refactor: JSON message with less allocations

This was found during experimenting `-Zbuild-analysis` with ndjson.

From me tracing the code with `cargo expand`, basically there shouldn't
have any significant performance difference between `serde(flatten)`
and inlining all the fields. Here the differences between them

* flatten one calls `Serialize::serialize_map` without fields size
  hint so cannot pre-allocate Vec with `Vec::with_capacity`,
  whereas inline case calls `Serialize::serialize_struct` with a
  known length of fields.
* flatten would end up calling `Serializer::serialize_map`
  and line calls `Serializer::serialize_struct`. And in serde_json
  serializer `serialize_struct` actually call `serailze_map`.
  So no difference on serializer side.
* There might be some function calls not inlined I like
  `FlatMapSerializer` but I doubt it is costly than allocation.

Here is the `cargo-expand`'d result:

```rust
#[derive(Serialize)]
pub struct Foo<D: Serialize> {
    id: u8,
    #[serde(flatten)]
    data: D,
}

#[derive(Serialize)]
struct Bar {
    a: bool,
}

// Expand to

extern crate serde as _serde;
impl<D: Serialize> _serde::Serialize for Foo<D>
where
    D: _serde::Serialize,
{
    fn serialize<__S>(
        &self,
        __serializer: __S,
    ) -> _serde::__private228::Result<__S::Ok, __S::Error>
    where
        __S: _serde::Serializer,
    {
        let mut __serde_state = _serde::Serializer::serialize_map(
            __serializer,
            _serde::__private228::None,
        )?;
        _serde::ser::SerializeMap::serialize_entry(
            &mut __serde_state,
            "id",
            &self.id,
        )?;
        _serde::Serialize::serialize(
            &&self.data,
            _serde::__private228::ser::FlatMapSerializer(&mut __serde_state),
        )?;
        _serde::ser::SerializeMap::end(__serde_state)
    }
}

impl _serde::Serialize for Bar {
    fn serialize<__S>(
        &self,
        __serializer: __S,
    ) -> _serde::__private228::Result<__S::Ok, __S::Error>
    where
        __S: _serde::Serializer,
    {
        let mut __serde_state = _serde::Serializer::serialize_struct(
            __serializer,
            "Bar",
            false as usize + 1,
        )?;
        _serde::ser::SerializeStruct::serialize_field(
            &mut __serde_state,
            "a",
            &self.a,
        )?;
        _serde::ser::SerializeStruct::end(__serde_state)
    }
}
```

```rust
#[derive(Serialize)]
pub struct Foo<D: Serialize> {
    id: u8,
    a: bool,
}

// Expand to

impl<D: Serialize> _serde::Serialize for Foo<D> {
    fn serialize<__S>(
        &self,
        __serializer: __S,
    ) -> _serde::__private228::Result<__S::Ok, __S::Error>
    where
        __S: _serde::Serializer,
    {
        let mut __serde_state = _serde::Serializer::serialize_struct(
            __serializer,
            "Foo",
            false as usize + 1 + 1,
        )?;
        _serde::ser::SerializeStruct::serialize_field(
            &mut __serde_state,
            "id",
            &self.id,
        )?;
        _serde::ser::SerializeStruct::serialize_field(
            &mut __serde_state,
            "a",
            &self.a,
        )?;
        _serde::ser::SerializeStruct::end(__serde_state)
    }
}
```

Author: weihanglo

src/cargo/util/machine_message.rs

@@ -3,7 +3,7 @@ use std::path::{Path, PathBuf};
 use cargo_util_schemas::core::PackageIdSpec;
 use serde::Serialize;
 use serde::ser;
-use serde_json::{json, value::RawValue};
+use serde_json::value::RawValue;
 
 use crate::core::Target;
 use crate::core::compiler::{CompilationSection, CompileMode};
@@ -12,10 +12,17 @@ pub trait Message: ser::Serialize {
     fn reason(&self) -> &str;
 
     fn to_json_string(&self) -> String {
-        let json = serde_json::to_string(self).unwrap();
-        assert!(json.starts_with("{\""));
-        let reason = json!(self.reason());
-        format!("{{\"reason\":{},{}", reason, &json[1..])
+        #[derive(Serialize)]
+        struct WithReason<'a, S: Serialize> {
+            reason: &'a str,
+            #[serde(flatten)]
+            msg: &'a S,
+        }
+        let with_reason = WithReason {
+            reason: self.reason(),
+            msg: &self,
+        };
+        serde_json::to_string(&with_reason).unwrap()
     }
 }