Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

24 files changed, 6006 insertions, 0 deletions

diff --git a/third_party/rust/prost-derive/.cargo-checksum.json b/third_party/rust/prost-derive/.cargo-checksum.json
new file mode 100644
index 0000000000..e64d4cd29b
--- /dev/null
+++ b/third_party/rust/prost-derive/.cargo-checksum.json
@@ -0,0 +1 @@
+{"files":{"Cargo.toml":"4af10c29150183b223cd13b9eb0375fdbef6daba35e43a654c55c0491ca78ba2","README.md":"6c67fa1e48f14adfaf834f520f798ddfb79f90804f46cc215ee391a7d57913a4","src/field/group.rs":"dfd31b34008741abc857dc11362c11a06afd8eddceb62924443bf2bea117460a","src/field/map.rs":"69e1fa14cfeb4d4407b9c343a2a72b4ba68c3ecb7af912965a63b50e91dd61fb","src/field/message.rs":"fb910a563eba0b0c68a7a3855f7877535931faef4d788b05a3402f0cdd037e3e","src/field/mod.rs":"ca917a3f673623f0946e9e37ebad2916f3c1f8d163a1844143ef7d21d245394c","src/field/oneof.rs":"4fc488445b05e464070fadd8799cafb806db5c23d1494c4300cb293394863012","src/field/scalar.rs":"03d69f74daa2037a0210a086bd873ae9beca6b615e2973c96c43a6d0d30aa525","src/lib.rs":"8cfa6f1fc707e8931df82f9df67fa07e1b822b7581e4c9733e3ce24febabf708"},"package":"600d2f334aa05acb02a755e217ef1ab6dea4d51b58b7846588b747edec04efba"}
\ No newline at end of file
diff --git a/third_party/rust/prost-derive/Cargo.toml b/third_party/rust/prost-derive/Cargo.toml
new file mode 100644
index 0000000000..d6c2d3e74c
--- /dev/null
+++ b/third_party/rust/prost-derive/Cargo.toml
@@ -0,0 +1,40 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "prost-derive"
+version = "0.8.0"
+authors = ["Dan Burkert <dan@danburkert.com>", "Tokio Contributors <team@tokio.rs>"]
+description = "A Protocol Buffers implementation for the Rust Language."
+documentation = "https://docs.rs/prost-derive"
+readme = "README.md"
+license = "Apache-2.0"
+repository = "https://github.com/tokio-rs/prost"
+
+[lib]
+proc_macro = true
+[dependencies.anyhow]
+version = "1"
+
+[dependencies.itertools]
+version = "0.10"
+
+[dependencies.proc-macro2]
+version = "1"
+
+[dependencies.quote]
+version = "1"
+
+[dependencies.syn]
+version = "1"
+features = ["extra-traits"]
diff --git a/third_party/rust/prost-derive/README.md b/third_party/rust/prost-derive/README.md
new file mode 100644
index 0000000000..a51050e7e6
--- /dev/null
+++ b/third_party/rust/prost-derive/README.md
@@ -0,0 +1,16 @@
+[![Documentation](https://docs.rs/prost-derive/badge.svg)](https://docs.rs/prost-derive/)
+[![Crate](https://img.shields.io/crates/v/prost-derive.svg)](https://crates.io/crates/prost-derive)
+
+# prost-derive
+
+`prost-derive` handles generating encoding and decoding implementations for Rust
+types annotated with `prost` annotation. For the most part, users of `prost`
+shouldn't need to interact with `prost-derive` directly.
+
+## License
+
+`prost-derive` is distributed under the terms of the Apache License (Version 2.0).
+
+See [LICENSE](../LICENSE) for details.
+
+Copyright 2017 Dan Burkert
diff --git a/third_party/rust/prost-derive/src/field/group.rs b/third_party/rust/prost-derive/src/field/group.rs
new file mode 100644
index 0000000000..6396128802
--- /dev/null
+++ b/third_party/rust/prost-derive/src/field/group.rs
@@ -0,0 +1,134 @@
+use anyhow::{bail, Error};
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::Meta;
+
+use crate::field::{set_bool, set_option, tag_attr, word_attr, Label};
+
+#[derive(Clone)]
+pub struct Field {
+    pub label: Label,
+    pub tag: u32,
+}
+
+impl Field {
+    pub fn new(attrs: &[Meta], inferred_tag: Option<u32>) -> Result<Option<Field>, Error> {
+        let mut group = false;
+        let mut label = None;
+        let mut tag = None;
+        let mut boxed = false;
+
+        let mut unknown_attrs = Vec::new();
+
+        for attr in attrs {
+            if word_attr("group", attr) {
+                set_bool(&mut group, "duplicate group attributes")?;
+            } else if word_attr("boxed", attr) {
+                set_bool(&mut boxed, "duplicate boxed attributes")?;
+            } else if let Some(t) = tag_attr(attr)? {
+                set_option(&mut tag, t, "duplicate tag attributes")?;
+            } else if let Some(l) = Label::from_attr(attr) {
+                set_option(&mut label, l, "duplicate label attributes")?;
+            } else {
+                unknown_attrs.push(attr);
+            }
+        }
+
+        if !group {
+            return Ok(None);
+        }
+
+        match unknown_attrs.len() {
+            0 => (),
+            1 => bail!("unknown attribute for group field: {:?}", unknown_attrs[0]),
+            _ => bail!("unknown attributes for group field: {:?}", unknown_attrs),
+        }
+
+        let tag = match tag.or(inferred_tag) {
+            Some(tag) => tag,
+            None => bail!("group field is missing a tag attribute"),
+        };
+
+        Ok(Some(Field {
+            label: label.unwrap_or(Label::Optional),
+            tag,
+        }))
+    }
+
+    pub fn new_oneof(attrs: &[Meta]) -> Result<Option<Field>, Error> {
+        if let Some(mut field) = Field::new(attrs, None)? {
+            if let Some(attr) = attrs.iter().find(|attr| Label::from_attr(attr).is_some()) {
+                bail!(
+                    "invalid attribute for oneof field: {}",
+                    attr.path().into_token_stream()
+                );
+            }
+            field.label = Label::Required;
+            Ok(Some(field))
+        } else {
+            Ok(None)
+        }
+    }
+
+    pub fn encode(&self, ident: TokenStream) -> TokenStream {
+        let tag = self.tag;
+        match self.label {
+            Label::Optional => quote! {
+                if let Some(ref msg) = #ident {
+                    ::prost::encoding::group::encode(#tag, msg, buf);
+                }
+            },
+            Label::Required => quote! {
+                ::prost::encoding::group::encode(#tag, &#ident, buf);
+            },
+            Label::Repeated => quote! {
+                for msg in &#ident {
+                    ::prost::encoding::group::encode(#tag, msg, buf);
+                }
+            },
+        }
+    }
+
+    pub fn merge(&self, ident: TokenStream) -> TokenStream {
+        match self.label {
+            Label::Optional => quote! {
+                ::prost::encoding::group::merge(
+                    tag,
+                    wire_type,
+                    #ident.get_or_insert_with(Default::default),
+                    buf,
+                    ctx,
+                )
+            },
+            Label::Required => quote! {
+                ::prost::encoding::group::merge(tag, wire_type, #ident, buf, ctx)
+            },
+            Label::Repeated => quote! {
+                ::prost::encoding::group::merge_repeated(tag, wire_type, #ident, buf, ctx)
+            },
+        }
+    }
+
+    pub fn encoded_len(&self, ident: TokenStream) -> TokenStream {
+        let tag = self.tag;
+        match self.label {
+            Label::Optional => quote! {
+                #ident.as_ref().map_or(0, |msg| ::prost::encoding::group::encoded_len(#tag, msg))
+            },
+            Label::Required => quote! {
+                ::prost::encoding::group::encoded_len(#tag, &#ident)
+            },
+            Label::Repeated => quote! {
+                ::prost::encoding::group::encoded_len_repeated(#tag, &#ident)
+            },
+        }
+    }
+
+    pub fn clear(&self, ident: TokenStream) -> TokenStream {
+        match self.label {
+            Label::Optional => quote!(#ident = ::core::option::Option::None),
+            Label::Required => quote!(#ident.clear()),
+            Label::Repeated => quote!(#ident.clear()),
+        }
+    }
+}
diff --git a/third_party/rust/prost-derive/src/field/map.rs b/third_party/rust/prost-derive/src/field/map.rs
new file mode 100644
index 0000000000..1228a6fae2
--- /dev/null
+++ b/third_party/rust/prost-derive/src/field/map.rs
@@ -0,0 +1,394 @@
+use anyhow::{bail, Error};
+use proc_macro2::{Span, TokenStream};
+use quote::quote;
+use syn::{Ident, Lit, Meta, MetaNameValue, NestedMeta};
+
+use crate::field::{scalar, set_option, tag_attr};
+
+#[derive(Clone, Debug)]
+pub enum MapTy {
+    HashMap,
+    BTreeMap,
+}
+
+impl MapTy {
+    fn from_str(s: &str) -> Option<MapTy> {
+        match s {
+            "map" | "hash_map" => Some(MapTy::HashMap),
+            "btree_map" => Some(MapTy::BTreeMap),
+            _ => None,
+        }
+    }
+
+    fn module(&self) -> Ident {
+        match *self {
+            MapTy::HashMap => Ident::new("hash_map", Span::call_site()),
+            MapTy::BTreeMap => Ident::new("btree_map", Span::call_site()),
+        }
+    }
+
+    fn lib(&self) -> TokenStream {
+        match self {
+            MapTy::HashMap => quote! { std },
+            MapTy::BTreeMap => quote! { prost::alloc },
+        }
+    }
+}
+
+fn fake_scalar(ty: scalar::Ty) -> scalar::Field {
+    let kind = scalar::Kind::Plain(scalar::DefaultValue::new(&ty));
+    scalar::Field {
+        ty,
+        kind,
+        tag: 0, // Not used here
+    }
+}
+
+#[derive(Clone)]
+pub struct Field {
+    pub map_ty: MapTy,
+    pub key_ty: scalar::Ty,
+    pub value_ty: ValueTy,
+    pub tag: u32,
+}
+
+impl Field {
+    pub fn new(attrs: &[Meta], inferred_tag: Option<u32>) -> Result<Option<Field>, Error> {
+        let mut types = None;
+        let mut tag = None;
+
+        for attr in attrs {
+            if let Some(t) = tag_attr(attr)? {
+                set_option(&mut tag, t, "duplicate tag attributes")?;
+            } else if let Some(map_ty) = attr
+                .path()
+                .get_ident()
+                .and_then(|i| MapTy::from_str(&i.to_string()))
+            {
+                let (k, v): (String, String) = match &*attr {
+                    Meta::NameValue(MetaNameValue {
+                        lit: Lit::Str(lit), ..
+                    }) => {
+                        let items = lit.value();
+                        let mut items = items.split(',').map(ToString::to_string);
+                        let k = items.next().unwrap();
+                        let v = match items.next() {
+                            Some(k) => k,
+                            None => bail!("invalid map attribute: must have key and value types"),
+                        };
+                        if items.next().is_some() {
+                            bail!("invalid map attribute: {:?}", attr);
+                        }
+                        (k, v)
+                    }
+                    Meta::List(meta_list) => {
+                        // TODO(rustlang/rust#23121): slice pattern matching would make this much nicer.
+                        if meta_list.nested.len() != 2 {
+                            bail!("invalid map attribute: must contain key and value types");
+                        }
+                        let k = match &meta_list.nested[0] {
+                            NestedMeta::Meta(Meta::Path(k)) if k.get_ident().is_some() => {
+                                k.get_ident().unwrap().to_string()
+                            }
+                            _ => bail!("invalid map attribute: key must be an identifier"),
+                        };
+                        let v = match &meta_list.nested[1] {
+                            NestedMeta::Meta(Meta::Path(v)) if v.get_ident().is_some() => {
+                                v.get_ident().unwrap().to_string()
+                            }
+                            _ => bail!("invalid map attribute: value must be an identifier"),
+                        };
+                        (k, v)
+                    }
+                    _ => return Ok(None),
+                };
+                set_option(
+                    &mut types,
+                    (map_ty, key_ty_from_str(&k)?, ValueTy::from_str(&v)?),
+                    "duplicate map type attribute",
+                )?;
+            } else {
+                return Ok(None);
+            }
+        }
+
+        Ok(match (types, tag.or(inferred_tag)) {
+            (Some((map_ty, key_ty, value_ty)), Some(tag)) => Some(Field {
+                map_ty,
+                key_ty,
+                value_ty,
+                tag,
+            }),
+            _ => None,
+        })
+    }
+
+    pub fn new_oneof(attrs: &[Meta]) -> Result<Option<Field>, Error> {
+        Field::new(attrs, None)
+    }
+
+    /// Returns a statement which encodes the map field.
+    pub fn encode(&self, ident: TokenStream) -> TokenStream {
+        let tag = self.tag;
+        let key_mod = self.key_ty.module();
+        let ke = quote!(::prost::encoding::#key_mod::encode);
+        let kl = quote!(::prost::encoding::#key_mod::encoded_len);
+        let module = self.map_ty.module();
+        match &self.value_ty {
+            ValueTy::Scalar(scalar::Ty::Enumeration(ty)) => {
+                let default = quote!(#ty::default() as i32);
+                quote! {
+                    ::prost::encoding::#module::encode_with_default(
+                        #ke,
+                        #kl,
+                        ::prost::encoding::int32::encode,
+                        ::prost::encoding::int32::encoded_len,
+                        &(#default),
+                        #tag,
+                        &#ident,
+                        buf,
+                    );
+                }
+            }
+            ValueTy::Scalar(value_ty) => {
+                let val_mod = value_ty.module();
+                let ve = quote!(::prost::encoding::#val_mod::encode);
+                let vl = quote!(::prost::encoding::#val_mod::encoded_len);
+                quote! {
+                    ::prost::encoding::#module::encode(
+                        #ke,
+                        #kl,
+                        #ve,
+                        #vl,
+                        #tag,
+                        &#ident,
+                        buf,
+                    );
+                }
+            }
+            ValueTy::Message => quote! {
+                ::prost::encoding::#module::encode(
+                    #ke,
+                    #kl,
+                    ::prost::encoding::message::encode,
+                    ::prost::encoding::message::encoded_len,
+                    #tag,
+                    &#ident,
+                    buf,
+                );
+            },
+        }
+    }
+
+    /// Returns an expression which evaluates to the result of merging a decoded key value pair
+    /// into the map.
+    pub fn merge(&self, ident: TokenStream) -> TokenStream {
+        let key_mod = self.key_ty.module();
+        let km = quote!(::prost::encoding::#key_mod::merge);
+        let module = self.map_ty.module();
+        match &self.value_ty {
+            ValueTy::Scalar(scalar::Ty::Enumeration(ty)) => {
+                let default = quote!(#ty::default() as i32);
+                quote! {
+                    ::prost::encoding::#module::merge_with_default(
+                        #km,
+                        ::prost::encoding::int32::merge,
+                        #default,
+                        &mut #ident,
+                        buf,
+                        ctx,
+                    )
+                }
+            }
+            ValueTy::Scalar(value_ty) => {
+                let val_mod = value_ty.module();
+                let vm = quote!(::prost::encoding::#val_mod::merge);
+                quote!(::prost::encoding::#module::merge(#km, #vm, &mut #ident, buf, ctx))
+            }
+            ValueTy::Message => quote! {
+                ::prost::encoding::#module::merge(
+                    #km,
+                    ::prost::encoding::message::merge,
+                    &mut #ident,
+                    buf,
+                    ctx,
+                )
+            },
+        }
+    }
+
+    /// Returns an expression which evaluates to the encoded length of the map.
+    pub fn encoded_len(&self, ident: TokenStream) -> TokenStream {
+        let tag = self.tag;
+        let key_mod = self.key_ty.module();
+        let kl = quote!(::prost::encoding::#key_mod::encoded_len);
+        let module = self.map_ty.module();
+        match &self.value_ty {
+            ValueTy::Scalar(scalar::Ty::Enumeration(ty)) => {
+                let default = quote!(#ty::default() as i32);
+                quote! {
+                    ::prost::encoding::#module::encoded_len_with_default(
+                        #kl,
+                        ::prost::encoding::int32::encoded_len,
+                        &(#default),
+                        #tag,
+                        &#ident,
+                    )
+                }
+            }
+            ValueTy::Scalar(value_ty) => {
+                let val_mod = value_ty.module();
+                let vl = quote!(::prost::encoding::#val_mod::encoded_len);
+                quote!(::prost::encoding::#module::encoded_len(#kl, #vl, #tag, &#ident))
+            }
+            ValueTy::Message => quote! {
+                ::prost::encoding::#module::encoded_len(
+                    #kl,
+                    ::prost::encoding::message::encoded_len,
+                    #tag,
+                    &#ident,
+                )
+            },
+        }
+    }
+
+    pub fn clear(&self, ident: TokenStream) -> TokenStream {
+        quote!(#ident.clear())
+    }
+
+    /// Returns methods to embed in the message.
+    pub fn methods(&self, ident: &Ident) -> Option<TokenStream> {
+        if let ValueTy::Scalar(scalar::Ty::Enumeration(ty)) = &self.value_ty {
+            let key_ty = self.key_ty.rust_type();
+            let key_ref_ty = self.key_ty.rust_ref_type();
+
+            let get = Ident::new(&format!("get_{}", ident), Span::call_site());
+            let insert = Ident::new(&format!("insert_{}", ident), Span::call_site());
+            let take_ref = if self.key_ty.is_numeric() {
+                quote!(&)
+            } else {
+                quote!()
+            };
+
+            let get_doc = format!(
+                "Returns the enum value for the corresponding key in `{}`, \
+                 or `None` if the entry does not exist or it is not a valid enum value.",
+                ident,
+            );
+            let insert_doc = format!("Inserts a key value pair into `{}`.", ident);
+            Some(quote! {
+                #[doc=#get_doc]
+                pub fn #get(&self, key: #key_ref_ty) -> ::core::option::Option<#ty> {
+                    self.#ident.get(#take_ref key).cloned().and_then(#ty::from_i32)
+                }
+                #[doc=#insert_doc]
+                pub fn #insert(&mut self, key: #key_ty, value: #ty) -> ::core::option::Option<#ty> {
+                    self.#ident.insert(key, value as i32).and_then(#ty::from_i32)
+                }
+            })
+        } else {
+            None
+        }
+    }
+
+    /// Returns a newtype wrapper around the map, implementing nicer Debug
+    ///
+    /// The Debug tries to convert any enumerations met into the variants if possible, instead of
+    /// outputting the raw numbers.
+    pub fn debug(&self, wrapper_name: TokenStream) -> TokenStream {
+        let type_name = match self.map_ty {
+            MapTy::HashMap => Ident::new("HashMap", Span::call_site()),
+            MapTy::BTreeMap => Ident::new("BTreeMap", Span::call_site()),
+        };
+
+        // A fake field for generating the debug wrapper
+        let key_wrapper = fake_scalar(self.key_ty.clone()).debug(quote!(KeyWrapper));
+        let key = self.key_ty.rust_type();
+        let value_wrapper = self.value_ty.debug();
+        let libname = self.map_ty.lib();
+        let fmt = quote! {
+            fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                #key_wrapper
+                #value_wrapper
+                let mut builder = f.debug_map();
+                for (k, v) in self.0 {
+                    builder.entry(&KeyWrapper(k), &ValueWrapper(v));
+                }
+                builder.finish()
+            }
+        };
+        match &self.value_ty {
+            ValueTy::Scalar(ty) => {
+                let value = ty.rust_type();
+                quote! {
+                    struct #wrapper_name<'a>(&'a ::#libname::collections::#type_name<#key, #value>);
+                    impl<'a> ::core::fmt::Debug for #wrapper_name<'a> {
+                        #fmt
+                    }
+                }
+            }
+            ValueTy::Message => quote! {
+                struct #wrapper_name<'a, V: 'a>(&'a ::#libname::collections::#type_name<#key, V>);
+                impl<'a, V> ::core::fmt::Debug for #wrapper_name<'a, V>
+                where
+                    V: ::core::fmt::Debug + 'a,
+                {
+                    #fmt
+                }
+            },
+        }
+    }
+}
+
+fn key_ty_from_str(s: &str) -> Result<scalar::Ty, Error> {
+    let ty = scalar::Ty::from_str(s)?;
+    match ty {
+        scalar::Ty::Int32
+        | scalar::Ty::Int64
+        | scalar::Ty::Uint32
+        | scalar::Ty::Uint64
+        | scalar::Ty::Sint32
+        | scalar::Ty::Sint64
+        | scalar::Ty::Fixed32
+        | scalar::Ty::Fixed64
+        | scalar::Ty::Sfixed32
+        | scalar::Ty::Sfixed64
+        | scalar::Ty::Bool
+        | scalar::Ty::String => Ok(ty),
+        _ => bail!("invalid map key type: {}", s),
+    }
+}
+
+/// A map value type.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum ValueTy {
+    Scalar(scalar::Ty),
+    Message,
+}
+
+impl ValueTy {
+    fn from_str(s: &str) -> Result<ValueTy, Error> {
+        if let Ok(ty) = scalar::Ty::from_str(s) {
+            Ok(ValueTy::Scalar(ty))
+        } else if s.trim() == "message" {
+            Ok(ValueTy::Message)
+        } else {
+            bail!("invalid map value type: {}", s);
+        }
+    }
+
+    /// Returns a newtype wrapper around the ValueTy for nicer debug.
+    ///
+    /// If the contained value is enumeration, it tries to convert it to the variant. If not, it
+    /// just forwards the implementation.
+    fn debug(&self) -> TokenStream {
+        match self {
+            ValueTy::Scalar(ty) => fake_scalar(ty.clone()).debug(quote!(ValueWrapper)),
+            ValueTy::Message => quote!(
+                fn ValueWrapper<T>(v: T) -> T {
+                    v
+                }
+            ),
+        }
+    }
+}
diff --git a/third_party/rust/prost-derive/src/field/message.rs b/third_party/rust/prost-derive/src/field/message.rs
new file mode 100644
index 0000000000..1ff7c37983
--- /dev/null
+++ b/third_party/rust/prost-derive/src/field/message.rs
@@ -0,0 +1,134 @@
+use anyhow::{bail, Error};
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::Meta;
+
+use crate::field::{set_bool, set_option, tag_attr, word_attr, Label};
+
+#[derive(Clone)]
+pub struct Field {
+    pub label: Label,
+    pub tag: u32,
+}
+
+impl Field {
+    pub fn new(attrs: &[Meta], inferred_tag: Option<u32>) -> Result<Option<Field>, Error> {
+        let mut message = false;
+        let mut label = None;
+        let mut tag = None;
+        let mut boxed = false;
+
+        let mut unknown_attrs = Vec::new();
+
+        for attr in attrs {
+            if word_attr("message", attr) {
+                set_bool(&mut message, "duplicate message attribute")?;
+            } else if word_attr("boxed", attr) {
+                set_bool(&mut boxed, "duplicate boxed attribute")?;
+            } else if let Some(t) = tag_attr(attr)? {
+                set_option(&mut tag, t, "duplicate tag attributes")?;
+            } else if let Some(l) = Label::from_attr(attr) {
+                set_option(&mut label, l, "duplicate label attributes")?;
+            } else {
+                unknown_attrs.push(attr);
+            }
+        }
+
+        if !message {
+            return Ok(None);
+        }
+
+        match unknown_attrs.len() {
+            0 => (),
+            1 => bail!(
+                "unknown attribute for message field: {:?}",
+                unknown_attrs[0]
+            ),
+            _ => bail!("unknown attributes for message field: {:?}", unknown_attrs),
+        }
+
+        let tag = match tag.or(inferred_tag) {
+            Some(tag) => tag,
+            None => bail!("message field is missing a tag attribute"),
+        };
+
+        Ok(Some(Field {
+            label: label.unwrap_or(Label::Optional),
+            tag,
+        }))
+    }
+
+    pub fn new_oneof(attrs: &[Meta]) -> Result<Option<Field>, Error> {
+        if let Some(mut field) = Field::new(attrs, None)? {
+            if let Some(attr) = attrs.iter().find(|attr| Label::from_attr(attr).is_some()) {
+                bail!(
+                    "invalid attribute for oneof field: {}",
+                    attr.path().into_token_stream()
+                );
+            }
+            field.label = Label::Required;
+            Ok(Some(field))
+        } else {
+            Ok(None)
+        }
+    }
+
+    pub fn encode(&self, ident: TokenStream) -> TokenStream {
+        let tag = self.tag;
+        match self.label {
+            Label::Optional => quote! {
+                if let Some(ref msg) = #ident {
+                    ::prost::encoding::message::encode(#tag, msg, buf);
+                }
+            },
+            Label::Required => quote! {
+                ::prost::encoding::message::encode(#tag, &#ident, buf);
+            },
+            Label::Repeated => quote! {
+                for msg in &#ident {
+                    ::prost::encoding::message::encode(#tag, msg, buf);
+                }
+            },
+        }
+    }
+
+    pub fn merge(&self, ident: TokenStream) -> TokenStream {
+        match self.label {
+            Label::Optional => quote! {
+                ::prost::encoding::message::merge(wire_type,
+                                                 #ident.get_or_insert_with(Default::default),
+                                                 buf,
+                                                 ctx)
+            },
+            Label::Required => quote! {
+                ::prost::encoding::message::merge(wire_type, #ident, buf, ctx)
+            },
+            Label::Repeated => quote! {
+                ::prost::encoding::message::merge_repeated(wire_type, #ident, buf, ctx)
+            },
+        }
+    }
+
+    pub fn encoded_len(&self, ident: TokenStream) -> TokenStream {
+        let tag = self.tag;
+        match self.label {
+            Label::Optional => quote! {
+                #ident.as_ref().map_or(0, |msg| ::prost::encoding::message::encoded_len(#tag, msg))
+            },
+            Label::Required => quote! {
+                ::prost::encoding::message::encoded_len(#tag, &#ident)
+            },
+            Label::Repeated => quote! {
+                ::prost::encoding::message::encoded_len_repeated(#tag, &#ident)
+            },
+        }
+    }
+
+    pub fn clear(&self, ident: TokenStream) -> TokenStream {
+        match self.label {
+            Label::Optional => quote!(#ident = ::core::option::Option::None),
+            Label::Required => quote!(#ident.clear()),
+            Label::Repeated => quote!(#ident.clear()),
+        }
+    }
+}
diff --git a/third_party/rust/prost-derive/src/field/mod.rs b/third_party/rust/prost-derive/src/field/mod.rs
new file mode 100644
index 0000000000..09fef830ef
--- /dev/null
+++ b/third_party/rust/prost-derive/src/field/mod.rs
@@ -0,0 +1,366 @@
+mod group;
+mod map;
+mod message;
+mod oneof;
+mod scalar;
+
+use std::fmt;
+use std::slice;
+
+use anyhow::{bail, Error};
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{Attribute, Ident, Lit, LitBool, Meta, MetaList, MetaNameValue, NestedMeta};
+
+#[derive(Clone)]
+pub enum Field {
+    /// A scalar field.
+    Scalar(scalar::Field),
+    /// A message field.
+    Message(message::Field),
+    /// A map field.
+    Map(map::Field),
+    /// A oneof field.
+    Oneof(oneof::Field),
+    /// A group field.
+    Group(group::Field),
+}
+
+impl Field {
+    /// Creates a new `Field` from an iterator of field attributes.
+    ///
+    /// If the meta items are invalid, an error will be returned.
+    /// If the field should be ignored, `None` is returned.
+    pub fn new(attrs: Vec<Attribute>, inferred_tag: Option<u32>) -> Result<Option<Field>, Error> {
+        let attrs = prost_attrs(attrs);
+
+        // TODO: check for ignore attribute.
+
+        let field = if let Some(field) = scalar::Field::new(&attrs, inferred_tag)? {
+            Field::Scalar(field)
+        } else if let Some(field) = message::Field::new(&attrs, inferred_tag)? {
+            Field::Message(field)
+        } else if let Some(field) = map::Field::new(&attrs, inferred_tag)? {
+            Field::Map(field)
+        } else if let Some(field) = oneof::Field::new(&attrs)? {
+            Field::Oneof(field)
+        } else if let Some(field) = group::Field::new(&attrs, inferred_tag)? {
+            Field::Group(field)
+        } else {
+            bail!("no type attribute");
+        };
+
+        Ok(Some(field))
+    }
+
+    /// Creates a new oneof `Field` from an iterator of field attributes.
+    ///
+    /// If the meta items are invalid, an error will be returned.
+    /// If the field should be ignored, `None` is returned.
+    pub fn new_oneof(attrs: Vec<Attribute>) -> Result<Option<Field>, Error> {
+        let attrs = prost_attrs(attrs);
+
+        // TODO: check for ignore attribute.
+
+        let field = if let Some(field) = scalar::Field::new_oneof(&attrs)? {
+            Field::Scalar(field)
+        } else if let Some(field) = message::Field::new_oneof(&attrs)? {
+            Field::Message(field)
+        } else if let Some(field) = map::Field::new_oneof(&attrs)? {
+            Field::Map(field)
+        } else if let Some(field) = group::Field::new_oneof(&attrs)? {
+            Field::Group(field)
+        } else {
+            bail!("no type attribute for oneof field");
+        };
+
+        Ok(Some(field))
+    }
+
+    pub fn tags(&self) -> Vec<u32> {
+        match *self {
+            Field::Scalar(ref scalar) => vec![scalar.tag],
+            Field::Message(ref message) => vec![message.tag],
+            Field::Map(ref map) => vec![map.tag],
+            Field::Oneof(ref oneof) => oneof.tags.clone(),
+            Field::Group(ref group) => vec![group.tag],
+        }
+    }
+
+    /// Returns a statement which encodes the field.
+    pub fn encode(&self, ident: TokenStream) -> TokenStream {
+        match *self {
+            Field::Scalar(ref scalar) => scalar.encode(ident),
+            Field::Message(ref message) => message.encode(ident),
+            Field::Map(ref map) => map.encode(ident),
+            Field::Oneof(ref oneof) => oneof.encode(ident),
+            Field::Group(ref group) => group.encode(ident),
+        }
+    }
+
+    /// Returns an expression which evaluates to the result of merging a decoded
+    /// value into the field.
+    pub fn merge(&self, ident: TokenStream) -> TokenStream {
+        match *self {
+            Field::Scalar(ref scalar) => scalar.merge(ident),
+            Field::Message(ref message) => message.merge(ident),
+            Field::Map(ref map) => map.merge(ident),
+            Field::Oneof(ref oneof) => oneof.merge(ident),
+            Field::Group(ref group) => group.merge(ident),
+        }
+    }
+
+    /// Returns an expression which evaluates to the encoded length of the field.
+    pub fn encoded_len(&self, ident: TokenStream) -> TokenStream {
+        match *self {
+            Field::Scalar(ref scalar) => scalar.encoded_len(ident),
+            Field::Map(ref map) => map.encoded_len(ident),
+            Field::Message(ref msg) => msg.encoded_len(ident),
+            Field::Oneof(ref oneof) => oneof.encoded_len(ident),
+            Field::Group(ref group) => group.encoded_len(ident),
+        }
+    }
+
+    /// Returns a statement which clears the field.
+    pub fn clear(&self, ident: TokenStream) -> TokenStream {
+        match *self {
+            Field::Scalar(ref scalar) => scalar.clear(ident),
+            Field::Message(ref message) => message.clear(ident),
+            Field::Map(ref map) => map.clear(ident),
+            Field::Oneof(ref oneof) => oneof.clear(ident),
+            Field::Group(ref group) => group.clear(ident),
+        }
+    }
+
+    pub fn default(&self) -> TokenStream {
+        match *self {
+            Field::Scalar(ref scalar) => scalar.default(),
+            _ => quote!(::core::default::Default::default()),
+        }
+    }
+
+    /// Produces the fragment implementing debug for the given field.
+    pub fn debug(&self, ident: TokenStream) -> TokenStream {
+        match *self {
+            Field::Scalar(ref scalar) => {
+                let wrapper = scalar.debug(quote!(ScalarWrapper));
+                quote! {
+                    {
+                        #wrapper
+                        ScalarWrapper(&#ident)
+                    }
+                }
+            }
+            Field::Map(ref map) => {
+                let wrapper = map.debug(quote!(MapWrapper));
+                quote! {
+                    {
+                        #wrapper
+                        MapWrapper(&#ident)
+                    }
+                }
+            }
+            _ => quote!(&#ident),
+        }
+    }
+
+    pub fn methods(&self, ident: &Ident) -> Option<TokenStream> {
+        match *self {
+            Field::Scalar(ref scalar) => scalar.methods(ident),
+            Field::Map(ref map) => map.methods(ident),
+            _ => None,
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum Label {
+    /// An optional field.
+    Optional,
+    /// A required field.
+    Required,
+    /// A repeated field.
+    Repeated,
+}
+
+impl Label {
+    fn as_str(self) -> &'static str {
+        match self {
+            Label::Optional => "optional",
+            Label::Required => "required",
+            Label::Repeated => "repeated",
+        }
+    }
+
+    fn variants() -> slice::Iter<'static, Label> {
+        const VARIANTS: &[Label] = &[Label::Optional, Label::Required, Label::Repeated];
+        VARIANTS.iter()
+    }
+
+    /// Parses a string into a field label.
+    /// If the string doesn't match a field label, `None` is returned.
+    fn from_attr(attr: &Meta) -> Option<Label> {
+        if let Meta::Path(ref path) = *attr {
+            for &label in Label::variants() {
+                if path.is_ident(label.as_str()) {
+                    return Some(label);
+                }
+            }
+        }
+        None
+    }
+}
+
+impl fmt::Debug for Label {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+impl fmt::Display for Label {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+/// Get the items belonging to the 'prost' list attribute, e.g. `#[prost(foo, bar="baz")]`.
+fn prost_attrs(attrs: Vec<Attribute>) -> Vec<Meta> {
+    attrs
+        .iter()
+        .flat_map(Attribute::parse_meta)
+        .flat_map(|meta| match meta {
+            Meta::List(MetaList { path, nested, .. }) => {
+                if path.is_ident("prost") {
+                    nested.into_iter().collect()
+                } else {
+                    Vec::new()
+                }
+            }
+            _ => Vec::new(),
+        })
+        .flat_map(|attr| -> Result<_, _> {
+            match attr {
+                NestedMeta::Meta(attr) => Ok(attr),
+                NestedMeta::Lit(lit) => bail!("invalid prost attribute: {:?}", lit),
+            }
+        })
+        .collect()
+}
+
+pub fn set_option<T>(option: &mut Option<T>, value: T, message: &str) -> Result<(), Error>
+where
+    T: fmt::Debug,
+{
+    if let Some(ref existing) = *option {
+        bail!("{}: {:?} and {:?}", message, existing, value);
+    }
+    *option = Some(value);
+    Ok(())
+}
+
+pub fn set_bool(b: &mut bool, message: &str) -> Result<(), Error> {
+    if *b {
+        bail!("{}", message);
+    } else {
+        *b = true;
+        Ok(())
+    }
+}
+
+/// Unpacks an attribute into a (key, boolean) pair, returning the boolean value.
+/// If the key doesn't match the attribute, `None` is returned.
+fn bool_attr(key: &str, attr: &Meta) -> Result<Option<bool>, Error> {
+    if !attr.path().is_ident(key) {
+        return Ok(None);
+    }
+    match *attr {
+        Meta::Path(..) => Ok(Some(true)),
+        Meta::List(ref meta_list) => {
+            // TODO(rustlang/rust#23121): slice pattern matching would make this much nicer.
+            if meta_list.nested.len() == 1 {
+                if let NestedMeta::Lit(Lit::Bool(LitBool { value, .. })) = meta_list.nested[0] {
+                    return Ok(Some(value));
+                }
+            }
+            bail!("invalid {} attribute", key);
+        }
+        Meta::NameValue(MetaNameValue {
+            lit: Lit::Str(ref lit),
+            ..
+        }) => lit
+            .value()
+            .parse::<bool>()
+            .map_err(Error::from)
+            .map(Option::Some),
+        Meta::NameValue(MetaNameValue {
+            lit: Lit::Bool(LitBool { value, .. }),
+            ..
+        }) => Ok(Some(value)),
+        _ => bail!("invalid {} attribute", key),
+    }
+}
+
+/// Checks if an attribute matches a word.
+fn word_attr(key: &str, attr: &Meta) -> bool {
+    if let Meta::Path(ref path) = *attr {
+        path.is_ident(key)
+    } else {
+        false
+    }
+}
+
+pub(super) fn tag_attr(attr: &Meta) -> Result<Option<u32>, Error> {
+    if !attr.path().is_ident("tag") {
+        return Ok(None);
+    }
+    match *attr {
+        Meta::List(ref meta_list) => {
+            // TODO(rustlang/rust#23121): slice pattern matching would make this much nicer.
+            if meta_list.nested.len() == 1 {
+                if let NestedMeta::Lit(Lit::Int(ref lit)) = meta_list.nested[0] {
+                    return Ok(Some(lit.base10_parse()?));
+                }
+            }
+            bail!("invalid tag attribute: {:?}", attr);
+        }
+        Meta::NameValue(ref meta_name_value) => match meta_name_value.lit {
+            Lit::Str(ref lit) => lit
+                .value()
+                .parse::<u32>()
+                .map_err(Error::from)
+                .map(Option::Some),
+            Lit::Int(ref lit) => Ok(Some(lit.base10_parse()?)),
+            _ => bail!("invalid tag attribute: {:?}", attr),
+        },
+        _ => bail!("invalid tag attribute: {:?}", attr),
+    }
+}
+
+fn tags_attr(attr: &Meta) -> Result<Option<Vec<u32>>, Error> {
+    if !attr.path().is_ident("tags") {
+        return Ok(None);
+    }
+    match *attr {
+        Meta::List(ref meta_list) => {
+            let mut tags = Vec::with_capacity(meta_list.nested.len());
+            for item in &meta_list.nested {
+                if let NestedMeta::Lit(Lit::Int(ref lit)) = *item {
+                    tags.push(lit.base10_parse()?);
+                } else {
+                    bail!("invalid tag attribute: {:?}", attr);
+                }
+            }
+            Ok(Some(tags))
+        }
+        Meta::NameValue(MetaNameValue {
+            lit: Lit::Str(ref lit),
+            ..
+        }) => lit
+            .value()
+            .split(',')
+            .map(|s| s.trim().parse::<u32>().map_err(Error::from))
+            .collect::<Result<Vec<u32>, _>>()
+            .map(Some),
+        _ => bail!("invalid tag attribute: {:?}", attr),
+    }
+}
diff --git a/third_party/rust/prost-derive/src/field/oneof.rs b/third_party/rust/prost-derive/src/field/oneof.rs
new file mode 100644
index 0000000000..7e7f08671c
--- /dev/null
+++ b/third_party/rust/prost-derive/src/field/oneof.rs
@@ -0,0 +1,99 @@
+use anyhow::{bail, Error};
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{parse_str, Lit, Meta, MetaNameValue, NestedMeta, Path};
+
+use crate::field::{set_option, tags_attr};
+
+#[derive(Clone)]
+pub struct Field {
+    pub ty: Path,
+    pub tags: Vec<u32>,
+}
+
+impl Field {
+    pub fn new(attrs: &[Meta]) -> Result<Option<Field>, Error> {
+        let mut ty = None;
+        let mut tags = None;
+        let mut unknown_attrs = Vec::new();
+
+        for attr in attrs {
+            if attr.path().is_ident("oneof") {
+                let t = match *attr {
+                    Meta::NameValue(MetaNameValue {
+                        lit: Lit::Str(ref lit),
+                        ..
+                    }) => parse_str::<Path>(&lit.value())?,
+                    Meta::List(ref list) if list.nested.len() == 1 => {
+                        // TODO(rustlang/rust#23121): slice pattern matching would make this much nicer.
+                        if let NestedMeta::Meta(Meta::Path(ref path)) = list.nested[0] {
+                            if let Some(ident) = path.get_ident() {
+                                Path::from(ident.clone())
+                            } else {
+                                bail!("invalid oneof attribute: item must be an identifier");
+                            }
+                        } else {
+                            bail!("invalid oneof attribute: item must be an identifier");
+                        }
+                    }
+                    _ => bail!("invalid oneof attribute: {:?}", attr),
+                };
+                set_option(&mut ty, t, "duplicate oneof attribute")?;
+            } else if let Some(t) = tags_attr(attr)? {
+                set_option(&mut tags, t, "duplicate tags attributes")?;
+            } else {
+                unknown_attrs.push(attr);
+            }
+        }
+
+        let ty = match ty {
+            Some(ty) => ty,
+            None => return Ok(None),
+        };
+
+        match unknown_attrs.len() {
+            0 => (),
+            1 => bail!(
+                "unknown attribute for message field: {:?}",
+                unknown_attrs[0]
+            ),
+            _ => bail!("unknown attributes for message field: {:?}", unknown_attrs),
+        }
+
+        let tags = match tags {
+            Some(tags) => tags,
+            None => bail!("oneof field is missing a tags attribute"),
+        };
+
+        Ok(Some(Field { ty, tags }))
+    }
+
+    /// Returns a statement which encodes the oneof field.
+    pub fn encode(&self, ident: TokenStream) -> TokenStream {
+        quote! {
+            if let Some(ref oneof) = #ident {
+                oneof.encode(buf)
+            }
+        }
+    }
+
+    /// Returns an expression which evaluates to the result of decoding the oneof field.
+    pub fn merge(&self, ident: TokenStream) -> TokenStream {
+        let ty = &self.ty;
+        quote! {
+            #ty::merge(#ident, tag, wire_type, buf, ctx)
+        }
+    }
+
+    /// Returns an expression which evaluates to the encoded length of the oneof field.
+    pub fn encoded_len(&self, ident: TokenStream) -> TokenStream {
+        let ty = &self.ty;
+        quote! {
+            #ident.as_ref().map_or(0, #ty::encoded_len)
+        }
+    }
+
+    pub fn clear(&self, ident: TokenStream) -> TokenStream {
+        quote!(#ident = ::core::option::Option::None)
+    }
+}
diff --git a/third_party/rust/prost-derive/src/field/scalar.rs b/third_party/rust/prost-derive/src/field/scalar.rs
new file mode 100644
index 0000000000..75a4fe3a86
--- /dev/null
+++ b/third_party/rust/prost-derive/src/field/scalar.rs
@@ -0,0 +1,810 @@
+use std::convert::TryFrom;
+use std::fmt;
+
+use anyhow::{anyhow, bail, Error};
+use proc_macro2::{Span, TokenStream};
+use quote::{quote, ToTokens, TokenStreamExt};
+use syn::{parse_str, Ident, Lit, LitByteStr, Meta, MetaList, MetaNameValue, NestedMeta, Path};
+
+use crate::field::{bool_attr, set_option, tag_attr, Label};
+
+/// A scalar protobuf field.
+#[derive(Clone)]
+pub struct Field {
+    pub ty: Ty,
+    pub kind: Kind,
+    pub tag: u32,
+}
+
+impl Field {
+    pub fn new(attrs: &[Meta], inferred_tag: Option<u32>) -> Result<Option<Field>, Error> {
+        let mut ty = None;
+        let mut label = None;
+        let mut packed = None;
+        let mut default = None;
+        let mut tag = None;
+
+        let mut unknown_attrs = Vec::new();
+
+        for attr in attrs {
+            if let Some(t) = Ty::from_attr(attr)? {
+                set_option(&mut ty, t, "duplicate type attributes")?;
+            } else if let Some(p) = bool_attr("packed", attr)? {
+                set_option(&mut packed, p, "duplicate packed attributes")?;
+            } else if let Some(t) = tag_attr(attr)? {
+                set_option(&mut tag, t, "duplicate tag attributes")?;
+            } else if let Some(l) = Label::from_attr(attr) {
+                set_option(&mut label, l, "duplicate label attributes")?;
+            } else if let Some(d) = DefaultValue::from_attr(attr)? {
+                set_option(&mut default, d, "duplicate default attributes")?;
+            } else {
+                unknown_attrs.push(attr);
+            }
+        }
+
+        let ty = match ty {
+            Some(ty) => ty,
+            None => return Ok(None),
+        };
+
+        match unknown_attrs.len() {
+            0 => (),
+            1 => bail!("unknown attribute: {:?}", unknown_attrs[0]),
+            _ => bail!("unknown attributes: {:?}", unknown_attrs),
+        }
+
+        let tag = match tag.or(inferred_tag) {
+            Some(tag) => tag,
+            None => bail!("missing tag attribute"),
+        };
+
+        let has_default = default.is_some();
+        let default = default.map_or_else(
+            || Ok(DefaultValue::new(&ty)),
+            |lit| DefaultValue::from_lit(&ty, lit),
+        )?;
+
+        let kind = match (label, packed, has_default) {
+            (None, Some(true), _)
+            | (Some(Label::Optional), Some(true), _)
+            | (Some(Label::Required), Some(true), _) => {
+                bail!("packed attribute may only be applied to repeated fields");
+            }
+            (Some(Label::Repeated), Some(true), _) if !ty.is_numeric() => {
+                bail!("packed attribute may only be applied to numeric types");
+            }
+            (Some(Label::Repeated), _, true) => {
+                bail!("repeated fields may not have a default value");
+            }
+
+            (None, _, _) => Kind::Plain(default),
+            (Some(Label::Optional), _, _) => Kind::Optional(default),
+            (Some(Label::Required), _, _) => Kind::Required(default),
+            (Some(Label::Repeated), packed, false) if packed.unwrap_or_else(|| ty.is_numeric()) => {
+                Kind::Packed
+            }
+            (Some(Label::Repeated), _, false) => Kind::Repeated,
+        };
+
+        Ok(Some(Field { ty, kind, tag }))
+    }
+
+    pub fn new_oneof(attrs: &[Meta]) -> Result<Option<Field>, Error> {
+        if let Some(mut field) = Field::new(attrs, None)? {
+            match field.kind {
+                Kind::Plain(default) => {
+                    field.kind = Kind::Required(default);
+                    Ok(Some(field))
+                }
+                Kind::Optional(..) => bail!("invalid optional attribute on oneof field"),
+                Kind::Required(..) => bail!("invalid required attribute on oneof field"),
+                Kind::Packed | Kind::Repeated => bail!("invalid repeated attribute on oneof field"),
+            }
+        } else {
+            Ok(None)
+        }
+    }
+
+    pub fn encode(&self, ident: TokenStream) -> TokenStream {
+        let module = self.ty.module();
+        let encode_fn = match self.kind {
+            Kind::Plain(..) | Kind::Optional(..) | Kind::Required(..) => quote!(encode),
+            Kind::Repeated => quote!(encode_repeated),
+            Kind::Packed => quote!(encode_packed),
+        };
+        let encode_fn = quote!(::prost::encoding::#module::#encode_fn);
+        let tag = self.tag;
+
+        match self.kind {
+            Kind::Plain(ref default) => {
+                let default = default.typed();
+                quote! {
+                    if #ident != #default {
+                        #encode_fn(#tag, &#ident, buf);
+                    }
+                }
+            }
+            Kind::Optional(..) => quote! {
+                if let ::core::option::Option::Some(ref value) = #ident {
+                    #encode_fn(#tag, value, buf);
+                }
+            },
+            Kind::Required(..) | Kind::Repeated | Kind::Packed => quote! {
+                #encode_fn(#tag, &#ident, buf);
+            },
+        }
+    }
+
+    /// Returns an expression which evaluates to the result of merging a decoded
+    /// scalar value into the field.
+    pub fn merge(&self, ident: TokenStream) -> TokenStream {
+        let module = self.ty.module();
+        let merge_fn = match self.kind {
+            Kind::Plain(..) | Kind::Optional(..) | Kind::Required(..) => quote!(merge),
+            Kind::Repeated | Kind::Packed => quote!(merge_repeated),
+        };
+        let merge_fn = quote!(::prost::encoding::#module::#merge_fn);
+
+        match self.kind {
+            Kind::Plain(..) | Kind::Required(..) | Kind::Repeated | Kind::Packed => quote! {
+                #merge_fn(wire_type, #ident, buf, ctx)
+            },
+            Kind::Optional(..) => quote! {
+                #merge_fn(wire_type,
+                          #ident.get_or_insert_with(Default::default),
+                          buf,
+                          ctx)
+            },
+        }
+    }
+
+    /// Returns an expression which evaluates to the encoded length of the field.
+    pub fn encoded_len(&self, ident: TokenStream) -> TokenStream {
+        let module = self.ty.module();
+        let encoded_len_fn = match self.kind {
+            Kind::Plain(..) | Kind::Optional(..) | Kind::Required(..) => quote!(encoded_len),
+            Kind::Repeated => quote!(encoded_len_repeated),
+            Kind::Packed => quote!(encoded_len_packed),
+        };
+        let encoded_len_fn = quote!(::prost::encoding::#module::#encoded_len_fn);
+        let tag = self.tag;
+
+        match self.kind {
+            Kind::Plain(ref default) => {
+                let default = default.typed();
+                quote! {
+                    if #ident != #default {
+                        #encoded_len_fn(#tag, &#ident)
+                    } else {
+                        0
+                    }
+                }
+            }
+            Kind::Optional(..) => quote! {
+                #ident.as_ref().map_or(0, |value| #encoded_len_fn(#tag, value))
+            },
+            Kind::Required(..) | Kind::Repeated | Kind::Packed => quote! {
+                #encoded_len_fn(#tag, &#ident)
+            },
+        }
+    }
+
+    pub fn clear(&self, ident: TokenStream) -> TokenStream {
+        match self.kind {
+            Kind::Plain(ref default) | Kind::Required(ref default) => {
+                let default = default.typed();
+                match self.ty {
+                    Ty::String | Ty::Bytes(..) => quote!(#ident.clear()),
+                    _ => quote!(#ident = #default),
+                }
+            }
+            Kind::Optional(_) => quote!(#ident = ::core::option::Option::None),
+            Kind::Repeated | Kind::Packed => quote!(#ident.clear()),
+        }
+    }
+
+    /// Returns an expression which evaluates to the default value of the field.
+    pub fn default(&self) -> TokenStream {
+        match self.kind {
+            Kind::Plain(ref value) | Kind::Required(ref value) => value.owned(),
+            Kind::Optional(_) => quote!(::core::option::Option::None),
+            Kind::Repeated | Kind::Packed => quote!(::prost::alloc::vec::Vec::new()),
+        }
+    }
+
+    /// An inner debug wrapper, around the base type.
+    fn debug_inner(&self, wrap_name: TokenStream) -> TokenStream {
+        if let Ty::Enumeration(ref ty) = self.ty {
+            quote! {
+                struct #wrap_name<'a>(&'a i32);
+                impl<'a> ::core::fmt::Debug for #wrap_name<'a> {
+                    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                        match #ty::from_i32(*self.0) {
+                            None => ::core::fmt::Debug::fmt(&self.0, f),
+                            Some(en) => ::core::fmt::Debug::fmt(&en, f),
+                        }
+                    }
+                }
+            }
+        } else {
+            quote! {
+                fn #wrap_name<T>(v: T) -> T { v }
+            }
+        }
+    }
+
+    /// Returns a fragment for formatting the field `ident` in `Debug`.
+    pub fn debug(&self, wrapper_name: TokenStream) -> TokenStream {
+        let wrapper = self.debug_inner(quote!(Inner));
+        let inner_ty = self.ty.rust_type();
+        match self.kind {
+            Kind::Plain(_) | Kind::Required(_) => self.debug_inner(wrapper_name),
+            Kind::Optional(_) => quote! {
+                struct #wrapper_name<'a>(&'a ::core::option::Option<#inner_ty>);
+                impl<'a> ::core::fmt::Debug for #wrapper_name<'a> {
+                    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                        #wrapper
+                        ::core::fmt::Debug::fmt(&self.0.as_ref().map(Inner), f)
+                    }
+                }
+            },
+            Kind::Repeated | Kind::Packed => {
+                quote! {
+                    struct #wrapper_name<'a>(&'a ::prost::alloc::vec::Vec<#inner_ty>);
+                    impl<'a> ::core::fmt::Debug for #wrapper_name<'a> {
+                        fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                            let mut vec_builder = f.debug_list();
+                            for v in self.0 {
+                                #wrapper
+                                vec_builder.entry(&Inner(v));
+                            }
+                            vec_builder.finish()
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /// Returns methods to embed in the message.
+    pub fn methods(&self, ident: &Ident) -> Option<TokenStream> {
+        let mut ident_str = ident.to_string();
+        if ident_str.starts_with("r#") {
+            ident_str = ident_str[2..].to_owned();
+        }
+
+        if let Ty::Enumeration(ref ty) = self.ty {
+            let set = Ident::new(&format!("set_{}", ident_str), Span::call_site());
+            let set_doc = format!("Sets `{}` to the provided enum value.", ident_str);
+            Some(match self.kind {
+                Kind::Plain(ref default) | Kind::Required(ref default) => {
+                    let get_doc = format!(
+                        "Returns the enum value of `{}`, \
+                         or the default if the field is set to an invalid enum value.",
+                        ident_str,
+                    );
+                    quote! {
+                        #[doc=#get_doc]
+                        pub fn #ident(&self) -> #ty {
+                            #ty::from_i32(self.#ident).unwrap_or(#default)
+                        }
+
+                        #[doc=#set_doc]
+                        pub fn #set(&mut self, value: #ty) {
+                            self.#ident = value as i32;
+                        }
+                    }
+                }
+                Kind::Optional(ref default) => {
+                    let get_doc = format!(
+                        "Returns the enum value of `{}`, \
+                         or the default if the field is unset or set to an invalid enum value.",
+                        ident_str,
+                    );
+                    quote! {
+                        #[doc=#get_doc]
+                        pub fn #ident(&self) -> #ty {
+                            self.#ident.and_then(#ty::from_i32).unwrap_or(#default)
+                        }
+
+                        #[doc=#set_doc]
+                        pub fn #set(&mut self, value: #ty) {
+                            self.#ident = ::core::option::Option::Some(value as i32);
+                        }
+                    }
+                }
+                Kind::Repeated | Kind::Packed => {
+                    let iter_doc = format!(
+                        "Returns an iterator which yields the valid enum values contained in `{}`.",
+                        ident_str,
+                    );
+                    let push = Ident::new(&format!("push_{}", ident_str), Span::call_site());
+                    let push_doc = format!("Appends the provided enum value to `{}`.", ident_str);
+                    quote! {
+                        #[doc=#iter_doc]
+                        pub fn #ident(&self) -> ::core::iter::FilterMap<
+                            ::core::iter::Cloned<::core::slice::Iter<i32>>,
+                            fn(i32) -> ::core::option::Option<#ty>,
+                        > {
+                            self.#ident.iter().cloned().filter_map(#ty::from_i32)
+                        }
+                        #[doc=#push_doc]
+                        pub fn #push(&mut self, value: #ty) {
+                            self.#ident.push(value as i32);
+                        }
+                    }
+                }
+            })
+        } else if let Kind::Optional(ref default) = self.kind {
+            let ty = self.ty.rust_ref_type();
+
+            let match_some = if self.ty.is_numeric() {
+                quote!(::core::option::Option::Some(val) => val,)
+            } else {
+                quote!(::core::option::Option::Some(ref val) => &val[..],)
+            };
+
+            let get_doc = format!(
+                "Returns the value of `{0}`, or the default value if `{0}` is unset.",
+                ident_str,
+            );
+
+            Some(quote! {
+                #[doc=#get_doc]
+                pub fn #ident(&self) -> #ty {
+                    match self.#ident {
+                        #match_some
+                        ::core::option::Option::None => #default,
+                    }
+                }
+            })
+        } else {
+            None
+        }
+    }
+}
+
+/// A scalar protobuf field type.
+#[derive(Clone, PartialEq, Eq)]
+pub enum Ty {
+    Double,
+    Float,
+    Int32,
+    Int64,
+    Uint32,
+    Uint64,
+    Sint32,
+    Sint64,
+    Fixed32,
+    Fixed64,
+    Sfixed32,
+    Sfixed64,
+    Bool,
+    String,
+    Bytes(BytesTy),
+    Enumeration(Path),
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum BytesTy {
+    Vec,
+    Bytes,
+}
+
+impl BytesTy {
+    fn try_from_str(s: &str) -> Result<Self, Error> {
+        match s {
+            "vec" => Ok(BytesTy::Vec),
+            "bytes" => Ok(BytesTy::Bytes),
+            _ => bail!("Invalid bytes type: {}", s),
+        }
+    }
+
+    fn rust_type(&self) -> TokenStream {
+        match self {
+            BytesTy::Vec => quote! { ::prost::alloc::vec::Vec<u8> },
+            BytesTy::Bytes => quote! { ::prost::bytes::Bytes },
+        }
+    }
+}
+
+impl Ty {
+    pub fn from_attr(attr: &Meta) -> Result<Option<Ty>, Error> {
+        let ty = match *attr {
+            Meta::Path(ref name) if name.is_ident("float") => Ty::Float,
+            Meta::Path(ref name) if name.is_ident("double") => Ty::Double,
+            Meta::Path(ref name) if name.is_ident("int32") => Ty::Int32,
+            Meta::Path(ref name) if name.is_ident("int64") => Ty::Int64,
+            Meta::Path(ref name) if name.is_ident("uint32") => Ty::Uint32,
+            Meta::Path(ref name) if name.is_ident("uint64") => Ty::Uint64,
+            Meta::Path(ref name) if name.is_ident("sint32") => Ty::Sint32,
+            Meta::Path(ref name) if name.is_ident("sint64") => Ty::Sint64,
+            Meta::Path(ref name) if name.is_ident("fixed32") => Ty::Fixed32,
+            Meta::Path(ref name) if name.is_ident("fixed64") => Ty::Fixed64,
+            Meta::Path(ref name) if name.is_ident("sfixed32") => Ty::Sfixed32,
+            Meta::Path(ref name) if name.is_ident("sfixed64") => Ty::Sfixed64,
+            Meta::Path(ref name) if name.is_ident("bool") => Ty::Bool,
+            Meta::Path(ref name) if name.is_ident("string") => Ty::String,
+            Meta::Path(ref name) if name.is_ident("bytes") => Ty::Bytes(BytesTy::Vec),
+            Meta::NameValue(MetaNameValue {
+                ref path,
+                lit: Lit::Str(ref l),
+                ..
+            }) if path.is_ident("bytes") => Ty::Bytes(BytesTy::try_from_str(&l.value())?),
+            Meta::NameValue(MetaNameValue {
+                ref path,
+                lit: Lit::Str(ref l),
+                ..
+            }) if path.is_ident("enumeration") => Ty::Enumeration(parse_str::<Path>(&l.value())?),
+            Meta::List(MetaList {
+                ref path,
+                ref nested,
+                ..
+            }) if path.is_ident("enumeration") => {
+                // TODO(rustlang/rust#23121): slice pattern matching would make this much nicer.
+                if nested.len() == 1 {
+                    if let NestedMeta::Meta(Meta::Path(ref path)) = nested[0] {
+                        Ty::Enumeration(path.clone())
+                    } else {
+                        bail!("invalid enumeration attribute: item must be an identifier");
+                    }
+                } else {
+                    bail!("invalid enumeration attribute: only a single identifier is supported");
+                }
+            }
+            _ => return Ok(None),
+        };
+        Ok(Some(ty))
+    }
+
+    pub fn from_str(s: &str) -> Result<Ty, Error> {
+        let enumeration_len = "enumeration".len();
+        let error = Err(anyhow!("invalid type: {}", s));
+        let ty = match s.trim() {
+            "float" => Ty::Float,
+            "double" => Ty::Double,
+            "int32" => Ty::Int32,
+            "int64" => Ty::Int64,
+            "uint32" => Ty::Uint32,
+            "uint64" => Ty::Uint64,
+            "sint32" => Ty::Sint32,
+            "sint64" => Ty::Sint64,
+            "fixed32" => Ty::Fixed32,
+            "fixed64" => Ty::Fixed64,
+            "sfixed32" => Ty::Sfixed32,
+            "sfixed64" => Ty::Sfixed64,
+            "bool" => Ty::Bool,
+            "string" => Ty::String,
+            "bytes" => Ty::Bytes(BytesTy::Vec),
+            s if s.len() > enumeration_len && &s[..enumeration_len] == "enumeration" => {
+                let s = &s[enumeration_len..].trim();
+                match s.chars().next() {
+                    Some('<') | Some('(') => (),
+                    _ => return error,
+                }
+                match s.chars().next_back() {
+                    Some('>') | Some(')') => (),
+                    _ => return error,
+                }
+
+                Ty::Enumeration(parse_str::<Path>(s[1..s.len() - 1].trim())?)
+            }
+            _ => return error,
+        };
+        Ok(ty)
+    }
+
+    /// Returns the type as it appears in protobuf field declarations.
+    pub fn as_str(&self) -> &'static str {
+        match *self {
+            Ty::Double => "double",
+            Ty::Float => "float",
+            Ty::Int32 => "int32",
+            Ty::Int64 => "int64",
+            Ty::Uint32 => "uint32",
+            Ty::Uint64 => "uint64",
+            Ty::Sint32 => "sint32",
+            Ty::Sint64 => "sint64",
+            Ty::Fixed32 => "fixed32",
+            Ty::Fixed64 => "fixed64",
+            Ty::Sfixed32 => "sfixed32",
+            Ty::Sfixed64 => "sfixed64",
+            Ty::Bool => "bool",
+            Ty::String => "string",
+            Ty::Bytes(..) => "bytes",
+            Ty::Enumeration(..) => "enum",
+        }
+    }
+
+    // TODO: rename to 'owned_type'.
+    pub fn rust_type(&self) -> TokenStream {
+        match self {
+            Ty::String => quote!(::prost::alloc::string::String),
+            Ty::Bytes(ty) => ty.rust_type(),
+            _ => self.rust_ref_type(),
+        }
+    }
+
+    // TODO: rename to 'ref_type'
+    pub fn rust_ref_type(&self) -> TokenStream {
+        match *self {
+            Ty::Double => quote!(f64),
+            Ty::Float => quote!(f32),
+            Ty::Int32 => quote!(i32),
+            Ty::Int64 => quote!(i64),
+            Ty::Uint32 => quote!(u32),
+            Ty::Uint64 => quote!(u64),
+            Ty::Sint32 => quote!(i32),
+            Ty::Sint64 => quote!(i64),
+            Ty::Fixed32 => quote!(u32),
+            Ty::Fixed64 => quote!(u64),
+            Ty::Sfixed32 => quote!(i32),
+            Ty::Sfixed64 => quote!(i64),
+            Ty::Bool => quote!(bool),
+            Ty::String => quote!(&str),
+            Ty::Bytes(..) => quote!(&[u8]),
+            Ty::Enumeration(..) => quote!(i32),
+        }
+    }
+
+    pub fn module(&self) -> Ident {
+        match *self {
+            Ty::Enumeration(..) => Ident::new("int32", Span::call_site()),
+            _ => Ident::new(self.as_str(), Span::call_site()),
+        }
+    }
+
+    /// Returns false if the scalar type is length delimited (i.e., `string` or `bytes`).
+    pub fn is_numeric(&self) -> bool {
+        !matches!(self, Ty::String | Ty::Bytes(..))
+    }
+}
+
+impl fmt::Debug for Ty {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+impl fmt::Display for Ty {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+/// Scalar Protobuf field types.
+#[derive(Clone)]
+pub enum Kind {
+    /// A plain proto3 scalar field.
+    Plain(DefaultValue),
+    /// An optional scalar field.
+    Optional(DefaultValue),
+    /// A required proto2 scalar field.
+    Required(DefaultValue),
+    /// A repeated scalar field.
+    Repeated,
+    /// A packed repeated scalar field.
+    Packed,
+}
+
+/// Scalar Protobuf field default value.
+#[derive(Clone, Debug)]
+pub enum DefaultValue {
+    F64(f64),
+    F32(f32),
+    I32(i32),
+    I64(i64),
+    U32(u32),
+    U64(u64),
+    Bool(bool),
+    String(String),
+    Bytes(Vec<u8>),
+    Enumeration(TokenStream),
+    Path(Path),
+}
+
+impl DefaultValue {
+    pub fn from_attr(attr: &Meta) -> Result<Option<Lit>, Error> {
+        if !attr.path().is_ident("default") {
+            Ok(None)
+        } else if let Meta::NameValue(ref name_value) = *attr {
+            Ok(Some(name_value.lit.clone()))
+        } else {
+            bail!("invalid default value attribute: {:?}", attr)
+        }
+    }
+
+    pub fn from_lit(ty: &Ty, lit: Lit) -> Result<DefaultValue, Error> {
+        let is_i32 = *ty == Ty::Int32 || *ty == Ty::Sint32 || *ty == Ty::Sfixed32;
+        let is_i64 = *ty == Ty::Int64 || *ty == Ty::Sint64 || *ty == Ty::Sfixed64;
+
+        let is_u32 = *ty == Ty::Uint32 || *ty == Ty::Fixed32;
+        let is_u64 = *ty == Ty::Uint64 || *ty == Ty::Fixed64;
+
+        let empty_or_is = |expected, actual: &str| expected == actual || actual.is_empty();
+
+        let default = match lit {
+            Lit::Int(ref lit) if is_i32 && empty_or_is("i32", lit.suffix()) => {
+                DefaultValue::I32(lit.base10_parse()?)
+            }
+            Lit::Int(ref lit) if is_i64 && empty_or_is("i64", lit.suffix()) => {
+                DefaultValue::I64(lit.base10_parse()?)
+            }
+            Lit::Int(ref lit) if is_u32 && empty_or_is("u32", lit.suffix()) => {
+                DefaultValue::U32(lit.base10_parse()?)
+            }
+            Lit::Int(ref lit) if is_u64 && empty_or_is("u64", lit.suffix()) => {
+                DefaultValue::U64(lit.base10_parse()?)
+            }
+
+            Lit::Float(ref lit) if *ty == Ty::Float && empty_or_is("f32", lit.suffix()) => {
+                DefaultValue::F32(lit.base10_parse()?)
+            }
+            Lit::Int(ref lit) if *ty == Ty::Float => DefaultValue::F32(lit.base10_parse()?),
+
+            Lit::Float(ref lit) if *ty == Ty::Double && empty_or_is("f64", lit.suffix()) => {
+                DefaultValue::F64(lit.base10_parse()?)
+            }
+            Lit::Int(ref lit) if *ty == Ty::Double => DefaultValue::F64(lit.base10_parse()?),
+
+            Lit::Bool(ref lit) if *ty == Ty::Bool => DefaultValue::Bool(lit.value),
+            Lit::Str(ref lit) if *ty == Ty::String => DefaultValue::String(lit.value()),
+            Lit::ByteStr(ref lit)
+                if *ty == Ty::Bytes(BytesTy::Bytes) || *ty == Ty::Bytes(BytesTy::Vec) =>
+            {
+                DefaultValue::Bytes(lit.value())
+            }
+
+            Lit::Str(ref lit) => {
+                let value = lit.value();
+                let value = value.trim();
+
+                if let Ty::Enumeration(ref path) = *ty {
+                    let variant = Ident::new(value, Span::call_site());
+                    return Ok(DefaultValue::Enumeration(quote!(#path::#variant)));
+                }
+
+                // Parse special floating point values.
+                if *ty == Ty::Float {
+                    match value {
+                        "inf" => {
+                            return Ok(DefaultValue::Path(parse_str::<Path>(
+                                "::core::f32::INFINITY",
+                            )?));
+                        }
+                        "-inf" => {
+                            return Ok(DefaultValue::Path(parse_str::<Path>(
+                                "::core::f32::NEG_INFINITY",
+                            )?));
+                        }
+                        "nan" => {
+                            return Ok(DefaultValue::Path(parse_str::<Path>("::core::f32::NAN")?));
+                        }
+                        _ => (),
+                    }
+                }
+                if *ty == Ty::Double {
+                    match value {
+                        "inf" => {
+                            return Ok(DefaultValue::Path(parse_str::<Path>(
+                                "::core::f64::INFINITY",
+                            )?));
+                        }
+                        "-inf" => {
+                            return Ok(DefaultValue::Path(parse_str::<Path>(
+                                "::core::f64::NEG_INFINITY",
+                            )?));
+                        }
+                        "nan" => {
+                            return Ok(DefaultValue::Path(parse_str::<Path>("::core::f64::NAN")?));
+                        }
+                        _ => (),
+                    }
+                }
+
+                // Rust doesn't have a negative literals, so they have to be parsed specially.
+                if let Some(Ok(lit)) = value.strip_prefix('-').map(syn::parse_str::<Lit>) {
+                    match lit {
+                        Lit::Int(ref lit) if is_i32 && empty_or_is("i32", lit.suffix()) => {
+                            // Initially parse into an i64, so that i32::MIN does not overflow.
+                            let value: i64 = -lit.base10_parse()?;
+                            return Ok(i32::try_from(value).map(DefaultValue::I32)?);
+                        }
+                        Lit::Int(ref lit) if is_i64 && empty_or_is("i64", lit.suffix()) => {
+                            // Initially parse into an i128, so that i64::MIN does not overflow.
+                            let value: i128 = -lit.base10_parse()?;
+                            return Ok(i64::try_from(value).map(DefaultValue::I64)?);
+                        }
+                        Lit::Float(ref lit)
+                            if *ty == Ty::Float && empty_or_is("f32", lit.suffix()) =>
+                        {
+                            return Ok(DefaultValue::F32(-lit.base10_parse()?));
+                        }
+                        Lit::Float(ref lit)
+                            if *ty == Ty::Double && empty_or_is("f64", lit.suffix()) =>
+                        {
+                            return Ok(DefaultValue::F64(-lit.base10_parse()?));
+                        }
+                        Lit::Int(ref lit) if *ty == Ty::Float && lit.suffix().is_empty() => {
+                            return Ok(DefaultValue::F32(-lit.base10_parse()?));
+                        }
+                        Lit::Int(ref lit) if *ty == Ty::Double && lit.suffix().is_empty() => {
+                            return Ok(DefaultValue::F64(-lit.base10_parse()?));
+                        }
+                        _ => (),
+                    }
+                }
+                match syn::parse_str::<Lit>(&value) {
+                    Ok(Lit::Str(_)) => (),
+                    Ok(lit) => return DefaultValue::from_lit(ty, lit),
+                    _ => (),
+                }
+                bail!("invalid default value: {}", quote!(#value));
+            }
+            _ => bail!("invalid default value: {}", quote!(#lit)),
+        };
+
+        Ok(default)
+    }
+
+    pub fn new(ty: &Ty) -> DefaultValue {
+        match *ty {
+            Ty::Float => DefaultValue::F32(0.0),
+            Ty::Double => DefaultValue::F64(0.0),
+            Ty::Int32 | Ty::Sint32 | Ty::Sfixed32 => DefaultValue::I32(0),
+            Ty::Int64 | Ty::Sint64 | Ty::Sfixed64 => DefaultValue::I64(0),
+            Ty::Uint32 | Ty::Fixed32 => DefaultValue::U32(0),
+            Ty::Uint64 | Ty::Fixed64 => DefaultValue::U64(0),
+
+            Ty::Bool => DefaultValue::Bool(false),
+            Ty::String => DefaultValue::String(String::new()),
+            Ty::Bytes(..) => DefaultValue::Bytes(Vec::new()),
+            Ty::Enumeration(ref path) => DefaultValue::Enumeration(quote!(#path::default())),
+        }
+    }
+
+    pub fn owned(&self) -> TokenStream {
+        match *self {
+            DefaultValue::String(ref value) if value.is_empty() => {
+                quote!(::prost::alloc::string::String::new())
+            }
+            DefaultValue::String(ref value) => quote!(#value.into()),
+            DefaultValue::Bytes(ref value) if value.is_empty() => quote!(Default::default()),
+            DefaultValue::Bytes(ref value) => {
+                let lit = LitByteStr::new(value, Span::call_site());
+                quote!(#lit.as_ref().into())
+            }
+
+            ref other => other.typed(),
+        }
+    }
+
+    pub fn typed(&self) -> TokenStream {
+        if let DefaultValue::Enumeration(_) = *self {
+            quote!(#self as i32)
+        } else {
+            quote!(#self)
+        }
+    }
+}
+
+impl ToTokens for DefaultValue {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match *self {
+            DefaultValue::F64(value) => value.to_tokens(tokens),
+            DefaultValue::F32(value) => value.to_tokens(tokens),
+            DefaultValue::I32(value) => value.to_tokens(tokens),
+            DefaultValue::I64(value) => value.to_tokens(tokens),
+            DefaultValue::U32(value) => value.to_tokens(tokens),
+            DefaultValue::U64(value) => value.to_tokens(tokens),
+            DefaultValue::Bool(value) => value.to_tokens(tokens),
+            DefaultValue::String(ref value) => value.to_tokens(tokens),
+            DefaultValue::Bytes(ref value) => {
+                let byte_str = LitByteStr::new(value, Span::call_site());
+                tokens.append_all(quote!(#byte_str as &[u8]));
+            }
+            DefaultValue::Enumeration(ref value) => value.to_tokens(tokens),
+            DefaultValue::Path(ref value) => value.to_tokens(tokens),
+        }
+    }
+}
diff --git a/third_party/rust/prost-derive/src/lib.rs b/third_party/rust/prost-derive/src/lib.rs
new file mode 100644
index 0000000000..a517c41e80
--- /dev/null
+++ b/third_party/rust/prost-derive/src/lib.rs
@@ -0,0 +1,470 @@
+#![doc(html_root_url = "https://docs.rs/prost-derive/0.8.0")]
+// The `quote!` macro requires deep recursion.
+#![recursion_limit = "4096"]
+
+extern crate alloc;
+extern crate proc_macro;
+
+use anyhow::{bail, Error};
+use itertools::Itertools;
+use proc_macro::TokenStream;
+use proc_macro2::Span;
+use quote::quote;
+use syn::{
+    punctuated::Punctuated, Data, DataEnum, DataStruct, DeriveInput, Expr, Fields, FieldsNamed,
+    FieldsUnnamed, Ident, Variant,
+};
+
+mod field;
+use crate::field::Field;
+
+fn try_message(input: TokenStream) -> Result<TokenStream, Error> {
+    let input: DeriveInput = syn::parse(input)?;
+
+    let ident = input.ident;
+
+    let variant_data = match input.data {
+        Data::Struct(variant_data) => variant_data,
+        Data::Enum(..) => bail!("Message can not be derived for an enum"),
+        Data::Union(..) => bail!("Message can not be derived for a union"),
+    };
+
+    let generics = &input.generics;
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let fields = match variant_data {
+        DataStruct {
+            fields: Fields::Named(FieldsNamed { named: fields, .. }),
+            ..
+        }
+        | DataStruct {
+            fields:
+                Fields::Unnamed(FieldsUnnamed {
+                    unnamed: fields, ..
+                }),
+            ..
+        } => fields.into_iter().collect(),
+        DataStruct {
+            fields: Fields::Unit,
+            ..
+        } => Vec::new(),
+    };
+
+    let mut next_tag: u32 = 1;
+    let mut fields = fields
+        .into_iter()
+        .enumerate()
+        .flat_map(|(idx, field)| {
+            let field_ident = field
+                .ident
+                .unwrap_or_else(|| Ident::new(&idx.to_string(), Span::call_site()));
+            match Field::new(field.attrs, Some(next_tag)) {
+                Ok(Some(field)) => {
+                    next_tag = field.tags().iter().max().map(|t| t + 1).unwrap_or(next_tag);
+                    Some(Ok((field_ident, field)))
+                }
+                Ok(None) => None,
+                Err(err) => Some(Err(
+                    err.context(format!("invalid message field {}.{}", ident, field_ident))
+                )),
+            }
+        })
+        .collect::<Result<Vec<_>, _>>()?;
+
+    // We want Debug to be in declaration order
+    let unsorted_fields = fields.clone();
+
+    // Sort the fields by tag number so that fields will be encoded in tag order.
+    // TODO: This encodes oneof fields in the position of their lowest tag,
+    // regardless of the currently occupied variant, is that consequential?
+    // See: https://developers.google.com/protocol-buffers/docs/encoding#order
+    fields.sort_by_key(|&(_, ref field)| field.tags().into_iter().min().unwrap());
+    let fields = fields;
+
+    let mut tags = fields
+        .iter()
+        .flat_map(|&(_, ref field)| field.tags())
+        .collect::<Vec<_>>();
+    let num_tags = tags.len();
+    tags.sort_unstable();
+    tags.dedup();
+    if tags.len() != num_tags {
+        bail!("message {} has fields with duplicate tags", ident);
+    }
+
+    let encoded_len = fields
+        .iter()
+        .map(|&(ref field_ident, ref field)| field.encoded_len(quote!(self.#field_ident)));
+
+    let encode = fields
+        .iter()
+        .map(|&(ref field_ident, ref field)| field.encode(quote!(self.#field_ident)));
+
+    let merge = fields.iter().map(|&(ref field_ident, ref field)| {
+        let merge = field.merge(quote!(value));
+        let tags = field.tags().into_iter().map(|tag| quote!(#tag));
+        let tags = Itertools::intersperse(tags, quote!(|));
+
+        quote! {
+            #(#tags)* => {
+                let mut value = &mut self.#field_ident;
+                #merge.map_err(|mut error| {
+                    error.push(STRUCT_NAME, stringify!(#field_ident));
+                    error
+                })
+            },
+        }
+    });
+
+    let struct_name = if fields.is_empty() {
+        quote!()
+    } else {
+        quote!(
+            const STRUCT_NAME: &'static str = stringify!(#ident);
+        )
+    };
+
+    // TODO
+    let is_struct = true;
+
+    let clear = fields
+        .iter()
+        .map(|&(ref field_ident, ref field)| field.clear(quote!(self.#field_ident)));
+
+    let default = fields.iter().map(|&(ref field_ident, ref field)| {
+        let value = field.default();
+        quote!(#field_ident: #value,)
+    });
+
+    let methods = fields
+        .iter()
+        .flat_map(|&(ref field_ident, ref field)| field.methods(field_ident))
+        .collect::<Vec<_>>();
+    let methods = if methods.is_empty() {
+        quote!()
+    } else {
+        quote! {
+            #[allow(dead_code)]
+            impl #impl_generics #ident #ty_generics #where_clause {
+                #(#methods)*
+            }
+        }
+    };
+
+    let debugs = unsorted_fields.iter().map(|&(ref field_ident, ref field)| {
+        let wrapper = field.debug(quote!(self.#field_ident));
+        let call = if is_struct {
+            quote!(builder.field(stringify!(#field_ident), &wrapper))
+        } else {
+            quote!(builder.field(&wrapper))
+        };
+        quote! {
+             let builder = {
+                 let wrapper = #wrapper;
+                 #call
+             };
+        }
+    });
+    let debug_builder = if is_struct {
+        quote!(f.debug_struct(stringify!(#ident)))
+    } else {
+        quote!(f.debug_tuple(stringify!(#ident)))
+    };
+
+    let expanded = quote! {
+        impl #impl_generics ::prost::Message for #ident #ty_generics #where_clause {
+            #[allow(unused_variables)]
+            fn encode_raw<B>(&self, buf: &mut B) where B: ::prost::bytes::BufMut {
+                #(#encode)*
+            }
+
+            #[allow(unused_variables)]
+            fn merge_field<B>(
+                &mut self,
+                tag: u32,
+                wire_type: ::prost::encoding::WireType,
+                buf: &mut B,
+                ctx: ::prost::encoding::DecodeContext,
+            ) -> ::core::result::Result<(), ::prost::DecodeError>
+            where B: ::prost::bytes::Buf {
+                #struct_name
+                match tag {
+                    #(#merge)*
+                    _ => ::prost::encoding::skip_field(wire_type, tag, buf, ctx),
+                }
+            }
+
+            #[inline]
+            fn encoded_len(&self) -> usize {
+                0 #(+ #encoded_len)*
+            }
+
+            fn clear(&mut self) {
+                #(#clear;)*
+            }
+        }
+
+        impl #impl_generics Default for #ident #ty_generics #where_clause {
+            fn default() -> Self {
+                #ident {
+                    #(#default)*
+                }
+            }
+        }
+
+        impl #impl_generics ::core::fmt::Debug for #ident #ty_generics #where_clause {
+            fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                let mut builder = #debug_builder;
+                #(#debugs;)*
+                builder.finish()
+            }
+        }
+
+        #methods
+    };
+
+    Ok(expanded.into())
+}
+
+#[proc_macro_derive(Message, attributes(prost))]
+pub fn message(input: TokenStream) -> TokenStream {
+    try_message(input).unwrap()
+}
+
+fn try_enumeration(input: TokenStream) -> Result<TokenStream, Error> {
+    let input: DeriveInput = syn::parse(input)?;
+    let ident = input.ident;
+
+    let generics = &input.generics;
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let punctuated_variants = match input.data {
+        Data::Enum(DataEnum { variants, .. }) => variants,
+        Data::Struct(_) => bail!("Enumeration can not be derived for a struct"),
+        Data::Union(..) => bail!("Enumeration can not be derived for a union"),
+    };
+
+    // Map the variants into 'fields'.
+    let mut variants: Vec<(Ident, Expr)> = Vec::new();
+    for Variant {
+        ident,
+        fields,
+        discriminant,
+        ..
+    } in punctuated_variants
+    {
+        match fields {
+            Fields::Unit => (),
+            Fields::Named(_) | Fields::Unnamed(_) => {
+                bail!("Enumeration variants may not have fields")
+            }
+        }
+
+        match discriminant {
+            Some((_, expr)) => variants.push((ident, expr)),
+            None => bail!("Enumeration variants must have a disriminant"),
+        }
+    }
+
+    if variants.is_empty() {
+        panic!("Enumeration must have at least one variant");
+    }
+
+    let default = variants[0].0.clone();
+
+    let is_valid = variants
+        .iter()
+        .map(|&(_, ref value)| quote!(#value => true));
+    let from = variants.iter().map(
+        |&(ref variant, ref value)| quote!(#value => ::core::option::Option::Some(#ident::#variant)),
+    );
+
+    let is_valid_doc = format!("Returns `true` if `value` is a variant of `{}`.", ident);
+    let from_i32_doc = format!(
+        "Converts an `i32` to a `{}`, or `None` if `value` is not a valid variant.",
+        ident
+    );
+
+    let expanded = quote! {
+        impl #impl_generics #ident #ty_generics #where_clause {
+            #[doc=#is_valid_doc]
+            pub fn is_valid(value: i32) -> bool {
+                match value {
+                    #(#is_valid,)*
+                    _ => false,
+                }
+            }
+
+            #[doc=#from_i32_doc]
+            pub fn from_i32(value: i32) -> ::core::option::Option<#ident> {
+                match value {
+                    #(#from,)*
+                    _ => ::core::option::Option::None,
+                }
+            }
+        }
+
+        impl #impl_generics ::core::default::Default for #ident #ty_generics #where_clause {
+            fn default() -> #ident {
+                #ident::#default
+            }
+        }
+
+        impl #impl_generics ::core::convert::From::<#ident> for i32 #ty_generics #where_clause {
+            fn from(value: #ident) -> i32 {
+                value as i32
+            }
+        }
+    };
+
+    Ok(expanded.into())
+}
+
+#[proc_macro_derive(Enumeration, attributes(prost))]
+pub fn enumeration(input: TokenStream) -> TokenStream {
+    try_enumeration(input).unwrap()
+}
+
+fn try_oneof(input: TokenStream) -> Result<TokenStream, Error> {
+    let input: DeriveInput = syn::parse(input)?;
+
+    let ident = input.ident;
+
+    let variants = match input.data {
+        Data::Enum(DataEnum { variants, .. }) => variants,
+        Data::Struct(..) => bail!("Oneof can not be derived for a struct"),
+        Data::Union(..) => bail!("Oneof can not be derived for a union"),
+    };
+
+    let generics = &input.generics;
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    // Map the variants into 'fields'.
+    let mut fields: Vec<(Ident, Field)> = Vec::new();
+    for Variant {
+        attrs,
+        ident: variant_ident,
+        fields: variant_fields,
+        ..
+    } in variants
+    {
+        let variant_fields = match variant_fields {
+            Fields::Unit => Punctuated::new(),
+            Fields::Named(FieldsNamed { named: fields, .. })
+            | Fields::Unnamed(FieldsUnnamed {
+                unnamed: fields, ..
+            }) => fields,
+        };
+        if variant_fields.len() != 1 {
+            bail!("Oneof enum variants must have a single field");
+        }
+        match Field::new_oneof(attrs)? {
+            Some(field) => fields.push((variant_ident, field)),
+            None => bail!("invalid oneof variant: oneof variants may not be ignored"),
+        }
+    }
+
+    let mut tags = fields
+        .iter()
+        .flat_map(|&(ref variant_ident, ref field)| -> Result<u32, Error> {
+            if field.tags().len() > 1 {
+                bail!(
+                    "invalid oneof variant {}::{}: oneof variants may only have a single tag",
+                    ident,
+                    variant_ident
+                );
+            }
+            Ok(field.tags()[0])
+        })
+        .collect::<Vec<_>>();
+    tags.sort_unstable();
+    tags.dedup();
+    if tags.len() != fields.len() {
+        panic!("invalid oneof {}: variants have duplicate tags", ident);
+    }
+
+    let encode = fields.iter().map(|&(ref variant_ident, ref field)| {
+        let encode = field.encode(quote!(*value));
+        quote!(#ident::#variant_ident(ref value) => { #encode })
+    });
+
+    let merge = fields.iter().map(|&(ref variant_ident, ref field)| {
+        let tag = field.tags()[0];
+        let merge = field.merge(quote!(value));
+        quote! {
+            #tag => {
+                match field {
+                    ::core::option::Option::Some(#ident::#variant_ident(ref mut value)) => {
+                        #merge
+                    },
+                    _ => {
+                        let mut owned_value = ::core::default::Default::default();
+                        let value = &mut owned_value;
+                        #merge.map(|_| *field = ::core::option::Option::Some(#ident::#variant_ident(owned_value)))
+                    },
+                }
+            }
+        }
+    });
+
+    let encoded_len = fields.iter().map(|&(ref variant_ident, ref field)| {
+        let encoded_len = field.encoded_len(quote!(*value));
+        quote!(#ident::#variant_ident(ref value) => #encoded_len)
+    });
+
+    let debug = fields.iter().map(|&(ref variant_ident, ref field)| {
+        let wrapper = field.debug(quote!(*value));
+        quote!(#ident::#variant_ident(ref value) => {
+            let wrapper = #wrapper;
+            f.debug_tuple(stringify!(#variant_ident))
+                .field(&wrapper)
+                .finish()
+        })
+    });
+
+    let expanded = quote! {
+        impl #impl_generics #ident #ty_generics #where_clause {
+            pub fn encode<B>(&self, buf: &mut B) where B: ::prost::bytes::BufMut {
+                match *self {
+                    #(#encode,)*
+                }
+            }
+
+            pub fn merge<B>(
+                field: &mut ::core::option::Option<#ident #ty_generics>,
+                tag: u32,
+                wire_type: ::prost::encoding::WireType,
+                buf: &mut B,
+                ctx: ::prost::encoding::DecodeContext,
+            ) -> ::core::result::Result<(), ::prost::DecodeError>
+            where B: ::prost::bytes::Buf {
+                match tag {
+                    #(#merge,)*
+                    _ => unreachable!(concat!("invalid ", stringify!(#ident), " tag: {}"), tag),
+                }
+            }
+
+            #[inline]
+            pub fn encoded_len(&self) -> usize {
+                match *self {
+                    #(#encoded_len,)*
+                }
+            }
+        }
+
+        impl #impl_generics ::core::fmt::Debug for #ident #ty_generics #where_clause {
+            fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                match *self {
+                    #(#debug,)*
+                }
+            }
+        }
+    };
+
+    Ok(expanded.into())
+}
+
+#[proc_macro_derive(Oneof, attributes(prost))]
+pub fn oneof(input: TokenStream) -> TokenStream {
+    try_oneof(input).unwrap()
+}
diff --git a/third_party/rust/prost/.cargo-checksum.json b/third_party/rust/prost/.cargo-checksum.json
new file mode 100644
index 0000000000..addf3e74e4
--- /dev/null
+++ b/third_party/rust/prost/.cargo-checksum.json
@@ -0,0 +1 @@
+{"files":{"Cargo.toml":"181dc7aae816183fc30486a41d4efcffc9dff51604b8a285b5259193103283a0","FUZZING.md":"0223441ab805f2ccfda9f172955446c3287e1e125cc68814f77a46573935feba","LICENSE":"a60eea817514531668d7e00765731449fe14d059d3249e0bc93b36de45f759f2","README.md":"582696a33d4cd5da2797a0928d35689bbb0a51317ca03c94be235224cbadd6cc","benches/varint.rs":"25e28eadeb5092882281eaa16307ae72cbdabe04a308c9cfe15d3a62c8ead40b","clippy.toml":"10eea08f9e26e0dc498e431ac3a62b861bd74029d1cad8a394284af4cbc90532","prepare-release.sh":"67f42e0649d33269c88272e69a9bf48d02de42126a9942ac6298b6995adea8df","publish-release.sh":"a9ff9a5a65a6772fbe115b64051b1284b0b81825f839a65594d6834c53d7a78f","src/encoding.rs":"20dd077efd9f12e45657b802cfe57de0739de4b9b81b6e420a569b7459150ff2","src/error.rs":"60194cd97e5a6b0f985e74630d3f41c14e3ce1bd009a802c1540462e9a671bc5","src/lib.rs":"7bbc5d3d941ab687dc2bed0982289d28e5a4e1f66805cde3ce09dd97d39eb2df","src/message.rs":"19527eb5efa2a959d555c45f9391022a8f380984fc6bbf35ab23069a0a6091ce","src/types.rs":"edfefaf56ab4bc12c98cbcd9d0b3ca18c10352217556c193be6314990ecffd9c"},"package":"de5e2533f59d08fcf364fd374ebda0692a70bd6d7e66ef97f306f45c6c5d8020"}
\ No newline at end of file
diff --git a/third_party/rust/prost/Cargo.toml b/third_party/rust/prost/Cargo.toml
new file mode 100644
index 0000000000..c1112fdc20
--- /dev/null
+++ b/third_party/rust/prost/Cargo.toml
@@ -0,0 +1,60 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "prost"
+version = "0.8.0"
+authors = ["Dan Burkert <dan@danburkert.com>", "Tokio Contributors <team@tokio.rs>"]
+description = "A Protocol Buffers implementation for the Rust Language."
+documentation = "https://docs.rs/prost"
+readme = "README.md"
+keywords = ["protobuf", "serialization"]
+categories = ["encoding"]
+license = "Apache-2.0"
+repository = "https://github.com/tokio-rs/prost"
+[profile.bench]
+debug = true
+
+[lib]
+bench = false
+
+[[bench]]
+name = "varint"
+harness = false
+[dependencies.bytes]
+version = "1"
+default-features = false
+
+[dependencies.prost-derive]
+version = "0.8.0"
+optional = true
+[dev-dependencies.criterion]
+version = "0.3"
+
+[dev-dependencies.env_logger]
+version = "0.8"
+default-features = false
+
+[dev-dependencies.log]
+version = "0.4"
+
+[dev-dependencies.proptest]
+version = "1"
+
+[dev-dependencies.rand]
+version = "0.8"
+
+[features]
+default = ["prost-derive", "std"]
+no-recursion-limit = []
+std = []
diff --git a/third_party/rust/prost/FUZZING.md b/third_party/rust/prost/FUZZING.md
new file mode 100644
index 0000000000..d47268d699
--- /dev/null
+++ b/third_party/rust/prost/FUZZING.md
@@ -0,0 +1,27 @@
+# Fuzzing
+
+Prost ships a few fuzz tests, using both libfuzzer and aflfuzz.
+
+
+## afl
+
+To run the afl fuzz tests, first install cargo-afl:
+
+    cargo install -f afl
+
+Then build a fuzz target and run afl on it:
+
+    cd afl/<target>/
+    cargo afl build --bin fuzz-target
+    cargo afl fuzz -i in -o out target/debug/fuzz-target
+
+To reproduce a crash:
+
+    cd afl/<target>/
+    cargo build --bin reproduce
+    cargo run --bin reproduce -- out/crashes/<crashfile>
+
+
+## libfuzzer
+
+TODO
diff --git a/third_party/rust/prost/LICENSE b/third_party/rust/prost/LICENSE
new file mode 100644
index 0000000000..16fe87b06e
--- /dev/null
+++ b/third_party/rust/prost/LICENSE
@@ -0,0 +1,201 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
+
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diff --git a/third_party/rust/prost/README.md b/third_party/rust/prost/README.md
new file mode 100644
index 0000000000..fc5d87b80d
--- /dev/null
+++ b/third_party/rust/prost/README.md
@@ -0,0 +1,462 @@
+![continuous integration](https://github.com/tokio-rs/prost/workflows/continuous%20integration/badge.svg)
+[![Documentation](https://docs.rs/prost/badge.svg)](https://docs.rs/prost/)
+[![Crate](https://img.shields.io/crates/v/prost.svg)](https://crates.io/crates/prost)
+[![Dependency Status](https://deps.rs/repo/github/tokio-rs/prost/status.svg)](https://deps.rs/repo/github/tokio-rs/prost)
+
+# *PROST!*
+
+`prost` is a [Protocol Buffers](https://developers.google.com/protocol-buffers/)
+implementation for the [Rust Language](https://www.rust-lang.org/). `prost`
+generates simple, idiomatic Rust code from `proto2` and `proto3` files.
+
+Compared to other Protocol Buffers implementations, `prost`
+
+* Generates simple, idiomatic, and readable Rust types by taking advantage of
+  Rust `derive` attributes.
+* Retains comments from `.proto` files in generated Rust code.
+* Allows existing Rust types (not generated from a `.proto`) to be serialized
+  and deserialized by adding attributes.
+* Uses the [`bytes::{Buf, BufMut}`](https://github.com/carllerche/bytes)
+  abstractions for serialization instead of `std::io::{Read, Write}`.
+* Respects the Protobuf `package` specifier when organizing generated code
+  into Rust modules.
+* Preserves unknown enum values during deserialization.
+* Does not include support for runtime reflection or message descriptors.
+
+## Using `prost` in a Cargo Project
+
+First, add `prost` and its public dependencies to your `Cargo.toml`:
+
+```
+[dependencies]
+prost = "0.8"
+# Only necessary if using Protobuf well-known types:
+prost-types = "0.8"
+```
+
+The recommended way to add `.proto` compilation to a Cargo project is to use the
+`prost-build` library. See the [`prost-build` documentation](prost-build) for
+more details and examples.
+
+## Generated Code
+
+`prost` generates Rust code from source `.proto` files using the `proto2` or
+`proto3` syntax. `prost`'s goal is to make the generated code as simple as
+possible.
+
+### Packages
+
+All `.proto` files used with `prost` must contain a
+[`package` specifier][package]. `prost` will translate the Protobuf package into
+a Rust module. For example, given the `package` specifier:
+
+[package]: https://developers.google.com/protocol-buffers/docs/proto#packages
+
+```proto
+package foo.bar;
+```
+
+All Rust types generated from the file will be in the `foo::bar` module.
+
+### Messages
+
+Given a simple message declaration:
+
+```proto
+// Sample message.
+message Foo {
+}
+```
+
+`prost` will generate the following Rust struct:
+
+```rust
+/// Sample message.
+#[derive(Clone, Debug, PartialEq, Message)]
+pub struct Foo {
+}
+```
+
+### Fields
+
+Fields in Protobuf messages are translated into Rust as public struct fields of the
+corresponding type.
+
+#### Scalar Values
+
+Scalar value types are converted as follows:
+
+| Protobuf Type | Rust Type |
+| --- | --- |
+| `double` | `f64` |
+| `float` | `f32` |
+| `int32` | `i32` |
+| `int64` | `i64` |
+| `uint32` | `u32` |
+| `uint64` | `u64` |
+| `sint32` | `i32` |
+| `sint64` | `i64` |
+| `fixed32` | `u32` |
+| `fixed64` | `u64` |
+| `sfixed32` | `i32` |
+| `sfixed64` | `i64` |
+| `bool` | `bool` |
+| `string` | `String` |
+| `bytes` | `Vec<u8>` |
+
+#### Enumerations
+
+All `.proto` enumeration types convert to the Rust `i32` type. Additionally,
+each enumeration type gets a corresponding Rust `enum` type. For example, this
+`proto` enum:
+
+```proto
+enum PhoneType {
+  MOBILE = 0;
+  HOME = 1;
+  WORK = 2;
+}
+```
+
+gets this corresponding Rust enum [1]:
+
+```rust
+pub enum PhoneType {
+    Mobile = 0,
+    Home = 1,
+    Work = 2,
+}
+```
+
+You can convert a `PhoneType` value to an `i32` by doing:
+
+```rust
+PhoneType::Mobile as i32
+```
+
+The `#[derive(::prost::Enumeration)]` annotation added to the generated
+`PhoneType` adds these associated functions to the type:
+
+```rust
+impl PhoneType {
+    pub fn is_valid(value: i32) -> bool { ... }
+    pub fn from_i32(value: i32) -> Option<PhoneType> { ... }
+}
+```
+
+so you can convert an `i32` to its corresponding `PhoneType` value by doing,
+for example:
+
+```rust
+let phone_type = 2i32;
+
+match PhoneType::from_i32(phone_type) {
+    Some(PhoneType::Mobile) => ...,
+    Some(PhoneType::Home) => ...,
+    Some(PhoneType::Work) => ...,
+    None => ...,
+}
+```
+
+Additionally, wherever a `proto` enum is used as a field in a `Message`, the
+message will have 'accessor' methods to get/set the value of the field as the
+Rust enum type. For instance, this proto `PhoneNumber` message that has a field
+named `type` of type `PhoneType`:
+
+```proto
+message PhoneNumber {
+  string number = 1;
+  PhoneType type = 2;
+}
+```
+
+will become the following Rust type [1] with methods `type` and `set_type`:
+
+```rust
+pub struct PhoneNumber {
+    pub number: String,
+    pub r#type: i32, // the `r#` is needed because `type` is a Rust keyword
+}
+
+impl PhoneNumber {
+    pub fn r#type(&self) -> PhoneType { ... }
+    pub fn set_type(&mut self, value: PhoneType) { ... }
+}
+```
+
+Note that the getter methods will return the Rust enum's default value if the
+field has an invalid `i32` value.
+
+The `enum` type isn't used directly as a field, because the Protobuf spec
+mandates that enumerations values are 'open', and decoding unrecognized
+enumeration values must be possible.
+
+[1] Annotations have been elided for clarity. See below for a full example.
+
+#### Field Modifiers
+
+Protobuf scalar value and enumeration message fields can have a modifier
+depending on the Protobuf version. Modifiers change the corresponding type of
+the Rust field:
+
+| `.proto` Version | Modifier | Rust Type |
+| --- | --- | --- |
+| `proto2` | `optional` | `Option<T>` |
+| `proto2` | `required` | `T` |
+| `proto3` | default | `T` |
+| `proto2`/`proto3` | repeated | `Vec<T>` |
+
+#### Map Fields
+
+Map fields are converted to a Rust `HashMap` with key and value type converted
+from the Protobuf key and value types.
+
+#### Message Fields
+
+Message fields are converted to the corresponding struct type. The table of
+field modifiers above applies to message fields, except that `proto3` message
+fields without a modifier (the default) will be wrapped in an `Option`.
+Typically message fields are unboxed. `prost` will automatically box a message
+field if the field type and the parent type are recursively nested in order to
+avoid an infinite sized struct.
+
+#### Oneof Fields
+
+Oneof fields convert to a Rust enum. Protobuf `oneof`s types are not named, so
+`prost` uses the name of the `oneof` field for the resulting Rust enum, and
+defines the enum in a module under the struct. For example, a `proto3` message
+such as:
+
+```proto
+message Foo {
+  oneof widget {
+    int32 quux = 1;
+    string bar = 2;
+  }
+}
+```
+
+generates the following Rust[1]:
+
+```rust
+pub struct Foo {
+    pub widget: Option<foo::Widget>,
+}
+pub mod foo {
+    pub enum Widget {
+        Quux(i32),
+        Bar(String),
+    }
+}
+```
+
+`oneof` fields are always wrapped in an `Option`.
+
+[1] Annotations have been elided for clarity. See below for a full example.
+
+### Services
+
+`prost-build` allows a custom code-generator to be used for processing `service`
+definitions. This can be used to output Rust traits according to an
+application's specific needs.
+
+### Generated Code Example
+
+Example `.proto` file:
+
+```proto
+syntax = "proto3";
+package tutorial;
+
+message Person {
+  string name = 1;
+  int32 id = 2;  // Unique ID number for this person.
+  string email = 3;
+
+  enum PhoneType {
+    MOBILE = 0;
+    HOME = 1;
+    WORK = 2;
+  }
+
+  message PhoneNumber {
+    string number = 1;
+    PhoneType type = 2;
+  }
+
+  repeated PhoneNumber phones = 4;
+}
+
+// Our address book file is just one of these.
+message AddressBook {
+  repeated Person people = 1;
+}
+```
+
+and the generated Rust code (`tutorial.rs`):
+
+```rust
+#[derive(Clone, PartialEq, ::prost::Message)]
+pub struct Person {
+    #[prost(string, tag="1")]
+    pub name: ::prost::alloc::string::String,
+    /// Unique ID number for this person.
+    #[prost(int32, tag="2")]
+    pub id: i32,
+    #[prost(string, tag="3")]
+    pub email: ::prost::alloc::string::String,
+    #[prost(message, repeated, tag="4")]
+    pub phones: ::prost::alloc::vec::Vec<person::PhoneNumber>,
+}
+/// Nested message and enum types in `Person`.
+pub mod person {
+    #[derive(Clone, PartialEq, ::prost::Message)]
+    pub struct PhoneNumber {
+        #[prost(string, tag="1")]
+        pub number: ::prost::alloc::string::String,
+        #[prost(enumeration="PhoneType", tag="2")]
+        pub r#type: i32,
+    }
+    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, ::prost::Enumeration)]
+    #[repr(i32)]
+    pub enum PhoneType {
+        Mobile = 0,
+        Home = 1,
+        Work = 2,
+    }
+}
+/// Our address book file is just one of these.
+#[derive(Clone, PartialEq, ::prost::Message)]
+pub struct AddressBook {
+    #[prost(message, repeated, tag="1")]
+    pub people: ::prost::alloc::vec::Vec<Person>,
+}
+```
+
+## Accessing the `protoc` `FileDescriptorSet`
+
+The `prost_build::Config::file_descriptor_set_path` option can be used to emit a file descriptor set
+during the build & code generation step. When used in conjunction with the `std::include_bytes`
+macro and the `prost_types::FileDescriptorSet` type, applications and libraries using Prost can
+implement introspection capabilities requiring details from the original `.proto` files.
+
+## Using `prost` in a `no_std` Crate
+
+`prost` is compatible with `no_std` crates. To enable `no_std` support, disable
+the `std` features in `prost` and `prost-types`:
+
+```
+[dependencies]
+prost = { version = "0.6", default-features = false, features = ["prost-derive"] }
+# Only necessary if using Protobuf well-known types:
+prost-types = { version = "0.6", default-features = false }
+```
+
+Additionally, configure `prost-build` to output `BTreeMap`s instead of `HashMap`s
+for all Protobuf `map` fields in your `build.rs`:
+
+```rust
+let mut config = prost_build::Config::new();
+config.btree_map(&["."]);
+```
+
+When using edition 2015, it may be necessary to add an `extern crate core;`
+directive to the crate which includes `prost`-generated code.
+
+## Serializing Existing Types
+
+`prost` uses a custom derive macro to handle encoding and decoding types, which
+means that if your existing Rust type is compatible with Protobuf types, you can
+serialize and deserialize it by adding the appropriate derive and field
+annotations.
+
+Currently the best documentation on adding annotations is to look at the
+generated code examples above.
+
+### Tag Inference for Existing Types
+
+Prost automatically infers tags for the struct.
+
+Fields are tagged sequentially in the order they
+are specified, starting with `1`.
+
+You may skip tags which have been reserved, or where there are gaps between
+sequentially occurring tag values by specifying the tag number to skip to with
+the `tag` attribute on the first field after the gap. The following fields will
+be tagged sequentially starting from the next number.
+
+```rust
+use prost;
+use prost::{Enumeration, Message};
+
+#[derive(Clone, PartialEq, Message)]
+struct Person {
+    #[prost(string, tag = "1")]
+    pub id: String, // tag=1
+    // NOTE: Old "name" field has been removed
+    // pub name: String, // tag=2 (Removed)
+    #[prost(string, tag = "6")]
+    pub given_name: String, // tag=6
+    #[prost(string)]
+    pub family_name: String, // tag=7
+    #[prost(string)]
+    pub formatted_name: String, // tag=8
+    #[prost(uint32, tag = "3")]
+    pub age: u32, // tag=3
+    #[prost(uint32)]
+    pub height: u32, // tag=4
+    #[prost(enumeration = "Gender")]
+    pub gender: i32, // tag=5
+    // NOTE: Skip to less commonly occurring fields
+    #[prost(string, tag = "16")]
+    pub name_prefix: String, // tag=16  (eg. mr/mrs/ms)
+    #[prost(string)]
+    pub name_suffix: String, // tag=17  (eg. jr/esq)
+    #[prost(string)]
+    pub maiden_name: String, // tag=18
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Enumeration)]
+pub enum Gender {
+    Unknown = 0,
+    Female = 1,
+    Male = 2,
+}
+```
+
+## FAQ
+
+1. **Could `prost` be implemented as a serializer for [Serde](https://serde.rs/)?**
+
+  Probably not, however I would like to hear from a Serde expert on the matter.
+  There are two complications with trying to serialize Protobuf messages with
+  Serde:
+
+  - Protobuf fields require a numbered tag, and currently there appears to be no
+    mechanism suitable for this in `serde`.
+  - The mapping of Protobuf type to Rust type is not 1-to-1. As a result,
+    trait-based approaches to dispatching don't work very well. Example: six
+    different Protobuf field types correspond to a Rust `Vec<i32>`: `repeated
+    int32`, `repeated sint32`, `repeated sfixed32`, and their packed
+    counterparts.
+
+  But it is possible to place `serde` derive tags onto the generated types, so
+  the same structure can support both `prost` and `Serde`.
+
+2. **I get errors when trying to run `cargo test` on MacOS**
+
+  If the errors are about missing `autoreconf` or similar, you can probably fix
+  them by running
+
+  ```
+  brew install automake
+  brew install libtool
+  ```
+
+## License
+
+`prost` is distributed under the terms of the Apache License (Version 2.0).
+
+See [LICENSE](LICENSE) for details.
+
+Copyright 2017 Dan Burkert
diff --git a/third_party/rust/prost/benches/varint.rs b/third_party/rust/prost/benches/varint.rs
new file mode 100644
index 0000000000..34951f5eaf
--- /dev/null
+++ b/third_party/rust/prost/benches/varint.rs
@@ -0,0 +1,99 @@
+use std::mem;
+
+use bytes::Buf;
+use criterion::{Criterion, Throughput};
+use prost::encoding::{decode_varint, encode_varint, encoded_len_varint};
+use rand::{rngs::StdRng, seq::SliceRandom, SeedableRng};
+
+fn benchmark_varint(criterion: &mut Criterion, name: &str, mut values: Vec<u64>) {
+    // Shuffle the values in a stable order.
+    values.shuffle(&mut StdRng::seed_from_u64(0));
+    let name = format!("varint/{}", name);
+
+    let encoded_len = values
+        .iter()
+        .cloned()
+        .map(encoded_len_varint)
+        .sum::<usize>() as u64;
+    let decoded_len = (values.len() * mem::size_of::<u64>()) as u64;
+
+    criterion
+        .benchmark_group(&name)
+        .bench_function("encode", {
+            let encode_values = values.clone();
+            move |b| {
+                let mut buf = Vec::<u8>::with_capacity(encode_values.len() * 10);
+                b.iter(|| {
+                    buf.clear();
+                    for &value in &encode_values {
+                        encode_varint(value, &mut buf);
+                    }
+                    criterion::black_box(&buf);
+                })
+            }
+        })
+        .throughput(Throughput::Bytes(encoded_len));
+
+    criterion
+        .benchmark_group(&name)
+        .bench_function("decode", {
+            let decode_values = values.clone();
+
+            move |b| {
+                let mut buf = Vec::with_capacity(decode_values.len() * 10);
+                for &value in &decode_values {
+                    encode_varint(value, &mut buf);
+                }
+
+                b.iter(|| {
+                    let mut buf = &mut buf.as_slice();
+                    while buf.has_remaining() {
+                        let result = decode_varint(&mut buf);
+                        debug_assert!(result.is_ok());
+                        criterion::black_box(&result);
+                    }
+                })
+            }
+        })
+        .throughput(Throughput::Bytes(decoded_len));
+
+    criterion
+        .benchmark_group(&name)
+        .bench_function("encoded_len", move |b| {
+            b.iter(|| {
+                let mut sum = 0;
+                for &value in &values {
+                    sum += encoded_len_varint(value);
+                }
+                criterion::black_box(sum);
+            })
+        })
+        .throughput(Throughput::Bytes(decoded_len));
+}
+
+fn main() {
+    let mut criterion = Criterion::default().configure_from_args();
+
+    // Benchmark encoding and decoding 100 small (1 byte) varints.
+    benchmark_varint(&mut criterion, "small", (0..100).collect());
+
+    // Benchmark encoding and decoding 100 medium (5 byte) varints.
+    benchmark_varint(&mut criterion, "medium", (1 << 28..).take(100).collect());
+
+    // Benchmark encoding and decoding 100 large (10 byte) varints.
+    benchmark_varint(&mut criterion, "large", (1 << 63..).take(100).collect());
+
+    // Benchmark encoding and decoding 100 varints of mixed width (average 5.5 bytes).
+    benchmark_varint(
+        &mut criterion,
+        "mixed",
+        (0..10)
+            .flat_map(move |width| {
+                let exponent = width * 7;
+                (0..10).map(move |offset| offset + (1 << exponent))
+            })
+            .collect(),
+    );
+
+    criterion.final_summary();
+}
diff --git a/third_party/rust/prost/clippy.toml b/third_party/rust/prost/clippy.toml
new file mode 100644
index 0000000000..5988e12d8f
--- /dev/null
+++ b/third_party/rust/prost/clippy.toml
@@ -0,0 +1 @@
+too-many-arguments-threshold=8
diff --git a/third_party/rust/prost/prepare-release.sh b/third_party/rust/prost/prepare-release.sh
new file mode 100755
index 0000000000..34e5202c24
--- /dev/null
+++ b/third_party/rust/prost/prepare-release.sh
@@ -0,0 +1,47 @@
+#!/bin/bash
+
+# Script which automates modifying source version fields, and creating a release
+# commit and tag. The commit and tag are not automatically pushed, nor are the
+# crates published (see publish-release.sh).
+
+set -ex
+
+if [ "$#" -ne 1 ]
+then
+  echo "Usage: $0 <version>"
+  exit 1
+fi
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+VERSION="$1"
+MINOR="$( echo ${VERSION} | cut -d\. -f1-2 )"
+
+VERSION_MATCHER="([a-z0-9\\.-]+)"
+PROST_CRATE_MATCHER="(prost|prost-[a-z]+)"
+
+# Update the README.md.
+sed -i -E "s/${PROST_CRATE_MATCHER} = \"${VERSION_MATCHER}\"/\1 = \"${MINOR}\"/" "$DIR/README.md"
+
+# Update html_root_url attributes.
+sed -i -E "s~html_root_url = \"https://docs\.rs/${PROST_CRATE_MATCHER}/$VERSION_MATCHER\"~html_root_url = \"https://docs.rs/\1/${VERSION}\"~" \
+  "$DIR/src/lib.rs" \
+  "$DIR/prost-derive/src/lib.rs" \
+  "$DIR/prost-build/src/lib.rs" \
+  "$DIR/prost-types/src/lib.rs"
+
+# Update Cargo.toml version fields.
+sed -i -E "s/^version = \"${VERSION_MATCHER}\"$/version = \"${VERSION}\"/" \
+  "$DIR/Cargo.toml" \
+  "$DIR/prost-derive/Cargo.toml" \
+  "$DIR/prost-build/Cargo.toml" \
+  "$DIR/prost-types/Cargo.toml"
+
+# Update Cargo.toml dependency versions.
+sed -i -E "s/^${PROST_CRATE_MATCHER} = \{ version = \"${VERSION_MATCHER}\"/\1 = { version = \"${VERSION}\"/" \
+  "$DIR/Cargo.toml" \
+  "$DIR/prost-derive/Cargo.toml" \
+  "$DIR/prost-build/Cargo.toml" \
+  "$DIR/prost-types/Cargo.toml"
+
+git commit -a -m "release ${VERSION}"
+git tag -a "v${VERSION}" -m "release ${VERSION}"
diff --git a/third_party/rust/prost/publish-release.sh b/third_party/rust/prost/publish-release.sh
new file mode 100755
index 0000000000..b2be598298
--- /dev/null
+++ b/third_party/rust/prost/publish-release.sh
@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# Script which automates publishing a crates.io release of the prost crates.
+
+set -ex
+
+if [ "$#" -ne 0 ]
+then
+  echo "Usage: $0"
+  exit 1
+fi
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+
+CRATES=( \
+  "prost-derive" \
+  "." \
+  "prost-types" \
+  "prost-build" \
+)
+
+for CRATE in "${CRATES[@]}"; do
+  pushd "$DIR/$CRATE"
+  cargo publish
+  popd
+done
diff --git a/third_party/rust/prost/src/encoding.rs b/third_party/rust/prost/src/encoding.rs
new file mode 100644
index 0000000000..252358685c
--- /dev/null
+++ b/third_party/rust/prost/src/encoding.rs
@@ -0,0 +1,1770 @@
+//! Utility functions and types for encoding and decoding Protobuf types.
+//!
+//! Meant to be used only from `Message` implementations.
+
+#![allow(clippy::implicit_hasher, clippy::ptr_arg)]
+
+use alloc::collections::BTreeMap;
+use alloc::format;
+use alloc::string::String;
+use alloc::vec::Vec;
+use core::cmp::min;
+use core::convert::TryFrom;
+use core::mem;
+use core::str;
+use core::u32;
+use core::usize;
+
+use ::bytes::{Buf, BufMut, Bytes};
+
+use crate::DecodeError;
+use crate::Message;
+
+/// Encodes an integer value into LEB128 variable length format, and writes it to the buffer.
+/// The buffer must have enough remaining space (maximum 10 bytes).
+#[inline]
+pub fn encode_varint<B>(mut value: u64, buf: &mut B)
+where
+    B: BufMut,
+{
+    // Safety notes:
+    //
+    // - ptr::write is an unsafe raw pointer write. The use here is safe since the length of the
+    //   uninit slice is checked.
+    // - advance_mut is unsafe because it could cause uninitialized memory to be advanced over. The
+    //   use here is safe since each byte which is advanced over has been written to in the
+    //   previous loop iteration.
+    unsafe {
+        let mut i;
+        'outer: loop {
+            i = 0;
+
+            let uninit_slice = buf.chunk_mut();
+            for offset in 0..uninit_slice.len() {
+                i += 1;
+                let ptr = uninit_slice.as_mut_ptr().add(offset);
+                if value < 0x80 {
+                    ptr.write(value as u8);
+                    break 'outer;
+                } else {
+                    ptr.write(((value & 0x7F) | 0x80) as u8);
+                    value >>= 7;
+                }
+            }
+
+            buf.advance_mut(i);
+            debug_assert!(buf.has_remaining_mut());
+        }
+
+        buf.advance_mut(i);
+    }
+}
+
+/// Decodes a LEB128-encoded variable length integer from the buffer.
+pub fn decode_varint<B>(buf: &mut B) -> Result<u64, DecodeError>
+where
+    B: Buf,
+{
+    let bytes = buf.chunk();
+    let len = bytes.len();
+    if len == 0 {
+        return Err(DecodeError::new("invalid varint"));
+    }
+
+    let byte = unsafe { *bytes.get_unchecked(0) };
+    if byte < 0x80 {
+        buf.advance(1);
+        Ok(u64::from(byte))
+    } else if len > 10 || bytes[len - 1] < 0x80 {
+        let (value, advance) = unsafe { decode_varint_slice(bytes) }?;
+        buf.advance(advance);
+        Ok(value)
+    } else {
+        decode_varint_slow(buf)
+    }
+}
+
+/// Decodes a LEB128-encoded variable length integer from the slice, returning the value and the
+/// number of bytes read.
+///
+/// Based loosely on [`ReadVarint64FromArray`][1].
+///
+/// ## Safety
+///
+/// The caller must ensure that `bytes` is non-empty and either `bytes.len() >= 10` or the last
+/// element in bytes is < `0x80`.
+///
+/// [1]: https://github.com/google/protobuf/blob/3.3.x/src/google/protobuf/io/coded_stream.cc#L365-L406
+#[inline]
+unsafe fn decode_varint_slice(bytes: &[u8]) -> Result<(u64, usize), DecodeError> {
+    // Fully unrolled varint decoding loop. Splitting into 32-bit pieces gives better performance.
+
+    let mut b: u8;
+    let mut part0: u32;
+    b = *bytes.get_unchecked(0);
+    part0 = u32::from(b);
+    if b < 0x80 {
+        return Ok((u64::from(part0), 1));
+    };
+    part0 -= 0x80;
+    b = *bytes.get_unchecked(1);
+    part0 += u32::from(b) << 7;
+    if b < 0x80 {
+        return Ok((u64::from(part0), 2));
+    };
+    part0 -= 0x80 << 7;
+    b = *bytes.get_unchecked(2);
+    part0 += u32::from(b) << 14;
+    if b < 0x80 {
+        return Ok((u64::from(part0), 3));
+    };
+    part0 -= 0x80 << 14;
+    b = *bytes.get_unchecked(3);
+    part0 += u32::from(b) << 21;
+    if b < 0x80 {
+        return Ok((u64::from(part0), 4));
+    };
+    part0 -= 0x80 << 21;
+    let value = u64::from(part0);
+
+    let mut part1: u32;
+    b = *bytes.get_unchecked(4);
+    part1 = u32::from(b);
+    if b < 0x80 {
+        return Ok((value + (u64::from(part1) << 28), 5));
+    };
+    part1 -= 0x80;
+    b = *bytes.get_unchecked(5);
+    part1 += u32::from(b) << 7;
+    if b < 0x80 {
+        return Ok((value + (u64::from(part1) << 28), 6));
+    };
+    part1 -= 0x80 << 7;
+    b = *bytes.get_unchecked(6);
+    part1 += u32::from(b) << 14;
+    if b < 0x80 {
+        return Ok((value + (u64::from(part1) << 28), 7));
+    };
+    part1 -= 0x80 << 14;
+    b = *bytes.get_unchecked(7);
+    part1 += u32::from(b) << 21;
+    if b < 0x80 {
+        return Ok((value + (u64::from(part1) << 28), 8));
+    };
+    part1 -= 0x80 << 21;
+    let value = value + ((u64::from(part1)) << 28);
+
+    let mut part2: u32;
+    b = *bytes.get_unchecked(8);
+    part2 = u32::from(b);
+    if b < 0x80 {
+        return Ok((value + (u64::from(part2) << 56), 9));
+    };
+    part2 -= 0x80;
+    b = *bytes.get_unchecked(9);
+    part2 += u32::from(b) << 7;
+    if b < 0x80 {
+        return Ok((value + (u64::from(part2) << 56), 10));
+    };
+
+    // We have overrun the maximum size of a varint (10 bytes). Assume the data is corrupt.
+    Err(DecodeError::new("invalid varint"))
+}
+
+/// Decodes a LEB128-encoded variable length integer from the buffer, advancing the buffer as
+/// necessary.
+#[inline(never)]
+fn decode_varint_slow<B>(buf: &mut B) -> Result<u64, DecodeError>
+where
+    B: Buf,
+{
+    let mut value = 0;
+    for count in 0..min(10, buf.remaining()) {
+        let byte = buf.get_u8();
+        value |= u64::from(byte & 0x7F) << (count * 7);
+        if byte <= 0x7F {
+            return Ok(value);
+        }
+    }
+
+    Err(DecodeError::new("invalid varint"))
+}
+
+/// Additional information passed to every decode/merge function.
+///
+/// The context should be passed by value and can be freely cloned. When passing
+/// to a function which is decoding a nested object, then use `enter_recursion`.
+#[derive(Clone, Debug)]
+pub struct DecodeContext {
+    /// How many times we can recurse in the current decode stack before we hit
+    /// the recursion limit.
+    ///
+    /// The recursion limit is defined by `RECURSION_LIMIT` and cannot be
+    /// customized. The recursion limit can be ignored by building the Prost
+    /// crate with the `no-recursion-limit` feature.
+    #[cfg(not(feature = "no-recursion-limit"))]
+    recurse_count: u32,
+}
+
+impl Default for DecodeContext {
+    #[cfg(not(feature = "no-recursion-limit"))]
+    #[inline]
+    fn default() -> DecodeContext {
+        DecodeContext {
+            recurse_count: crate::RECURSION_LIMIT,
+        }
+    }
+
+    #[cfg(feature = "no-recursion-limit")]
+    #[inline]
+    fn default() -> DecodeContext {
+        DecodeContext {}
+    }
+}
+
+impl DecodeContext {
+    /// Call this function before recursively decoding.
+    ///
+    /// There is no `exit` function since this function creates a new `DecodeContext`
+    /// to be used at the next level of recursion. Continue to use the old context
+    // at the previous level of recursion.
+    #[cfg(not(feature = "no-recursion-limit"))]
+    #[inline]
+    pub(crate) fn enter_recursion(&self) -> DecodeContext {
+        DecodeContext {
+            recurse_count: self.recurse_count - 1,
+        }
+    }
+
+    #[cfg(feature = "no-recursion-limit")]
+    #[inline]
+    pub(crate) fn enter_recursion(&self) -> DecodeContext {
+        DecodeContext {}
+    }
+
+    /// Checks whether the recursion limit has been reached in the stack of
+    /// decodes described by the `DecodeContext` at `self.ctx`.
+    ///
+    /// Returns `Ok<()>` if it is ok to continue recursing.
+    /// Returns `Err<DecodeError>` if the recursion limit has been reached.
+    #[cfg(not(feature = "no-recursion-limit"))]
+    #[inline]
+    pub(crate) fn limit_reached(&self) -> Result<(), DecodeError> {
+        if self.recurse_count == 0 {
+            Err(DecodeError::new("recursion limit reached"))
+        } else {
+            Ok(())
+        }
+    }
+
+    #[cfg(feature = "no-recursion-limit")]
+    #[inline]
+    #[allow(clippy::unnecessary_wraps)] // needed in other features
+    pub(crate) fn limit_reached(&self) -> Result<(), DecodeError> {
+        Ok(())
+    }
+}
+
+/// Returns the encoded length of the value in LEB128 variable length format.
+/// The returned value will be between 1 and 10, inclusive.
+#[inline]
+pub fn encoded_len_varint(value: u64) -> usize {
+    // Based on [VarintSize64][1].
+    // [1]: https://github.com/google/protobuf/blob/3.3.x/src/google/protobuf/io/coded_stream.h#L1301-L1309
+    ((((value | 1).leading_zeros() ^ 63) * 9 + 73) / 64) as usize
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+#[repr(u8)]
+pub enum WireType {
+    Varint = 0,
+    SixtyFourBit = 1,
+    LengthDelimited = 2,
+    StartGroup = 3,
+    EndGroup = 4,
+    ThirtyTwoBit = 5,
+}
+
+pub const MIN_TAG: u32 = 1;
+pub const MAX_TAG: u32 = (1 << 29) - 1;
+
+impl TryFrom<u64> for WireType {
+    type Error = DecodeError;
+
+    #[inline]
+    fn try_from(value: u64) -> Result<Self, Self::Error> {
+        match value {
+            0 => Ok(WireType::Varint),
+            1 => Ok(WireType::SixtyFourBit),
+            2 => Ok(WireType::LengthDelimited),
+            3 => Ok(WireType::StartGroup),
+            4 => Ok(WireType::EndGroup),
+            5 => Ok(WireType::ThirtyTwoBit),
+            _ => Err(DecodeError::new(format!(
+                "invalid wire type value: {}",
+                value
+            ))),
+        }
+    }
+}
+
+/// Encodes a Protobuf field key, which consists of a wire type designator and
+/// the field tag.
+#[inline]
+pub fn encode_key<B>(tag: u32, wire_type: WireType, buf: &mut B)
+where
+    B: BufMut,
+{
+    debug_assert!((MIN_TAG..=MAX_TAG).contains(&tag));
+    let key = (tag << 3) | wire_type as u32;
+    encode_varint(u64::from(key), buf);
+}
+
+/// Decodes a Protobuf field key, which consists of a wire type designator and
+/// the field tag.
+#[inline(always)]
+pub fn decode_key<B>(buf: &mut B) -> Result<(u32, WireType), DecodeError>
+where
+    B: Buf,
+{
+    let key = decode_varint(buf)?;
+    if key > u64::from(u32::MAX) {
+        return Err(DecodeError::new(format!("invalid key value: {}", key)));
+    }
+    let wire_type = WireType::try_from(key & 0x07)?;
+    let tag = key as u32 >> 3;
+
+    if tag < MIN_TAG {
+        return Err(DecodeError::new("invalid tag value: 0"));
+    }
+
+    Ok((tag, wire_type))
+}
+
+/// Returns the width of an encoded Protobuf field key with the given tag.
+/// The returned width will be between 1 and 5 bytes (inclusive).
+#[inline]
+pub fn key_len(tag: u32) -> usize {
+    encoded_len_varint(u64::from(tag << 3))
+}
+
+/// Checks that the expected wire type matches the actual wire type,
+/// or returns an error result.
+#[inline]
+pub fn check_wire_type(expected: WireType, actual: WireType) -> Result<(), DecodeError> {
+    if expected != actual {
+        return Err(DecodeError::new(format!(
+            "invalid wire type: {:?} (expected {:?})",
+            actual, expected
+        )));
+    }
+    Ok(())
+}
+
+/// Helper function which abstracts reading a length delimiter prefix followed
+/// by decoding values until the length of bytes is exhausted.
+pub fn merge_loop<T, M, B>(
+    value: &mut T,
+    buf: &mut B,
+    ctx: DecodeContext,
+    mut merge: M,
+) -> Result<(), DecodeError>
+where
+    M: FnMut(&mut T, &mut B, DecodeContext) -> Result<(), DecodeError>,
+    B: Buf,
+{
+    let len = decode_varint(buf)?;
+    let remaining = buf.remaining();
+    if len > remaining as u64 {
+        return Err(DecodeError::new("buffer underflow"));
+    }
+
+    let limit = remaining - len as usize;
+    while buf.remaining() > limit {
+        merge(value, buf, ctx.clone())?;
+    }
+
+    if buf.remaining() != limit {
+        return Err(DecodeError::new("delimited length exceeded"));
+    }
+    Ok(())
+}
+
+pub fn skip_field<B>(
+    wire_type: WireType,
+    tag: u32,
+    buf: &mut B,
+    ctx: DecodeContext,
+) -> Result<(), DecodeError>
+where
+    B: Buf,
+{
+    ctx.limit_reached()?;
+    let len = match wire_type {
+        WireType::Varint => decode_varint(buf).map(|_| 0)?,
+        WireType::ThirtyTwoBit => 4,
+        WireType::SixtyFourBit => 8,
+        WireType::LengthDelimited => decode_varint(buf)?,
+        WireType::StartGroup => loop {
+            let (inner_tag, inner_wire_type) = decode_key(buf)?;
+            match inner_wire_type {
+                WireType::EndGroup => {
+                    if inner_tag != tag {
+                        return Err(DecodeError::new("unexpected end group tag"));
+                    }
+                    break 0;
+                }
+                _ => skip_field(inner_wire_type, inner_tag, buf, ctx.enter_recursion())?,
+            }
+        },
+        WireType::EndGroup => return Err(DecodeError::new("unexpected end group tag")),
+    };
+
+    if len > buf.remaining() as u64 {
+        return Err(DecodeError::new("buffer underflow"));
+    }
+
+    buf.advance(len as usize);
+    Ok(())
+}
+
+/// Helper macro which emits an `encode_repeated` function for the type.
+macro_rules! encode_repeated {
+    ($ty:ty) => {
+        pub fn encode_repeated<B>(tag: u32, values: &[$ty], buf: &mut B)
+        where
+            B: BufMut,
+        {
+            for value in values {
+                encode(tag, value, buf);
+            }
+        }
+    };
+}
+
+/// Helper macro which emits a `merge_repeated` function for the numeric type.
+macro_rules! merge_repeated_numeric {
+    ($ty:ty,
+     $wire_type:expr,
+     $merge:ident,
+     $merge_repeated:ident) => {
+        pub fn $merge_repeated<B>(
+            wire_type: WireType,
+            values: &mut Vec<$ty>,
+            buf: &mut B,
+            ctx: DecodeContext,
+        ) -> Result<(), DecodeError>
+        where
+            B: Buf,
+        {
+            if wire_type == WireType::LengthDelimited {
+                // Packed.
+                merge_loop(values, buf, ctx, |values, buf, ctx| {
+                    let mut value = Default::default();
+                    $merge($wire_type, &mut value, buf, ctx)?;
+                    values.push(value);
+                    Ok(())
+                })
+            } else {
+                // Unpacked.
+                check_wire_type($wire_type, wire_type)?;
+                let mut value = Default::default();
+                $merge(wire_type, &mut value, buf, ctx)?;
+                values.push(value);
+                Ok(())
+            }
+        }
+    };
+}
+
+/// Macro which emits a module containing a set of encoding functions for a
+/// variable width numeric type.
+macro_rules! varint {
+    ($ty:ty,
+     $proto_ty:ident) => (
+        varint!($ty,
+                $proto_ty,
+                to_uint64(value) { *value as u64 },
+                from_uint64(value) { value as $ty });
+    );
+
+    ($ty:ty,
+     $proto_ty:ident,
+     to_uint64($to_uint64_value:ident) $to_uint64:expr,
+     from_uint64($from_uint64_value:ident) $from_uint64:expr) => (
+
+         pub mod $proto_ty {
+            use crate::encoding::*;
+
+            pub fn encode<B>(tag: u32, $to_uint64_value: &$ty, buf: &mut B) where B: BufMut {
+                encode_key(tag, WireType::Varint, buf);
+                encode_varint($to_uint64, buf);
+            }
+
+            pub fn merge<B>(wire_type: WireType, value: &mut $ty, buf: &mut B, _ctx: DecodeContext) -> Result<(), DecodeError> where B: Buf {
+                check_wire_type(WireType::Varint, wire_type)?;
+                let $from_uint64_value = decode_varint(buf)?;
+                *value = $from_uint64;
+                Ok(())
+            }
+
+            encode_repeated!($ty);
+
+            pub fn encode_packed<B>(tag: u32, values: &[$ty], buf: &mut B) where B: BufMut {
+                if values.is_empty() { return; }
+
+                encode_key(tag, WireType::LengthDelimited, buf);
+                let len: usize = values.iter().map(|$to_uint64_value| {
+                    encoded_len_varint($to_uint64)
+                }).sum();
+                encode_varint(len as u64, buf);
+
+                for $to_uint64_value in values {
+                    encode_varint($to_uint64, buf);
+                }
+            }
+
+            merge_repeated_numeric!($ty, WireType::Varint, merge, merge_repeated);
+
+            #[inline]
+            pub fn encoded_len(tag: u32, $to_uint64_value: &$ty) -> usize {
+                key_len(tag) + encoded_len_varint($to_uint64)
+            }
+
+            #[inline]
+            pub fn encoded_len_repeated(tag: u32, values: &[$ty]) -> usize {
+                key_len(tag) * values.len() + values.iter().map(|$to_uint64_value| {
+                    encoded_len_varint($to_uint64)
+                }).sum::<usize>()
+            }
+
+            #[inline]
+            pub fn encoded_len_packed(tag: u32, values: &[$ty]) -> usize {
+                if values.is_empty() {
+                    0
+                } else {
+                    let len = values.iter()
+                                    .map(|$to_uint64_value| encoded_len_varint($to_uint64))
+                                    .sum::<usize>();
+                    key_len(tag) + encoded_len_varint(len as u64) + len
+                }
+            }
+
+            #[cfg(test)]
+            mod test {
+                use proptest::prelude::*;
+
+                use crate::encoding::$proto_ty::*;
+                use crate::encoding::test::{
+                    check_collection_type,
+                    check_type,
+                };
+
+                proptest! {
+                    #[test]
+                    fn check(value: $ty, tag in MIN_TAG..=MAX_TAG) {
+                        check_type(value, tag, WireType::Varint,
+                                   encode, merge, encoded_len)?;
+                    }
+                    #[test]
+                    fn check_repeated(value: Vec<$ty>, tag in MIN_TAG..=MAX_TAG) {
+                        check_collection_type(value, tag, WireType::Varint,
+                                              encode_repeated, merge_repeated,
+                                              encoded_len_repeated)?;
+                    }
+                    #[test]
+                    fn check_packed(value: Vec<$ty>, tag in MIN_TAG..=MAX_TAG) {
+                        check_type(value, tag, WireType::LengthDelimited,
+                                   encode_packed, merge_repeated,
+                                   encoded_len_packed)?;
+                    }
+                }
+            }
+         }
+
+    );
+}
+varint!(bool, bool,
+        to_uint64(value) if *value { 1u64 } else { 0u64 },
+        from_uint64(value) value != 0);
+varint!(i32, int32);
+varint!(i64, int64);
+varint!(u32, uint32);
+varint!(u64, uint64);
+varint!(i32, sint32,
+to_uint64(value) {
+    ((value << 1) ^ (value >> 31)) as u32 as u64
+},
+from_uint64(value) {
+    let value = value as u32;
+    ((value >> 1) as i32) ^ (-((value & 1) as i32))
+});
+varint!(i64, sint64,
+to_uint64(value) {
+    ((value << 1) ^ (value >> 63)) as u64
+},
+from_uint64(value) {
+    ((value >> 1) as i64) ^ (-((value & 1) as i64))
+});
+
+/// Macro which emits a module containing a set of encoding functions for a
+/// fixed width numeric type.
+macro_rules! fixed_width {
+    ($ty:ty,
+     $width:expr,
+     $wire_type:expr,
+     $proto_ty:ident,
+     $put:ident,
+     $get:ident) => {
+        pub mod $proto_ty {
+            use crate::encoding::*;
+
+            pub fn encode<B>(tag: u32, value: &$ty, buf: &mut B)
+            where
+                B: BufMut,
+            {
+                encode_key(tag, $wire_type, buf);
+                buf.$put(*value);
+            }
+
+            pub fn merge<B>(
+                wire_type: WireType,
+                value: &mut $ty,
+                buf: &mut B,
+                _ctx: DecodeContext,
+            ) -> Result<(), DecodeError>
+            where
+                B: Buf,
+            {
+                check_wire_type($wire_type, wire_type)?;
+                if buf.remaining() < $width {
+                    return Err(DecodeError::new("buffer underflow"));
+                }
+                *value = buf.$get();
+                Ok(())
+            }
+
+            encode_repeated!($ty);
+
+            pub fn encode_packed<B>(tag: u32, values: &[$ty], buf: &mut B)
+            where
+                B: BufMut,
+            {
+                if values.is_empty() {
+                    return;
+                }
+
+                encode_key(tag, WireType::LengthDelimited, buf);
+                let len = values.len() as u64 * $width;
+                encode_varint(len as u64, buf);
+
+                for value in values {
+                    buf.$put(*value);
+                }
+            }
+
+            merge_repeated_numeric!($ty, $wire_type, merge, merge_repeated);
+
+            #[inline]
+            pub fn encoded_len(tag: u32, _: &$ty) -> usize {
+                key_len(tag) + $width
+            }
+
+            #[inline]
+            pub fn encoded_len_repeated(tag: u32, values: &[$ty]) -> usize {
+                (key_len(tag) + $width) * values.len()
+            }
+
+            #[inline]
+            pub fn encoded_len_packed(tag: u32, values: &[$ty]) -> usize {
+                if values.is_empty() {
+                    0
+                } else {
+                    let len = $width * values.len();
+                    key_len(tag) + encoded_len_varint(len as u64) + len
+                }
+            }
+
+            #[cfg(test)]
+            mod test {
+                use proptest::prelude::*;
+
+                use super::super::test::{check_collection_type, check_type};
+                use super::*;
+
+                proptest! {
+                    #[test]
+                    fn check(value: $ty, tag in MIN_TAG..=MAX_TAG) {
+                        check_type(value, tag, $wire_type,
+                                   encode, merge, encoded_len)?;
+                    }
+                    #[test]
+                    fn check_repeated(value: Vec<$ty>, tag in MIN_TAG..=MAX_TAG) {
+                        check_collection_type(value, tag, $wire_type,
+                                              encode_repeated, merge_repeated,
+                                              encoded_len_repeated)?;
+                    }
+                    #[test]
+                    fn check_packed(value: Vec<$ty>, tag in MIN_TAG..=MAX_TAG) {
+                        check_type(value, tag, WireType::LengthDelimited,
+                                   encode_packed, merge_repeated,
+                                   encoded_len_packed)?;
+                    }
+                }
+            }
+        }
+    };
+}
+fixed_width!(
+    f32,
+    4,
+    WireType::ThirtyTwoBit,
+    float,
+    put_f32_le,
+    get_f32_le
+);
+fixed_width!(
+    f64,
+    8,
+    WireType::SixtyFourBit,
+    double,
+    put_f64_le,
+    get_f64_le
+);
+fixed_width!(
+    u32,
+    4,
+    WireType::ThirtyTwoBit,
+    fixed32,
+    put_u32_le,
+    get_u32_le
+);
+fixed_width!(
+    u64,
+    8,
+    WireType::SixtyFourBit,
+    fixed64,
+    put_u64_le,
+    get_u64_le
+);
+fixed_width!(
+    i32,
+    4,
+    WireType::ThirtyTwoBit,
+    sfixed32,
+    put_i32_le,
+    get_i32_le
+);
+fixed_width!(
+    i64,
+    8,
+    WireType::SixtyFourBit,
+    sfixed64,
+    put_i64_le,
+    get_i64_le
+);
+
+/// Macro which emits encoding functions for a length-delimited type.
+macro_rules! length_delimited {
+    ($ty:ty) => {
+        encode_repeated!($ty);
+
+        pub fn merge_repeated<B>(
+            wire_type: WireType,
+            values: &mut Vec<$ty>,
+            buf: &mut B,
+            ctx: DecodeContext,
+        ) -> Result<(), DecodeError>
+        where
+            B: Buf,
+        {
+            check_wire_type(WireType::LengthDelimited, wire_type)?;
+            let mut value = Default::default();
+            merge(wire_type, &mut value, buf, ctx)?;
+            values.push(value);
+            Ok(())
+        }
+
+        #[inline]
+        pub fn encoded_len(tag: u32, value: &$ty) -> usize {
+            key_len(tag) + encoded_len_varint(value.len() as u64) + value.len()
+        }
+
+        #[inline]
+        pub fn encoded_len_repeated(tag: u32, values: &[$ty]) -> usize {
+            key_len(tag) * values.len()
+                + values
+                    .iter()
+                    .map(|value| encoded_len_varint(value.len() as u64) + value.len())
+                    .sum::<usize>()
+        }
+    };
+}
+
+pub mod string {
+    use super::*;
+
+    pub fn encode<B>(tag: u32, value: &String, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        encode_key(tag, WireType::LengthDelimited, buf);
+        encode_varint(value.len() as u64, buf);
+        buf.put_slice(value.as_bytes());
+    }
+    pub fn merge<B>(
+        wire_type: WireType,
+        value: &mut String,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        // ## Unsafety
+        //
+        // `string::merge` reuses `bytes::merge`, with an additional check of utf-8
+        // well-formedness. If the utf-8 is not well-formed, or if any other error occurs, then the
+        // string is cleared, so as to avoid leaking a string field with invalid data.
+        //
+        // This implementation uses the unsafe `String::as_mut_vec` method instead of the safe
+        // alternative of temporarily swapping an empty `String` into the field, because it results
+        // in up to 10% better performance on the protobuf message decoding benchmarks.
+        //
+        // It's required when using `String::as_mut_vec` that invalid utf-8 data not be leaked into
+        // the backing `String`. To enforce this, even in the event of a panic in `bytes::merge` or
+        // in the buf implementation, a drop guard is used.
+        unsafe {
+            struct DropGuard<'a>(&'a mut Vec<u8>);
+            impl<'a> Drop for DropGuard<'a> {
+                #[inline]
+                fn drop(&mut self) {
+                    self.0.clear();
+                }
+            }
+
+            let drop_guard = DropGuard(value.as_mut_vec());
+            bytes::merge(wire_type, drop_guard.0, buf, ctx)?;
+            match str::from_utf8(drop_guard.0) {
+                Ok(_) => {
+                    // Success; do not clear the bytes.
+                    mem::forget(drop_guard);
+                    Ok(())
+                }
+                Err(_) => Err(DecodeError::new(
+                    "invalid string value: data is not UTF-8 encoded",
+                )),
+            }
+        }
+    }
+
+    length_delimited!(String);
+
+    #[cfg(test)]
+    mod test {
+        use proptest::prelude::*;
+
+        use super::super::test::{check_collection_type, check_type};
+        use super::*;
+
+        proptest! {
+            #[test]
+            fn check(value: String, tag in MIN_TAG..=MAX_TAG) {
+                super::test::check_type(value, tag, WireType::LengthDelimited,
+                                        encode, merge, encoded_len)?;
+            }
+            #[test]
+            fn check_repeated(value: Vec<String>, tag in MIN_TAG..=MAX_TAG) {
+                super::test::check_collection_type(value, tag, WireType::LengthDelimited,
+                                                   encode_repeated, merge_repeated,
+                                                   encoded_len_repeated)?;
+            }
+        }
+    }
+}
+
+pub trait BytesAdapter: sealed::BytesAdapter {}
+
+mod sealed {
+    use super::{Buf, BufMut};
+
+    pub trait BytesAdapter: Default + Sized + 'static {
+        fn len(&self) -> usize;
+
+        /// Replace contents of this buffer with the contents of another buffer.
+        fn replace_with<B>(&mut self, buf: B)
+        where
+            B: Buf;
+
+        /// Appends this buffer to the (contents of) other buffer.
+        fn append_to<B>(&self, buf: &mut B)
+        where
+            B: BufMut;
+
+        fn is_empty(&self) -> bool {
+            self.len() == 0
+        }
+    }
+}
+
+impl BytesAdapter for Bytes {}
+
+impl sealed::BytesAdapter for Bytes {
+    fn len(&self) -> usize {
+        Buf::remaining(self)
+    }
+
+    fn replace_with<B>(&mut self, mut buf: B)
+    where
+        B: Buf,
+    {
+        *self = buf.copy_to_bytes(buf.remaining());
+    }
+
+    fn append_to<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        buf.put(self.clone())
+    }
+}
+
+impl BytesAdapter for Vec<u8> {}
+
+impl sealed::BytesAdapter for Vec<u8> {
+    fn len(&self) -> usize {
+        Vec::len(self)
+    }
+
+    fn replace_with<B>(&mut self, buf: B)
+    where
+        B: Buf,
+    {
+        self.clear();
+        self.reserve(buf.remaining());
+        self.put(buf);
+    }
+
+    fn append_to<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        buf.put(self.as_slice())
+    }
+}
+
+pub mod bytes {
+    use super::*;
+
+    pub fn encode<A, B>(tag: u32, value: &A, buf: &mut B)
+    where
+        A: BytesAdapter,
+        B: BufMut,
+    {
+        encode_key(tag, WireType::LengthDelimited, buf);
+        encode_varint(value.len() as u64, buf);
+        value.append_to(buf);
+    }
+
+    pub fn merge<A, B>(
+        wire_type: WireType,
+        value: &mut A,
+        buf: &mut B,
+        _ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        A: BytesAdapter,
+        B: Buf,
+    {
+        check_wire_type(WireType::LengthDelimited, wire_type)?;
+        let len = decode_varint(buf)?;
+        if len > buf.remaining() as u64 {
+            return Err(DecodeError::new("buffer underflow"));
+        }
+        let len = len as usize;
+
+        // Clear the existing value. This follows from the following rule in the encoding guide[1]:
+        //
+        // > Normally, an encoded message would never have more than one instance of a non-repeated
+        // > field. However, parsers are expected to handle the case in which they do. For numeric
+        // > types and strings, if the same field appears multiple times, the parser accepts the
+        // > last value it sees.
+        //
+        // [1]: https://developers.google.com/protocol-buffers/docs/encoding#optional
+        value.replace_with(buf.copy_to_bytes(len));
+        Ok(())
+    }
+
+    length_delimited!(impl BytesAdapter);
+
+    #[cfg(test)]
+    mod test {
+        use proptest::prelude::*;
+
+        use super::super::test::{check_collection_type, check_type};
+        use super::*;
+
+        proptest! {
+            #[test]
+            fn check_vec(value: Vec<u8>, tag in MIN_TAG..=MAX_TAG) {
+                super::test::check_type::<Vec<u8>, Vec<u8>>(value, tag, WireType::LengthDelimited,
+                                                            encode, merge, encoded_len)?;
+            }
+
+            #[test]
+            fn check_bytes(value: Vec<u8>, tag in MIN_TAG..=MAX_TAG) {
+                let value = Bytes::from(value);
+                super::test::check_type::<Bytes, Bytes>(value, tag, WireType::LengthDelimited,
+                                                        encode, merge, encoded_len)?;
+            }
+
+            #[test]
+            fn check_repeated_vec(value: Vec<Vec<u8>>, tag in MIN_TAG..=MAX_TAG) {
+                super::test::check_collection_type(value, tag, WireType::LengthDelimited,
+                                                   encode_repeated, merge_repeated,
+                                                   encoded_len_repeated)?;
+            }
+
+            #[test]
+            fn check_repeated_bytes(value: Vec<Vec<u8>>, tag in MIN_TAG..=MAX_TAG) {
+                let value = value.into_iter().map(Bytes::from).collect();
+                super::test::check_collection_type(value, tag, WireType::LengthDelimited,
+                                                   encode_repeated, merge_repeated,
+                                                   encoded_len_repeated)?;
+            }
+        }
+    }
+}
+
+pub mod message {
+    use super::*;
+
+    pub fn encode<M, B>(tag: u32, msg: &M, buf: &mut B)
+    where
+        M: Message,
+        B: BufMut,
+    {
+        encode_key(tag, WireType::LengthDelimited, buf);
+        encode_varint(msg.encoded_len() as u64, buf);
+        msg.encode_raw(buf);
+    }
+
+    pub fn merge<M, B>(
+        wire_type: WireType,
+        msg: &mut M,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        M: Message,
+        B: Buf,
+    {
+        check_wire_type(WireType::LengthDelimited, wire_type)?;
+        ctx.limit_reached()?;
+        merge_loop(
+            msg,
+            buf,
+            ctx.enter_recursion(),
+            |msg: &mut M, buf: &mut B, ctx| {
+                let (tag, wire_type) = decode_key(buf)?;
+                msg.merge_field(tag, wire_type, buf, ctx)
+            },
+        )
+    }
+
+    pub fn encode_repeated<M, B>(tag: u32, messages: &[M], buf: &mut B)
+    where
+        M: Message,
+        B: BufMut,
+    {
+        for msg in messages {
+            encode(tag, msg, buf);
+        }
+    }
+
+    pub fn merge_repeated<M, B>(
+        wire_type: WireType,
+        messages: &mut Vec<M>,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        M: Message + Default,
+        B: Buf,
+    {
+        check_wire_type(WireType::LengthDelimited, wire_type)?;
+        let mut msg = M::default();
+        merge(WireType::LengthDelimited, &mut msg, buf, ctx)?;
+        messages.push(msg);
+        Ok(())
+    }
+
+    #[inline]
+    pub fn encoded_len<M>(tag: u32, msg: &M) -> usize
+    where
+        M: Message,
+    {
+        let len = msg.encoded_len();
+        key_len(tag) + encoded_len_varint(len as u64) + len
+    }
+
+    #[inline]
+    pub fn encoded_len_repeated<M>(tag: u32, messages: &[M]) -> usize
+    where
+        M: Message,
+    {
+        key_len(tag) * messages.len()
+            + messages
+                .iter()
+                .map(Message::encoded_len)
+                .map(|len| len + encoded_len_varint(len as u64))
+                .sum::<usize>()
+    }
+}
+
+pub mod group {
+    use super::*;
+
+    pub fn encode<M, B>(tag: u32, msg: &M, buf: &mut B)
+    where
+        M: Message,
+        B: BufMut,
+    {
+        encode_key(tag, WireType::StartGroup, buf);
+        msg.encode_raw(buf);
+        encode_key(tag, WireType::EndGroup, buf);
+    }
+
+    pub fn merge<M, B>(
+        tag: u32,
+        wire_type: WireType,
+        msg: &mut M,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        M: Message,
+        B: Buf,
+    {
+        check_wire_type(WireType::StartGroup, wire_type)?;
+
+        ctx.limit_reached()?;
+        loop {
+            let (field_tag, field_wire_type) = decode_key(buf)?;
+            if field_wire_type == WireType::EndGroup {
+                if field_tag != tag {
+                    return Err(DecodeError::new("unexpected end group tag"));
+                }
+                return Ok(());
+            }
+
+            M::merge_field(msg, field_tag, field_wire_type, buf, ctx.enter_recursion())?;
+        }
+    }
+
+    pub fn encode_repeated<M, B>(tag: u32, messages: &[M], buf: &mut B)
+    where
+        M: Message,
+        B: BufMut,
+    {
+        for msg in messages {
+            encode(tag, msg, buf);
+        }
+    }
+
+    pub fn merge_repeated<M, B>(
+        tag: u32,
+        wire_type: WireType,
+        messages: &mut Vec<M>,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        M: Message + Default,
+        B: Buf,
+    {
+        check_wire_type(WireType::StartGroup, wire_type)?;
+        let mut msg = M::default();
+        merge(tag, WireType::StartGroup, &mut msg, buf, ctx)?;
+        messages.push(msg);
+        Ok(())
+    }
+
+    #[inline]
+    pub fn encoded_len<M>(tag: u32, msg: &M) -> usize
+    where
+        M: Message,
+    {
+        2 * key_len(tag) + msg.encoded_len()
+    }
+
+    #[inline]
+    pub fn encoded_len_repeated<M>(tag: u32, messages: &[M]) -> usize
+    where
+        M: Message,
+    {
+        2 * key_len(tag) * messages.len() + messages.iter().map(Message::encoded_len).sum::<usize>()
+    }
+}
+
+/// Rust doesn't have a `Map` trait, so macros are currently the best way to be
+/// generic over `HashMap` and `BTreeMap`.
+macro_rules! map {
+    ($map_ty:ident) => {
+        use crate::encoding::*;
+        use core::hash::Hash;
+
+        /// Generic protobuf map encode function.
+        pub fn encode<K, V, B, KE, KL, VE, VL>(
+            key_encode: KE,
+            key_encoded_len: KL,
+            val_encode: VE,
+            val_encoded_len: VL,
+            tag: u32,
+            values: &$map_ty<K, V>,
+            buf: &mut B,
+        ) where
+            K: Default + Eq + Hash + Ord,
+            V: Default + PartialEq,
+            B: BufMut,
+            KE: Fn(u32, &K, &mut B),
+            KL: Fn(u32, &K) -> usize,
+            VE: Fn(u32, &V, &mut B),
+            VL: Fn(u32, &V) -> usize,
+        {
+            encode_with_default(
+                key_encode,
+                key_encoded_len,
+                val_encode,
+                val_encoded_len,
+                &V::default(),
+                tag,
+                values,
+                buf,
+            )
+        }
+
+        /// Generic protobuf map merge function.
+        pub fn merge<K, V, B, KM, VM>(
+            key_merge: KM,
+            val_merge: VM,
+            values: &mut $map_ty<K, V>,
+            buf: &mut B,
+            ctx: DecodeContext,
+        ) -> Result<(), DecodeError>
+        where
+            K: Default + Eq + Hash + Ord,
+            V: Default,
+            B: Buf,
+            KM: Fn(WireType, &mut K, &mut B, DecodeContext) -> Result<(), DecodeError>,
+            VM: Fn(WireType, &mut V, &mut B, DecodeContext) -> Result<(), DecodeError>,
+        {
+            merge_with_default(key_merge, val_merge, V::default(), values, buf, ctx)
+        }
+
+        /// Generic protobuf map encode function.
+        pub fn encoded_len<K, V, KL, VL>(
+            key_encoded_len: KL,
+            val_encoded_len: VL,
+            tag: u32,
+            values: &$map_ty<K, V>,
+        ) -> usize
+        where
+            K: Default + Eq + Hash + Ord,
+            V: Default + PartialEq,
+            KL: Fn(u32, &K) -> usize,
+            VL: Fn(u32, &V) -> usize,
+        {
+            encoded_len_with_default(key_encoded_len, val_encoded_len, &V::default(), tag, values)
+        }
+
+        /// Generic protobuf map encode function with an overriden value default.
+        ///
+        /// This is necessary because enumeration values can have a default value other
+        /// than 0 in proto2.
+        pub fn encode_with_default<K, V, B, KE, KL, VE, VL>(
+            key_encode: KE,
+            key_encoded_len: KL,
+            val_encode: VE,
+            val_encoded_len: VL,
+            val_default: &V,
+            tag: u32,
+            values: &$map_ty<K, V>,
+            buf: &mut B,
+        ) where
+            K: Default + Eq + Hash + Ord,
+            V: PartialEq,
+            B: BufMut,
+            KE: Fn(u32, &K, &mut B),
+            KL: Fn(u32, &K) -> usize,
+            VE: Fn(u32, &V, &mut B),
+            VL: Fn(u32, &V) -> usize,
+        {
+            for (key, val) in values.iter() {
+                let skip_key = key == &K::default();
+                let skip_val = val == val_default;
+
+                let len = (if skip_key { 0 } else { key_encoded_len(1, key) })
+                    + (if skip_val { 0 } else { val_encoded_len(2, val) });
+
+                encode_key(tag, WireType::LengthDelimited, buf);
+                encode_varint(len as u64, buf);
+                if !skip_key {
+                    key_encode(1, key, buf);
+                }
+                if !skip_val {
+                    val_encode(2, val, buf);
+                }
+            }
+        }
+
+        /// Generic protobuf map merge function with an overriden value default.
+        ///
+        /// This is necessary because enumeration values can have a default value other
+        /// than 0 in proto2.
+        pub fn merge_with_default<K, V, B, KM, VM>(
+            key_merge: KM,
+            val_merge: VM,
+            val_default: V,
+            values: &mut $map_ty<K, V>,
+            buf: &mut B,
+            ctx: DecodeContext,
+        ) -> Result<(), DecodeError>
+        where
+            K: Default + Eq + Hash + Ord,
+            B: Buf,
+            KM: Fn(WireType, &mut K, &mut B, DecodeContext) -> Result<(), DecodeError>,
+            VM: Fn(WireType, &mut V, &mut B, DecodeContext) -> Result<(), DecodeError>,
+        {
+            let mut key = Default::default();
+            let mut val = val_default;
+            ctx.limit_reached()?;
+            merge_loop(
+                &mut (&mut key, &mut val),
+                buf,
+                ctx.enter_recursion(),
+                |&mut (ref mut key, ref mut val), buf, ctx| {
+                    let (tag, wire_type) = decode_key(buf)?;
+                    match tag {
+                        1 => key_merge(wire_type, key, buf, ctx),
+                        2 => val_merge(wire_type, val, buf, ctx),
+                        _ => skip_field(wire_type, tag, buf, ctx),
+                    }
+                },
+            )?;
+            values.insert(key, val);
+
+            Ok(())
+        }
+
+        /// Generic protobuf map encode function with an overriden value default.
+        ///
+        /// This is necessary because enumeration values can have a default value other
+        /// than 0 in proto2.
+        pub fn encoded_len_with_default<K, V, KL, VL>(
+            key_encoded_len: KL,
+            val_encoded_len: VL,
+            val_default: &V,
+            tag: u32,
+            values: &$map_ty<K, V>,
+        ) -> usize
+        where
+            K: Default + Eq + Hash + Ord,
+            V: PartialEq,
+            KL: Fn(u32, &K) -> usize,
+            VL: Fn(u32, &V) -> usize,
+        {
+            key_len(tag) * values.len()
+                + values
+                    .iter()
+                    .map(|(key, val)| {
+                        let len = (if key == &K::default() {
+                            0
+                        } else {
+                            key_encoded_len(1, key)
+                        }) + (if val == val_default {
+                            0
+                        } else {
+                            val_encoded_len(2, val)
+                        });
+                        encoded_len_varint(len as u64) + len
+                    })
+                    .sum::<usize>()
+        }
+    };
+}
+
+#[cfg(feature = "std")]
+pub mod hash_map {
+    use std::collections::HashMap;
+    map!(HashMap);
+}
+
+pub mod btree_map {
+    map!(BTreeMap);
+}
+
+#[cfg(test)]
+mod test {
+    use alloc::string::ToString;
+    use core::borrow::Borrow;
+    use core::fmt::Debug;
+    use core::u64;
+
+    use ::bytes::{Bytes, BytesMut};
+    use proptest::{prelude::*, test_runner::TestCaseResult};
+
+    use crate::encoding::*;
+
+    pub fn check_type<T, B>(
+        value: T,
+        tag: u32,
+        wire_type: WireType,
+        encode: fn(u32, &B, &mut BytesMut),
+        merge: fn(WireType, &mut T, &mut Bytes, DecodeContext) -> Result<(), DecodeError>,
+        encoded_len: fn(u32, &B) -> usize,
+    ) -> TestCaseResult
+    where
+        T: Debug + Default + PartialEq + Borrow<B>,
+        B: ?Sized,
+    {
+        prop_assume!(MIN_TAG <= tag && tag <= MAX_TAG);
+
+        let expected_len = encoded_len(tag, value.borrow());
+
+        let mut buf = BytesMut::with_capacity(expected_len);
+        encode(tag, value.borrow(), &mut buf);
+
+        let mut buf = buf.freeze();
+
+        prop_assert_eq!(
+            buf.remaining(),
+            expected_len,
+            "encoded_len wrong; expected: {}, actual: {}",
+            expected_len,
+            buf.remaining()
+        );
+
+        if !buf.has_remaining() {
+            // Short circuit for empty packed values.
+            return Ok(());
+        }
+
+        let (decoded_tag, decoded_wire_type) =
+            decode_key(&mut buf).map_err(|error| TestCaseError::fail(error.to_string()))?;
+        prop_assert_eq!(
+            tag,
+            decoded_tag,
+            "decoded tag does not match; expected: {}, actual: {}",
+            tag,
+            decoded_tag
+        );
+
+        prop_assert_eq!(
+            wire_type,
+            decoded_wire_type,
+            "decoded wire type does not match; expected: {:?}, actual: {:?}",
+            wire_type,
+            decoded_wire_type,
+        );
+
+        match wire_type {
+            WireType::SixtyFourBit if buf.remaining() != 8 => Err(TestCaseError::fail(format!(
+                "64bit wire type illegal remaining: {}, tag: {}",
+                buf.remaining(),
+                tag
+            ))),
+            WireType::ThirtyTwoBit if buf.remaining() != 4 => Err(TestCaseError::fail(format!(
+                "32bit wire type illegal remaining: {}, tag: {}",
+                buf.remaining(),
+                tag
+            ))),
+            _ => Ok(()),
+        }?;
+
+        let mut roundtrip_value = T::default();
+        merge(
+            wire_type,
+            &mut roundtrip_value,
+            &mut buf,
+            DecodeContext::default(),
+        )
+        .map_err(|error| TestCaseError::fail(error.to_string()))?;
+
+        prop_assert!(
+            !buf.has_remaining(),
+            "expected buffer to be empty, remaining: {}",
+            buf.remaining()
+        );
+
+        prop_assert_eq!(value, roundtrip_value);
+
+        Ok(())
+    }
+
+    pub fn check_collection_type<T, B, E, M, L>(
+        value: T,
+        tag: u32,
+        wire_type: WireType,
+        encode: E,
+        mut merge: M,
+        encoded_len: L,
+    ) -> TestCaseResult
+    where
+        T: Debug + Default + PartialEq + Borrow<B>,
+        B: ?Sized,
+        E: FnOnce(u32, &B, &mut BytesMut),
+        M: FnMut(WireType, &mut T, &mut Bytes, DecodeContext) -> Result<(), DecodeError>,
+        L: FnOnce(u32, &B) -> usize,
+    {
+        prop_assume!(MIN_TAG <= tag && tag <= MAX_TAG);
+
+        let expected_len = encoded_len(tag, value.borrow());
+
+        let mut buf = BytesMut::with_capacity(expected_len);
+        encode(tag, value.borrow(), &mut buf);
+
+        let mut buf = buf.freeze();
+
+        prop_assert_eq!(
+            buf.remaining(),
+            expected_len,
+            "encoded_len wrong; expected: {}, actual: {}",
+            expected_len,
+            buf.remaining()
+        );
+
+        let mut roundtrip_value = Default::default();
+        while buf.has_remaining() {
+            let (decoded_tag, decoded_wire_type) =
+                decode_key(&mut buf).map_err(|error| TestCaseError::fail(error.to_string()))?;
+
+            prop_assert_eq!(
+                tag,
+                decoded_tag,
+                "decoded tag does not match; expected: {}, actual: {}",
+                tag,
+                decoded_tag
+            );
+
+            prop_assert_eq!(
+                wire_type,
+                decoded_wire_type,
+                "decoded wire type does not match; expected: {:?}, actual: {:?}",
+                wire_type,
+                decoded_wire_type
+            );
+
+            merge(
+                wire_type,
+                &mut roundtrip_value,
+                &mut buf,
+                DecodeContext::default(),
+            )
+            .map_err(|error| TestCaseError::fail(error.to_string()))?;
+        }
+
+        prop_assert_eq!(value, roundtrip_value);
+
+        Ok(())
+    }
+
+    #[test]
+    fn string_merge_invalid_utf8() {
+        let mut s = String::new();
+        let buf = b"\x02\x80\x80";
+
+        let r = string::merge(
+            WireType::LengthDelimited,
+            &mut s,
+            &mut &buf[..],
+            DecodeContext::default(),
+        );
+        r.expect_err("must be an error");
+        assert!(s.is_empty());
+    }
+
+    #[test]
+    fn varint() {
+        fn check(value: u64, mut encoded: &[u8]) {
+            // TODO(rust-lang/rust-clippy#5494)
+            #![allow(clippy::clone_double_ref)]
+
+            // Small buffer.
+            let mut buf = Vec::with_capacity(1);
+            encode_varint(value, &mut buf);
+            assert_eq!(buf, encoded);
+
+            // Large buffer.
+            let mut buf = Vec::with_capacity(100);
+            encode_varint(value, &mut buf);
+            assert_eq!(buf, encoded);
+
+            assert_eq!(encoded_len_varint(value), encoded.len());
+
+            let roundtrip_value = decode_varint(&mut encoded.clone()).expect("decoding failed");
+            assert_eq!(value, roundtrip_value);
+
+            let roundtrip_value = decode_varint_slow(&mut encoded).expect("slow decoding failed");
+            assert_eq!(value, roundtrip_value);
+        }
+
+        check(2u64.pow(0) - 1, &[0x00]);
+        check(2u64.pow(0), &[0x01]);
+
+        check(2u64.pow(7) - 1, &[0x7F]);
+        check(2u64.pow(7), &[0x80, 0x01]);
+        check(300, &[0xAC, 0x02]);
+
+        check(2u64.pow(14) - 1, &[0xFF, 0x7F]);
+        check(2u64.pow(14), &[0x80, 0x80, 0x01]);
+
+        check(2u64.pow(21) - 1, &[0xFF, 0xFF, 0x7F]);
+        check(2u64.pow(21), &[0x80, 0x80, 0x80, 0x01]);
+
+        check(2u64.pow(28) - 1, &[0xFF, 0xFF, 0xFF, 0x7F]);
+        check(2u64.pow(28), &[0x80, 0x80, 0x80, 0x80, 0x01]);
+
+        check(2u64.pow(35) - 1, &[0xFF, 0xFF, 0xFF, 0xFF, 0x7F]);
+        check(2u64.pow(35), &[0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);
+
+        check(2u64.pow(42) - 1, &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F]);
+        check(2u64.pow(42), &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01]);
+
+        check(
+            2u64.pow(49) - 1,
+            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
+        );
+        check(
+            2u64.pow(49),
+            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
+        );
+
+        check(
+            2u64.pow(56) - 1,
+            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
+        );
+        check(
+            2u64.pow(56),
+            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
+        );
+
+        check(
+            2u64.pow(63) - 1,
+            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x7F],
+        );
+        check(
+            2u64.pow(63),
+            &[0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x01],
+        );
+
+        check(
+            u64::MAX,
+            &[0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x01],
+        );
+    }
+
+    /// This big bowl o' macro soup generates an encoding property test for each combination of map
+    /// type, scalar map key, and value type.
+    /// TODO: these tests take a long time to compile, can this be improved?
+    #[cfg(feature = "std")]
+    macro_rules! map_tests {
+        (keys: $keys:tt,
+         vals: $vals:tt) => {
+            mod hash_map {
+                map_tests!(@private HashMap, hash_map, $keys, $vals);
+            }
+            mod btree_map {
+                map_tests!(@private BTreeMap, btree_map, $keys, $vals);
+            }
+        };
+
+        (@private $map_type:ident,
+                  $mod_name:ident,
+                  [$(($key_ty:ty, $key_proto:ident)),*],
+                  $vals:tt) => {
+            $(
+                mod $key_proto {
+                    use std::collections::$map_type;
+
+                    use proptest::prelude::*;
+
+                    use crate::encoding::*;
+                    use crate::encoding::test::check_collection_type;
+
+                    map_tests!(@private $map_type, $mod_name, ($key_ty, $key_proto), $vals);
+                }
+            )*
+        };
+
+        (@private $map_type:ident,
+                  $mod_name:ident,
+                  ($key_ty:ty, $key_proto:ident),
+                  [$(($val_ty:ty, $val_proto:ident)),*]) => {
+            $(
+                proptest! {
+                    #[test]
+                    fn $val_proto(values: $map_type<$key_ty, $val_ty>, tag in MIN_TAG..=MAX_TAG) {
+                        check_collection_type(values, tag, WireType::LengthDelimited,
+                                              |tag, values, buf| {
+                                                  $mod_name::encode($key_proto::encode,
+                                                                    $key_proto::encoded_len,
+                                                                    $val_proto::encode,
+                                                                    $val_proto::encoded_len,
+                                                                    tag,
+                                                                    values,
+                                                                    buf)
+                                              },
+                                              |wire_type, values, buf, ctx| {
+                                                  check_wire_type(WireType::LengthDelimited, wire_type)?;
+                                                  $mod_name::merge($key_proto::merge,
+                                                                   $val_proto::merge,
+                                                                   values,
+                                                                   buf,
+                                                                   ctx)
+                                              },
+                                              |tag, values| {
+                                                  $mod_name::encoded_len($key_proto::encoded_len,
+                                                                         $val_proto::encoded_len,
+                                                                         tag,
+                                                                         values)
+                                              })?;
+                    }
+                }
+             )*
+        };
+    }
+
+    #[cfg(feature = "std")]
+    map_tests!(keys: [
+        (i32, int32),
+        (i64, int64),
+        (u32, uint32),
+        (u64, uint64),
+        (i32, sint32),
+        (i64, sint64),
+        (u32, fixed32),
+        (u64, fixed64),
+        (i32, sfixed32),
+        (i64, sfixed64),
+        (bool, bool),
+        (String, string)
+    ],
+    vals: [
+        (f32, float),
+        (f64, double),
+        (i32, int32),
+        (i64, int64),
+        (u32, uint32),
+        (u64, uint64),
+        (i32, sint32),
+        (i64, sint64),
+        (u32, fixed32),
+        (u64, fixed64),
+        (i32, sfixed32),
+        (i64, sfixed64),
+        (bool, bool),
+        (String, string),
+        (Vec<u8>, bytes)
+    ]);
+}
diff --git a/third_party/rust/prost/src/error.rs b/third_party/rust/prost/src/error.rs
new file mode 100644
index 0000000000..fc098299c8
--- /dev/null
+++ b/third_party/rust/prost/src/error.rs
@@ -0,0 +1,131 @@
+//! Protobuf encoding and decoding errors.
+
+use alloc::borrow::Cow;
+use alloc::boxed::Box;
+use alloc::vec::Vec;
+
+use core::fmt;
+
+/// A Protobuf message decoding error.
+///
+/// `DecodeError` indicates that the input buffer does not caontain a valid
+/// Protobuf message. The error details should be considered 'best effort': in
+/// general it is not possible to exactly pinpoint why data is malformed.
+#[derive(Clone, PartialEq, Eq)]
+pub struct DecodeError {
+    inner: Box<Inner>,
+}
+
+#[derive(Clone, PartialEq, Eq)]
+struct Inner {
+    /// A 'best effort' root cause description.
+    description: Cow<'static, str>,
+    /// A stack of (message, field) name pairs, which identify the specific
+    /// message type and field where decoding failed. The stack contains an
+    /// entry per level of nesting.
+    stack: Vec<(&'static str, &'static str)>,
+}
+
+impl DecodeError {
+    /// Creates a new `DecodeError` with a 'best effort' root cause description.
+    ///
+    /// Meant to be used only by `Message` implementations.
+    #[doc(hidden)]
+    #[cold]
+    pub fn new(description: impl Into<Cow<'static, str>>) -> DecodeError {
+        DecodeError {
+            inner: Box::new(Inner {
+                description: description.into(),
+                stack: Vec::new(),
+            }),
+        }
+    }
+
+    /// Pushes a (message, field) name location pair on to the location stack.
+    ///
+    /// Meant to be used only by `Message` implementations.
+    #[doc(hidden)]
+    pub fn push(&mut self, message: &'static str, field: &'static str) {
+        self.inner.stack.push((message, field));
+    }
+}
+
+impl fmt::Debug for DecodeError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("DecodeError")
+            .field("description", &self.inner.description)
+            .field("stack", &self.inner.stack)
+            .finish()
+    }
+}
+
+impl fmt::Display for DecodeError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("failed to decode Protobuf message: ")?;
+        for &(message, field) in &self.inner.stack {
+            write!(f, "{}.{}: ", message, field)?;
+        }
+        f.write_str(&self.inner.description)
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for DecodeError {}
+
+#[cfg(feature = "std")]
+impl From<DecodeError> for std::io::Error {
+    fn from(error: DecodeError) -> std::io::Error {
+        std::io::Error::new(std::io::ErrorKind::InvalidData, error)
+    }
+}
+
+/// A Protobuf message encoding error.
+///
+/// `EncodeError` always indicates that a message failed to encode because the
+/// provided buffer had insufficient capacity. Message encoding is otherwise
+/// infallible.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct EncodeError {
+    required: usize,
+    remaining: usize,
+}
+
+impl EncodeError {
+    /// Creates a new `EncodeError`.
+    pub(crate) fn new(required: usize, remaining: usize) -> EncodeError {
+        EncodeError {
+            required,
+            remaining,
+        }
+    }
+
+    /// Returns the required buffer capacity to encode the message.
+    pub fn required_capacity(&self) -> usize {
+        self.required
+    }
+
+    /// Returns the remaining length in the provided buffer at the time of encoding.
+    pub fn remaining(&self) -> usize {
+        self.remaining
+    }
+}
+
+impl fmt::Display for EncodeError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "failed to encode Protobuf messsage; insufficient buffer capacity (required: {}, remaining: {})",
+            self.required, self.remaining
+        )
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for EncodeError {}
+
+#[cfg(feature = "std")]
+impl From<EncodeError> for std::io::Error {
+    fn from(error: EncodeError) -> std::io::Error {
+        std::io::Error::new(std::io::ErrorKind::InvalidInput, error)
+    }
+}
diff --git a/third_party/rust/prost/src/lib.rs b/third_party/rust/prost/src/lib.rs
new file mode 100644
index 0000000000..9d4069e76e
--- /dev/null
+++ b/third_party/rust/prost/src/lib.rs
@@ -0,0 +1,93 @@
+#![doc(html_root_url = "https://docs.rs/prost/0.8.0")]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+// Re-export the alloc crate for use within derived code.
+#[doc(hidden)]
+pub extern crate alloc;
+
+// Re-export the bytes crate for use within derived code.
+#[doc(hidden)]
+pub use bytes;
+
+mod error;
+mod message;
+mod types;
+
+#[doc(hidden)]
+pub mod encoding;
+
+pub use crate::error::{DecodeError, EncodeError};
+pub use crate::message::Message;
+
+use bytes::{Buf, BufMut};
+
+use crate::encoding::{decode_varint, encode_varint, encoded_len_varint};
+
+// See `encoding::DecodeContext` for more info.
+// 100 is the default recursion limit in the C++ implementation.
+#[cfg(not(feature = "no-recursion-limit"))]
+const RECURSION_LIMIT: u32 = 100;
+
+/// Encodes a length delimiter to the buffer.
+///
+/// See [Message.encode_length_delimited] for more info.
+///
+/// An error will be returned if the buffer does not have sufficient capacity to encode the
+/// delimiter.
+pub fn encode_length_delimiter<B>(length: usize, buf: &mut B) -> Result<(), EncodeError>
+where
+    B: BufMut,
+{
+    let length = length as u64;
+    let required = encoded_len_varint(length);
+    let remaining = buf.remaining_mut();
+    if required > remaining {
+        return Err(EncodeError::new(required, remaining));
+    }
+    encode_varint(length, buf);
+    Ok(())
+}
+
+/// Returns the encoded length of a length delimiter.
+///
+/// Applications may use this method to ensure sufficient buffer capacity before calling
+/// `encode_length_delimiter`. The returned size will be between 1 and 10, inclusive.
+pub fn length_delimiter_len(length: usize) -> usize {
+    encoded_len_varint(length as u64)
+}
+
+/// Decodes a length delimiter from the buffer.
+///
+/// This method allows the length delimiter to be decoded independently of the message, when the
+/// message is encoded with [Message.encode_length_delimited].
+///
+/// An error may be returned in two cases:
+///
+///  * If the supplied buffer contains fewer than 10 bytes, then an error indicates that more
+///    input is required to decode the full delimiter.
+///  * If the supplied buffer contains more than 10 bytes, then the buffer contains an invalid
+///    delimiter, and typically the buffer should be considered corrupt.
+pub fn decode_length_delimiter<B>(mut buf: B) -> Result<usize, DecodeError>
+where
+    B: Buf,
+{
+    let length = decode_varint(&mut buf)?;
+    if length > usize::max_value() as u64 {
+        return Err(DecodeError::new(
+            "length delimiter exceeds maximum usize value",
+        ));
+    }
+    Ok(length as usize)
+}
+
+// Re-export #[derive(Message, Enumeration, Oneof)].
+// Based on serde's equivalent re-export [1], but enabled by default.
+//
+// [1]: https://github.com/serde-rs/serde/blob/v1.0.89/serde/src/lib.rs#L245-L256
+#[cfg(feature = "prost-derive")]
+#[allow(unused_imports)]
+#[macro_use]
+extern crate prost_derive;
+#[cfg(feature = "prost-derive")]
+#[doc(hidden)]
+pub use prost_derive::*;
diff --git a/third_party/rust/prost/src/message.rs b/third_party/rust/prost/src/message.rs
new file mode 100644
index 0000000000..112c7b89f1
--- /dev/null
+++ b/third_party/rust/prost/src/message.rs
@@ -0,0 +1,200 @@
+use alloc::boxed::Box;
+use core::fmt::Debug;
+use core::usize;
+
+use bytes::{Buf, BufMut};
+
+use crate::encoding::{
+    decode_key, encode_varint, encoded_len_varint, message, DecodeContext, WireType,
+};
+use crate::DecodeError;
+use crate::EncodeError;
+
+/// A Protocol Buffers message.
+pub trait Message: Debug + Send + Sync {
+    /// Encodes the message to a buffer.
+    ///
+    /// This method will panic if the buffer has insufficient capacity.
+    ///
+    /// Meant to be used only by `Message` implementations.
+    #[doc(hidden)]
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+        Self: Sized;
+
+    /// Decodes a field from a buffer, and merges it into `self`.
+    ///
+    /// Meant to be used only by `Message` implementations.
+    #[doc(hidden)]
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+        Self: Sized;
+
+    /// Returns the encoded length of the message without a length delimiter.
+    fn encoded_len(&self) -> usize;
+
+    /// Encodes the message to a buffer.
+    ///
+    /// An error will be returned if the buffer does not have sufficient capacity.
+    fn encode<B>(&self, buf: &mut B) -> Result<(), EncodeError>
+    where
+        B: BufMut,
+        Self: Sized,
+    {
+        let required = self.encoded_len();
+        let remaining = buf.remaining_mut();
+        if required > buf.remaining_mut() {
+            return Err(EncodeError::new(required, remaining));
+        }
+
+        self.encode_raw(buf);
+        Ok(())
+    }
+
+    #[cfg(feature = "std")]
+    /// Encodes the message to a newly allocated buffer.
+    fn encode_to_vec(&self) -> Vec<u8>
+    where
+        Self: Sized,
+    {
+        let mut buf = Vec::with_capacity(self.encoded_len());
+
+        self.encode_raw(&mut buf);
+        buf
+    }
+
+    /// Encodes the message with a length-delimiter to a buffer.
+    ///
+    /// An error will be returned if the buffer does not have sufficient capacity.
+    fn encode_length_delimited<B>(&self, buf: &mut B) -> Result<(), EncodeError>
+    where
+        B: BufMut,
+        Self: Sized,
+    {
+        let len = self.encoded_len();
+        let required = len + encoded_len_varint(len as u64);
+        let remaining = buf.remaining_mut();
+        if required > remaining {
+            return Err(EncodeError::new(required, remaining));
+        }
+        encode_varint(len as u64, buf);
+        self.encode_raw(buf);
+        Ok(())
+    }
+
+    #[cfg(feature = "std")]
+    /// Encodes the message with a length-delimiter to a newly allocated buffer.
+    fn encode_length_delimited_to_vec(&self) -> Vec<u8>
+    where
+        Self: Sized,
+    {
+        let len = self.encoded_len();
+        let mut buf = Vec::with_capacity(len + encoded_len_varint(len as u64));
+
+        encode_varint(len as u64, &mut buf);
+        self.encode_raw(&mut buf);
+        buf
+    }
+
+    /// Decodes an instance of the message from a buffer.
+    ///
+    /// The entire buffer will be consumed.
+    fn decode<B>(mut buf: B) -> Result<Self, DecodeError>
+    where
+        B: Buf,
+        Self: Default,
+    {
+        let mut message = Self::default();
+        Self::merge(&mut message, &mut buf).map(|_| message)
+    }
+
+    /// Decodes a length-delimited instance of the message from the buffer.
+    fn decode_length_delimited<B>(buf: B) -> Result<Self, DecodeError>
+    where
+        B: Buf,
+        Self: Default,
+    {
+        let mut message = Self::default();
+        message.merge_length_delimited(buf)?;
+        Ok(message)
+    }
+
+    /// Decodes an instance of the message from a buffer, and merges it into `self`.
+    ///
+    /// The entire buffer will be consumed.
+    fn merge<B>(&mut self, mut buf: B) -> Result<(), DecodeError>
+    where
+        B: Buf,
+        Self: Sized,
+    {
+        let ctx = DecodeContext::default();
+        while buf.has_remaining() {
+            let (tag, wire_type) = decode_key(&mut buf)?;
+            self.merge_field(tag, wire_type, &mut buf, ctx.clone())?;
+        }
+        Ok(())
+    }
+
+    /// Decodes a length-delimited instance of the message from buffer, and
+    /// merges it into `self`.
+    fn merge_length_delimited<B>(&mut self, mut buf: B) -> Result<(), DecodeError>
+    where
+        B: Buf,
+        Self: Sized,
+    {
+        message::merge(
+            WireType::LengthDelimited,
+            self,
+            &mut buf,
+            DecodeContext::default(),
+        )
+    }
+
+    /// Clears the message, resetting all fields to their default.
+    fn clear(&mut self);
+}
+
+impl<M> Message for Box<M>
+where
+    M: Message,
+{
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        (**self).encode_raw(buf)
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        (**self).merge_field(tag, wire_type, buf, ctx)
+    }
+    fn encoded_len(&self) -> usize {
+        (**self).encoded_len()
+    }
+    fn clear(&mut self) {
+        (**self).clear()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const _MESSAGE_IS_OBJECT_SAFE: Option<&dyn Message> = None;
+}
diff --git a/third_party/rust/prost/src/types.rs b/third_party/rust/prost/src/types.rs
new file mode 100644
index 0000000000..864a2adda1
--- /dev/null
+++ b/third_party/rust/prost/src/types.rs
@@ -0,0 +1,424 @@
+//! Protocol Buffers well-known wrapper types.
+//!
+//! This module provides implementations of `Message` for Rust standard library types which
+//! correspond to a Protobuf well-known wrapper type. The remaining well-known types are defined in
+//! the `prost-types` crate in order to avoid a cyclic dependency between `prost` and
+//! `prost-build`.
+
+use alloc::string::String;
+use alloc::vec::Vec;
+
+use ::bytes::{Buf, BufMut, Bytes};
+
+use crate::{
+    encoding::{
+        bool, bytes, double, float, int32, int64, skip_field, string, uint32, uint64,
+        DecodeContext, WireType,
+    },
+    DecodeError, Message,
+};
+
+/// `google.protobuf.BoolValue`
+impl Message for bool {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self {
+            bool::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            bool::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self {
+            2
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = false;
+    }
+}
+
+/// `google.protobuf.UInt32Value`
+impl Message for u32 {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self != 0 {
+            uint32::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            uint32::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self != 0 {
+            uint32::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = 0;
+    }
+}
+
+/// `google.protobuf.UInt64Value`
+impl Message for u64 {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self != 0 {
+            uint64::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            uint64::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self != 0 {
+            uint64::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = 0;
+    }
+}
+
+/// `google.protobuf.Int32Value`
+impl Message for i32 {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self != 0 {
+            int32::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            int32::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self != 0 {
+            int32::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = 0;
+    }
+}
+
+/// `google.protobuf.Int64Value`
+impl Message for i64 {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self != 0 {
+            int64::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            int64::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self != 0 {
+            int64::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = 0;
+    }
+}
+
+/// `google.protobuf.FloatValue`
+impl Message for f32 {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self != 0.0 {
+            float::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            float::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self != 0.0 {
+            float::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = 0.0;
+    }
+}
+
+/// `google.protobuf.DoubleValue`
+impl Message for f64 {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if *self != 0.0 {
+            double::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            double::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if *self != 0.0 {
+            double::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        *self = 0.0;
+    }
+}
+
+/// `google.protobuf.StringValue`
+impl Message for String {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if !self.is_empty() {
+            string::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            string::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if !self.is_empty() {
+            string::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        self.clear();
+    }
+}
+
+/// `google.protobuf.BytesValue`
+impl Message for Vec<u8> {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if !self.is_empty() {
+            bytes::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            bytes::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if !self.is_empty() {
+            bytes::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        self.clear();
+    }
+}
+
+/// `google.protobuf.BytesValue`
+impl Message for Bytes {
+    fn encode_raw<B>(&self, buf: &mut B)
+    where
+        B: BufMut,
+    {
+        if !self.is_empty() {
+            bytes::encode(1, self, buf)
+        }
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        if tag == 1 {
+            bytes::merge(wire_type, self, buf, ctx)
+        } else {
+            skip_field(wire_type, tag, buf, ctx)
+        }
+    }
+    fn encoded_len(&self) -> usize {
+        if !self.is_empty() {
+            bytes::encoded_len(1, self)
+        } else {
+            0
+        }
+    }
+    fn clear(&mut self) {
+        self.clear();
+    }
+}
+
+/// `google.protobuf.Empty`
+impl Message for () {
+    fn encode_raw<B>(&self, _buf: &mut B)
+    where
+        B: BufMut,
+    {
+    }
+    fn merge_field<B>(
+        &mut self,
+        tag: u32,
+        wire_type: WireType,
+        buf: &mut B,
+        ctx: DecodeContext,
+    ) -> Result<(), DecodeError>
+    where
+        B: Buf,
+    {
+        skip_field(wire_type, tag, buf, ctx)
+    }
+    fn encoded_len(&self) -> usize {
+        0
+    }
+    fn clear(&mut self) {}
+}