the trait bound Post: serde::Serialize is not satisfied（明明 derive 了）

# oldlib/Cargo.toml
[package]
name = "oldlib"
version = "0.1.0"
edition = "2015"
publish = false

[dependencies]
serde = "0.9"
serde_derive = "0.9"

// A library that has not been upgraded: it derives serde 0.9's Serialize.
#[macro_use]
extern crate serde_derive;

#[derive(Serialize, Debug)]
pub struct Post {
  pub title: String,
}

[dependencies]
oldlib = { path = "oldlib" }
newlib = { path = "newlib" }
serde_json = "=1.0.154"

// oldlib::Post implements serde 0.9's Serialize; serde_json 1.x wants serde 1.x's.
fn main() {
  let p = oldlib::Post { title: "hi".into() };
  println!("{}", serde_json::to_string(&p).unwrap());
}

error[E0277]: the trait bound `Post: serde::Serialize` is not satisfied
    --> src/main.rs:4:40
     |
   4 |   println!("{}", serde_json::to_string(&p).unwrap());
     |                  --------------------- ^^ the trait `serde_core::ser::Serialize` is not implemented for `Post`
     |                  |
     |                  required by a bound introduced by this call
     |
     = note: for local types consider adding `#[derive(serde::Serialize)]` to your `Post` type
     = note: for types from other crates check whether the crate offers a `serde` feature flag
     = note: `Post` implements similarly named trait `serde::ser::Serialize`, but not `serde_core::ser::Serialize`
     = help: the following other types implement trait `serde_core::ser::Serialize`:
               &'a T
               &'a mut T
               ()
               (T,)
               (T0, T1)
               (T0, T1, T2)
               (T0, T1, T2, T3)
               (T0, T1, T2, T3, T4)
             and 129 others
note: required by a bound in `serde_json::to_string`
    --> ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_json-1.0.154/src/ser.rs:2247:17
     |
2245 | pub fn to_string<T>(value: &T) -> Result<String>
     |        --------- required by a bound in this function
2246 | where
2247 |     T: ?Sized + Serialize,
     |                 ^^^^^^^^^ required by this bound in `to_string`

For more information about this error, try `rustc --explain E0277`.

warning: non-local `impl` definition, `impl` blocks should be written at the same level as their item
 --> oldlib/src/lib.rs:5:10
  |
5 | #[derive(Serialize, Debug)]
  |          ^^^^^^^^^
  |          |
  |          `Post` is not local
  |          `Serialize` is not local
  |          move the `impl` block outside of this constant `_IMPL_SERIALIZE_FOR_Post`
  |
  = note: the derive macro `Serialize` defines the non-local `impl`, and may need to be changed
  = note: the derive macro `Serialize` may come from an old version of the `serde_derive` crate, try updating your dependency with `cargo update -p serde_derive`
  = note: an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`
  = note: items in an anonymous const item (`const _: () = { ... }`) are treated as in the same scope as the anonymous const's declaration for the purpose of this lint
  = note: `#[warn(non_local_definitions)]` on by default
  = note: this warning originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

#[cfg_attr(
    not(no_diagnostic_namespace),
    diagnostic::on_unimplemented(
        // Prevents `serde_core::ser::Serialize` appearing in the error message
        // in projects with no direct dependency on serde_core.
        message = "the trait bound `{Self}: serde::Serialize` is not satisfied",
        note = "for local types consider adding `#[derive(serde::Serialize)]` to your `{Self}` type",
        note = "for types from other crates check whether the crate offers a `serde` feature flag",
    )
)]
pub trait Serialize {

error[E0277]: the trait bound `Post: serde::ser::Serialize` is not satisfied
    --> src/main.rs:4:40
     |
   4 |   println!("{}", serde_json::to_string(&p).unwrap());
     |                  --------------------- ^^ the trait `serde::ser::Serialize` is not implemented for `Post`
     |                  |
     |                  required by a bound introduced by this call
     |
     = note: for local types consider adding `#[derive(serde::Serialize)]` to your `Post` type
     = note: for types from other crates check whether the crate offers a `serde` feature flag
note: there are multiple different versions of crate `serde` in the dependency graph
    --> ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-1.0.219/src/ser/mod.rs:225:1
     |
 225 | pub trait Serialize {
     | ^^^^^^^^^^^^^^^^^^^ this is the expected trait
     |
    ::: ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde-0.9.15/src/ser/mod.rs:179:1
     |
 179 | pub trait Serialize {
     | ------------------- this is the found trait
     = help: you can use `cargo tree` to explore your dependency tree
     = help: the following other types implement trait `serde::ser::Serialize`:
               &'a T
               &'a mut T
               ()
               (T,)
               (T0, T1)
               (T0, T1, T2)
               (T0, T1, T2, T3)
               (T0, T1, T2, T3, T4)
             and 131 others
note: required by a bound in `serde_json::to_string`
    --> ~/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_json-1.0.140/src/ser.rs:2239:17
     |
2237 | pub fn to_string<T>(value: &T) -> Result<String>
     |        --------- required by a bound in this function
2238 | where
2239 |     T: ?Sized + Serialize,
     |                 ^^^^^^^^^ required by this bound in `to_string`

For more information about this error, try `rustc --explain E0277`.

error: failed to select a version for `serde_derive`.
    ... required by package `serde_core v1.0.228`
    ... which satisfies dependency `serde_core = "=1.0.228"` of package `serde v1.0.228`
    ... which satisfies dependency `serde = "=1.0.228"` of package `exp-serde-intro-mismatch v0.1.0 (<scratchpad>/exp-serde-intro/mismatch)`
versions that meet the requirements `=1.0.228` are: 1.0.228

all possible versions conflict with previously selected packages

  previously selected package `serde_derive v1.0.229`
    ... which satisfies dependency `serde_derive = "=1.0.229"` of package `exp-serde-intro-mismatch v0.1.0 (<scratchpad>/exp-serde-intro/mismatch)`

failed to select a version for `serde_derive` which could resolve this conflict

[dependencies]
serde = { version = "1", features = ["derive"] }

// The same library after upgrading to serde 1.
#[derive(serde::Serialize, Debug)]
pub struct Post {
  pub title: String,
}

{"title":"hi"}