Auto merge of #116016 - jhpratt:kill-rustc-serialize, r=ehuss

Soft-destabilize `RustcEncodable` & `RustcDecodable`, remove from prelude in next edition

cc rust-lang/libs-team#272

Any use of `RustcEncodable` and `RustcDecodable` now triggers a deny-by-default lint. The derives have been removed from the 2024 prelude. I specifically chose **not** to document this in the module-level documentation, as the presence in existing preludes is not documented (which I presume is intentional).

This does not implement the proposed change for `rustfix`, which I will be looking into shortly.

With regard to the items in the preludes being stable, this should not be an issue because #15702 has been resolved.

r? libs-api

3 files changed, 88 insertions, 10 deletions

diff --git a/tests/run-make-fulldeps/pretty-expanded/input.rs b/tests/run-make-fulldeps/pretty-expanded/input.rs
index af3d75b3bf2..02b235068a1 100644
--- a/tests/run-make-fulldeps/pretty-expanded/input.rs
+++ b/tests/run-make-fulldeps/pretty-expanded/input.rs
@@ -1,12 +1,8 @@
-#[crate_type="lib"]
-
 // #13544
 
-extern crate rustc_serialize;
-
-#[derive(RustcEncodable)] pub struct A;
-#[derive(RustcEncodable)] pub struct B(isize);
-#[derive(RustcEncodable)] pub struct C { x: isize }
-#[derive(RustcEncodable)] pub enum D {}
-#[derive(RustcEncodable)] pub enum E { y }
-#[derive(RustcEncodable)] pub enum F { z(isize) }
+#[derive(Debug)] pub struct A;
+#[derive(Debug)] pub struct B(isize);
+#[derive(Debug)] pub struct C { x: isize }
+#[derive(Debug)] pub enum D {}
+#[derive(Debug)] pub enum E { y }
+#[derive(Debug)] pub enum F { z(isize) }
diff --git a/tests/ui/feature-gates/feature-gate-rustc_encodable_decodable.rs b/tests/ui/feature-gates/feature-gate-rustc_encodable_decodable.rs
new file mode 100644
index 00000000000..13f8fd5fe22
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-rustc_encodable_decodable.rs
@@ -0,0 +1,16 @@
+#![crate_type = "lib"]
+
+// This isn't intended to compile, so it's easiest to just ignore this error.
+extern crate rustc_serialize; //~ERROR can't find crate for `rustc_serialize`
+
+#[derive(
+    RustcEncodable,
+    //~^   ERROR   use of unstable library feature 'rustc_encodable_decodable'
+    //~^^  WARNING this was previously accepted by the compiler
+    //~^^^ WARNING use of deprecated macro `RustcEncodable`
+    RustcDecodable,
+    //~^   ERROR   use of unstable library feature 'rustc_encodable_decodable'
+    //~^^  WARNING this was previously accepted by the compiler
+    //~^^^ WARNING use of deprecated macro `RustcDecodable`
+)]
+struct S;
diff --git a/tests/ui/feature-gates/feature-gate-rustc_encodable_decodable.stderr b/tests/ui/feature-gates/feature-gate-rustc_encodable_decodable.stderr
new file mode 100644
index 00000000000..02b74dacf4d
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-rustc_encodable_decodable.stderr
@@ -0,0 +1,66 @@
+error[E0463]: can't find crate for `rustc_serialize`
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:4:1
+   |
+LL | extern crate rustc_serialize;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate
+   |
+   = help: maybe you need to install the missing components with: `rustup component add rust-src rustc-dev llvm-tools-preview`
+
+error: use of unstable library feature 'rustc_encodable_decodable': derive macro for `rustc-serialize`; should not be used in new code
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:7:5
+   |
+LL |     RustcEncodable,
+   |     ^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
+   = note: `#[deny(soft_unstable)]` on by default
+
+warning: use of deprecated macro `RustcEncodable`: rustc-serialize is deprecated and no longer supported
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:7:5
+   |
+LL |     RustcEncodable,
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(deprecated)]` on by default
+
+error: use of unstable library feature 'rustc_encodable_decodable': derive macro for `rustc-serialize`; should not be used in new code
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:11:5
+   |
+LL |     RustcDecodable,
+   |     ^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
+
+warning: use of deprecated macro `RustcDecodable`: rustc-serialize is deprecated and no longer supported
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:11:5
+   |
+LL |     RustcDecodable,
+   |     ^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors; 2 warnings emitted
+
+For more information about this error, try `rustc --explain E0463`.
+Future incompatibility report: Future breakage diagnostic:
+error: use of unstable library feature 'rustc_encodable_decodable': derive macro for `rustc-serialize`; should not be used in new code
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:7:5
+   |
+LL |     RustcEncodable,
+   |     ^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
+   = note: `#[deny(soft_unstable)]` on by default
+
+Future breakage diagnostic:
+error: use of unstable library feature 'rustc_encodable_decodable': derive macro for `rustc-serialize`; should not be used in new code
+  --> $DIR/feature-gate-rustc_encodable_decodable.rs:11:5
+   |
+LL |     RustcDecodable,
+   |     ^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
+   = note: `#[deny(soft_unstable)]` on by default
+