Reflect serde_derive required compiler in build script and rust-versi…

…on metadata

serde/build.rs

@@ -78,11 +78,6 @@ fn main() {
         println!("cargo:rustc-cfg=no_num_nonzero");
     }
 
-    // Current minimum supported version of serde_derive crate is Rust 1.31.
-    if minor < 31 {
-        println!("cargo:rustc-cfg=no_serde_derive");
-    }
-
     // TryFrom, Atomic types, non-zero signed integers, and SystemTime::checked_add
     // stabilized in Rust 1.34:
     // https://blog.rust-lang.org/2019/04/11/Rust-1.34.0.html#tryfrom-and-tryinto
@@ -94,6 +89,11 @@ fn main() {
         println!("cargo:rustc-cfg=no_relaxed_trait_bounds");
     }
 
+    // Current minimum supported version of serde_derive crate is Rust 1.56.
+    if minor < 56 {
+        println!("cargo:rustc-cfg=no_serde_derive");
+    }
+
     // Support for #[cfg(target_has_atomic = "...")] stabilized in Rust 1.60.
     if minor < 60 {
         println!("cargo:rustc-cfg=no_target_has_atomic");

serde_derive/Cargo.toml

@@ -11,7 +11,7 @@ keywords = ["serde", "serialization", "no_std", "derive"]
 license = "MIT OR Apache-2.0"
 readme = "crates-io.md"
 repository = "https://github.com/serde-rs/serde"
-rust-version = "1.31"
+rust-version = "1.56"
 
 [features]
 default = []

serde_derive_internals/Cargo.toml

@@ -9,7 +9,7 @@ include = ["lib.rs", "src/**/*.rs", "LICENSE-APACHE", "LICENSE-MIT"]
 keywords = ["serde", "serialization"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/serde-rs/serde"
-rust-version = "1.31"
+rust-version = "1.56"
 
 [lib]
 path = "lib.rs"