warning: trait `AssertNotSerdeSerialize` is never used
      --> test_suite/tests/test_serde_path.rs:39:7
       |
    39 | trait AssertNotSerdeSerialize {}
       |       ^^^^^^^^^^^^^^^^^^^^^^^
       |
       = note: `#[warn(dead_code)]` on by default

    warning: trait `AssertNotSerdeDeserialize` is never used
      --> test_suite/tests/test_serde_path.rs:43:7
       |
    43 | trait AssertNotSerdeDeserialize<'a> {}
       |       ^^^^^^^^^^^^^^^^^^^^^^^^^

    warning: method `serialize` is never used
      --> test_suite/tests/test_serde_path.rs:31:12
       |
    30 |     pub trait Serialize {
       |               --------- method in this trait
    31 |         fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error>;
       |            ^^^^^^^^^

    warning: associated function `deserialize` is never used
      --> test_suite/tests/test_serde_path.rs:35:12
       |
    34 |     pub trait Deserialize<'a>: Sized {
       |               ----------- associated function in this trait
    35 |         fn deserialize<D: Deserializer<'a>>(deserializer: D) -> Result<Self, D::Error>;
       |            ^^^^^^^^^^^

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.35.0`
       --> serde/src/de/impls.rs:200:39
        |
    200 |                   Ok((v as Self::Value).copysign(sign))
        |                                         ^^^^^^^^^^^^^^
    ...
    374 | / impl_deserialize_num! {
    375 | |     f32, deserialize_f32
    376 | |     num_self!(f32:visit_f32);
    377 | |     num_as_copysign_self!(f64:visit_f64);
    378 | |     num_as_self!(i8:visit_i8 i16:visit_i16 i32:visit_i32 i64:visit_i64);
    379 | |     num_as_self!(u8:visit_u8 u16:visit_u16 u32:visit_u32 u64:visit_u64);
    380 | | }
        | |_- in this macro invocation
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
        = note: `-W clippy::incompatible-msrv` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::incompatible_msrv)]`
        = note: this warning originates in the macro `num_as_copysign_self` which comes from the expansion of the macro `impl_deserialize_num` (in Nightly builds, run with -Z macro-backtrace for more info)

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.35.0`
       --> serde/src/de/impls.rs:200:39
        |
    200 |                   Ok((v as Self::Value).copysign(sign))
        |                                         ^^^^^^^^^^^^^^
    ...
    382 | / impl_deserialize_num! {
    383 | |     f64, deserialize_f64
    384 | |     num_self!(f64:visit_f64);
    385 | |     num_as_copysign_self!(f32:visit_f32);
    386 | |     num_as_self!(i8:visit_i8 i16:visit_i16 i32:visit_i32 i64:visit_i64);
    387 | |     num_as_self!(u8:visit_u8 u16:visit_u16 u32:visit_u32 u64:visit_u64);
    388 | | }
        | |_- in this macro invocation
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
        = note: this warning originates in the macro `num_as_copysign_self` which comes from the expansion of the macro `impl_deserialize_num` (in Nightly builds, run with -Z macro-backtrace for more info)

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.34.0`
        --> serde/src/de/impls.rs:2308:14
         |
    2308 |             .checked_add(duration)
         |              ^^^^^^^^^^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.34.0`
       --> serde/src/ser/impls.rs:606:26
        |
    606 |                       self.get().serialize(serializer)
        |                            ^^^^^
    ...
    623 | / nonzero_integers! {
    624 | |     NonZeroI8,
    625 | |     NonZeroI16,
    626 | |     NonZeroI32,
    ...   |
    629 | |     NonZeroIsize,
    630 | | }
        | |_- in this macro invocation
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
        = note: this warning originates in the macro `nonzero_integers` (in Nightly builds, run with -Z macro-backtrace for more info)

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.34.0`
        --> serde/src/ser/impls.rs:1053:26
         |
    1053 |                       self.load(Ordering::Relaxed).serialize(serializer)
         |                            ^^^^^^^^^^^^^^^^^^^^^^^
    ...
    1061 | / atomic_impl! {
    1062 | |     AtomicBool "8"
    1063 | |     AtomicI8 "8"
    1064 | |     AtomicI16 "16"
    ...    |
    1070 | |     AtomicUsize "ptr"
    1071 | | }
         | |_- in this macro invocation
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
         = note: this warning originates in the macro `atomic_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

    warning: current MSRV (Minimum Supported Rust Version) is `1.31.0` but this item is stable since `1.34.0`
        --> serde/src/ser/impls.rs:1053:26
         |
    1053 |                       self.load(Ordering::Relaxed).serialize(serializer)
         |                            ^^^^^^^^^^^^^^^^^^^^^^^
    ...
    1074 | / atomic_impl! {
    1075 | |     AtomicI64 "64"
    1076 | |     AtomicU64 "64"
    1077 | | }
         | |_- in this macro invocation
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
         = note: this warning originates in the macro `atomic_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

Apparently `write!` generates more code than `write_str` when used with a simple string, so optimizing it.

A few minor `write_str` optimizations

    warning: the item `Into` is imported redundantly
       --> serde/src/lib.rs:184:47
        |
    184 |     pub use self::core::convert::{self, From, Into};
        |                                               ^^^^
        |
       ::: nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/prelude/mod.rs:115:13
        |
    115 |     pub use super::v1::*;
        |             --------- the item `Into` is already defined here
        |
        = note: `#[warn(unused_imports)]` on by default