Delete support for compilers without dynamically sized Rc construction

.github/workflows/ci.yml

@@ -77,7 +77,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [1.20.0, 1.21.0, 1.25.0, 1.26.0, 1.34.0]
+        rust: [1.21.0, 1.25.0, 1.26.0, 1.34.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v3

README.md

@@ -1,12 +1,12 @@
-# Serde   [![Build Status]][actions] [![Latest Version]][crates.io] [![serde: rustc 1.20+]][Rust 1.20] [![serde_derive: rustc 1.56+]][Rust 1.56]
+# Serde   [![Build Status]][actions] [![Latest Version]][crates.io] [![serde: rustc 1.21+]][Rust 1.21] [![serde_derive: rustc 1.56+]][Rust 1.56]
 
 [Build Status]: https://img.shields.io/github/actions/workflow/status/serde-rs/serde/ci.yml?branch=master
 [actions]: https://github.com/serde-rs/serde/actions?query=branch%3Amaster
 [Latest Version]: https://img.shields.io/crates/v/serde.svg
 [crates.io]: https://crates.io/crates/serde
-[serde: rustc 1.20+]: https://img.shields.io/badge/serde-rustc_1.20+-lightgray.svg
+[serde: rustc 1.21+]: https://img.shields.io/badge/serde-rustc_1.21+-lightgray.svg
 [serde_derive: rustc 1.56+]: https://img.shields.io/badge/serde_derive-rustc_1.56+-lightgray.svg
-[Rust 1.20]: https://blog.rust-lang.org/2017/08/31/Rust-1.20.html
+[Rust 1.21]: https://blog.rust-lang.org/2017/10/12/Rust-1.21.html
 [Rust 1.56]: https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html
 
 **Serde is a framework for *ser*ializing and *de*serializing Rust data structures efficiently and generically.**

serde/Cargo.toml

@@ -11,7 +11,7 @@ keywords = ["serde", "serialization", "no_std"]
 license = "MIT OR Apache-2.0"
 readme = "crates-io.md"
 repository = "https://github.com/serde-rs/serde"
-rust-version = "1.20"
+rust-version = "1.21"
 
 [dependencies]
 serde_derive = { version = "=1.0.179", optional = true, path = "../serde_derive" }

serde/build.rs

@@ -16,13 +16,6 @@ fn main() {
     let target = env::var("TARGET").unwrap();
     let emscripten = target == "asmjs-unknown-emscripten" || target == "wasm32-unknown-emscripten";
 
-    // From<Box<T>> for Rc<T> / Arc<T> stabilized in Rust 1.21:
-    // https://doc.rust-lang.org/std/rc/struct.Rc.html#impl-From<Box<T>>
-    // https://doc.rust-lang.org/std/sync/struct.Arc.html#impl-From<Box<T>>
-    if minor < 21 {
-        println!("cargo:rustc-cfg=no_de_rc_dst");
-    }
-
     // Duration available in core since Rust 1.25:
     // https://blog.rust-lang.org/2018/03/29/Rust-1.25.html#library-stabilizations
     if minor < 25 {

serde/src/de/impls.rs

@@ -1791,30 +1791,6 @@ forwarded_impl!((T), Box<[T]>, Vec::into_boxed_slice);
 #[cfg(any(feature = "std", feature = "alloc"))]
 forwarded_impl!((), Box<str>, String::into_boxed_str);
 
-#[cfg(all(no_de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
-forwarded_impl! {
-    /// This impl requires the [`"rc"`] Cargo feature of Serde.
-    ///
-    /// Deserializing a data structure containing `Arc` will not attempt to
-    /// deduplicate `Arc` references to the same data. Every deserialized `Arc`
-    /// will end up with a strong count of 1.
-    ///
-    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
-    (T), Arc<T>, Arc::new
-}
-
-#[cfg(all(no_de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
-forwarded_impl! {
-    /// This impl requires the [`"rc"`] Cargo feature of Serde.
-    ///
-    /// Deserializing a data structure containing `Rc` will not attempt to
-    /// deduplicate `Rc` references to the same data. Every deserialized `Rc`
-    /// will end up with a strong count of 1.
-    ///
-    /// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
-    (T), Rc<T>, Rc::new
-}
-
 #[cfg(any(feature = "std", feature = "alloc"))]
 impl<'de, 'a, T: ?Sized> Deserialize<'de> for Cow<'a, T>
 where
@@ -1870,11 +1846,7 @@ where
 
 ////////////////////////////////////////////////////////////////////////////////
 
-#[cfg(all(
-    not(no_de_rc_dst),
-    feature = "rc",
-    any(feature = "std", feature = "alloc")
-))]
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
 macro_rules! box_forwarded_impl {
     (
         $(#[doc = $doc:tt])*
@@ -1895,11 +1867,7 @@ macro_rules! box_forwarded_impl {
     };
 }
 
-#[cfg(all(
-    not(no_de_rc_dst),
-    feature = "rc",
-    any(feature = "std", feature = "alloc")
-))]
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
 box_forwarded_impl! {
     /// This impl requires the [`"rc"`] Cargo feature of Serde.
     ///
@@ -1911,11 +1879,7 @@ box_forwarded_impl! {
     Rc
 }
 
-#[cfg(all(
-    not(no_de_rc_dst),
-    feature = "rc",
-    any(feature = "std", feature = "alloc")
-))]
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
 box_forwarded_impl! {
     /// This impl requires the [`"rc"`] Cargo feature of Serde.
     ///