable::decode(d)?))
+    fn decode(d: &mut D) -> P<[T]> {
+        P::from_vec(Decodable::decode(d))
     }
 }
 

compiler/rustc_ast/src/tokenstream.rs

@@ -163,7 +163,7 @@ impl<S: Encoder> Encodable<S> for LazyTokenStream {
 }
 
 impl<D: Decoder> Decodable<D> for LazyTokenStream {
-    fn decode(_d: &mut D) -> Result<Self, D::Error> {
+    fn decode(_d: &mut D) -> Self {
         panic!("Attempted to decode LazyTokenStream");
     }
 }

compiler/rustc_data_structures/src/fingerprint.rs

@@ -149,10 +149,10 @@ impl<E: rustc_serialize::Encoder> Encodable<E> for Fingerprint {
 
 impl<D: rustc_serialize::Decoder> Decodable<D> for Fingerprint {
     #[inline]
-    fn decode(d: &mut D) -> Result<Self, D::Error> {
+    fn decode(d: &mut D) -> Self {
         let mut bytes = [0u8; 16];
-        d.read_raw_bytes_into(&mut bytes)?;
-        Ok(Fingerprint::from_le_bytes(bytes))
+        d.read_raw_bytes_into(&mut bytes);
+        Fingerprint::from_le_bytes(bytes)
     }
 }
 
@@ -195,8 +195,8 @@ impl<E: rustc_serialize::Encoder> Encodable<E> for PackedFingerprint {
 
 impl<D: rustc_serialize::Decoder> Decodable<D> for PackedFingerprint {
     #[inline]
-    fn decode(d: &mut D) -> Result<Self, D::Error> {
-        Fingerprint::decode(d).map(PackedFingerprint)
+    fn decode(d: &mut D) -> Self {
+        Self(Fingerprint::decode(d))
     }
 }
 

compiler/rustc_data_structures/src/svh.rs

@@ -55,8 +55,8 @@ impl<S: Encoder> Encodable<S> for Svh {
 }
 
 impl<D: Decoder> Decodable<D> for Svh {
-    fn decode(d: &mut D) -> Result<Svh, D::Error> {
-        d.read_u64().map(u64::from_le).map(Svh::new)
+    fn decode(d: &mut D) -> Svh {
+        Svh::new(u64::from_le(d.read_u64()))
     }
 }
 

compiler/rustc_driver/src/lib.rs

@@ -597,10 +597,7 @@ impl RustcDefaultCalls {
                 let rlink_data = fs::read_to_string(file).unwrap_or_else(|err| {
                     sess.fatal(&format!("failed to read rlink file: {}", err));
                 });
-                let codegen_results: CodegenResults =
-                    json::decode(&rlink_data).unwrap_or_else(|err| {
-                        sess.fatal(&format!("failed to decode rlink: {}", err));
-                    });
+                let codegen_results: CodegenResults = json::decode(&rlink_data);
                 let result = compiler.codegen_backend().link(sess, codegen_results, &outputs);
                 abort_on_err(result, sess);
             } else {

compiler/rustc_errors/src/json/tests.rs

@@ -64,7 +64,7 @@ fn test_positions(code: &str, span: (u32, u32), expected_output: SpanTestData) {
 
         let bytes = output.lock().unwrap();
         let actual_output = str::from_utf8(&bytes).unwrap();
-        let actual_output: TestData = decode(actual_output).unwrap();
+        let actual_output: TestData = decode(actual_output);
 
         assert_eq!(expected_output, actual_output)
     })

compiler/rustc_errors/src/lib.rs

@@ -99,8 +99,8 @@ impl Hash for ToolMetadata {
 
 // Doesn't really need to round-trip
 impl<D: Decoder> Decodable<D> for ToolMetadata {
-    fn decode(_d: &mut D) -> Result<Self, D::Error> {
-        Ok(ToolMetadata(None))
+    fn decode(_d: &mut D) -> Self {
+        ToolMetadata(None)
     }
 }
 

compiler/rustc_incremental/src/persist/load.rs

@@ -158,14 +158,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
             // Decode the list of work_products
             let mut work_product_decoder = Decoder::new(&work_products_data[..], start_pos);
             let work_products: Vec<SerializedWorkProduct> =
-                Decodable::decode(&mut work_product_decoder).unwrap_or_else(|e| {
-                    let msg = format!(
-                        "Error decoding `work-products` from incremental \
-                                    compilation session directory: {}",
-                        e
-                    );
-                    sess.fatal(&msg)
-                });
+                Decodable::decode(&mut work_product_decoder);
 
             for swp in work_products {
                 let mut all_files_exist = true;
@@ -203,8 +196,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
             LoadResult::Error { message } => LoadResult::Error { message },
             LoadResult::Ok { data: (bytes, start_pos) } => {
                 let mut decoder = Decoder::new(&bytes, start_pos);
-                let prev_commandline_args_hash = u64::decode(&mut decoder)
-                    .expect("Error reading commandline arg hash from cached dep-graph");
+                let prev_commandline_args_hash = u64::decode(&mut decoder);
 
                 if prev_commandline_args_hash != expected_hash {
                     if report_incremental_info {
@@ -220,8 +212,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
                     return LoadResult::DataOutOfDate;
                 }
 
-                let dep_graph = SerializedDepGraph::decode(&mut decoder)
-                    .expect("Error reading cached dep-graph");
+                let dep_graph = SerializedDepGraph::decode(&mut decoder);
 
                 LoadResult::Ok { data: (dep_graph, prev_work_products) }
             }

compiler/rustc_index/src/vec.rs

@@ -395,8 +395,8 @@ macro_rules! newtype_index {
 
     (@serializable $type:ident) => (
         impl<D: ::rustc_serialize::Decoder> ::rustc_serialize::Decodable<D> for $type {
-            fn decode(d: &mut D) -> Result<Self, D::Error> {
-                d.read_u32().map(Self::from_u32)
+            fn decode(d: &mut D) -> Self {
+                Self::from_u32(d.read_u32())
             }
         }
         impl<E: ::rustc_serialize::Encoder> ::rustc_serialize::Encodable<E> for $type {
@@ -527,8 +527,8 @@ impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for &IndexVec<I, T> {
 }
 
 impl<D: Decoder, I: Idx, T: Decodable<D>> Decodable<D> for IndexVec<I, T> {
-    fn decode(d: &mut D) -> Result<Self, D::Error> {
-        Decodable::decode(d).map(|v| IndexVec { raw: v, _marker: PhantomData })
+    fn decode(d: &mut D) -> Self {
+        IndexVec { raw: Decodable::decode(d), _marker: PhantomData }
     }
 }
 

compiler/rustc_macros/src/serialize.rs

@@ -47,7 +47,7 @@ fn decodable_body(
             quote! {
                 ::rustc_serialize::Decoder::read_struct(
                     __decoder,
-                    |__decoder| { ::std::result::Result::Ok(#construct) },
+                    |__decoder| { #construct },
                 )
             }
         }
@@ -57,7 +57,7 @@ fn decodable_body(
                 .enumerate()
                 .map(|(idx, vi)| {
                     let construct = vi.construct(|field, index| decode_field(field, index, false));
-                    quote! { #idx => { ::std::result::Result::Ok(#construct) } }
+                    quote! { #idx => { #construct } }
                 })
                 .collect();
             let names: TokenStream = variants
@@ -82,8 +82,7 @@ fn decodable_body(
                             |__decoder, __variant_idx| {
                                 match __variant_idx {
                                     #match_inner
-                                    _ => return ::std::result::Result::Err(
-                                        ::rustc_serialize::Decoder::error(__decoder, #message)),
+                                    _ => panic!(#message),
                                 }
                             })
                     }
@@ -95,9 +94,7 @@ fn decodable_body(
     s.bound_impl(
         quote!(::rustc_serialize::Decodable<#decoder_ty>),
         quote! {
-            fn decode(
-                __decoder: &mut #decoder_ty,
-            ) -> ::std::result::Result<Self, <#decoder_ty as ::rustc_serialize::Decoder>::Error> {
+            fn decode(__decoder: &mut #decoder_ty) -> Self {
                 #decode_body
             }
         },
@@ -127,12 +124,7 @@ fn decode_field(field: &syn::Field, index: usize, is_struct: bool) -> proc_macro
                 #__decoder, #opt_field_name #decode_inner_method)
     };
 
-    quote! {
-        match #decode_call  {
-            ::std::result::Result::Ok(__res) => __res,
-            ::std::result::Result::Err(__err) => return ::std::result::Result::Err(__err),
-        }
-    }
+    quote! { #decode_call }
 }
 
 pub fn type_encodable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {