Avoid expanding to unstable internal method

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -149,7 +149,22 @@ fn decodable_substructure(
                 arms.push(cx.arm(v_span, cx.pat_lit(v_span, cx.expr_usize(v_span, i)), decoded));
             }
 
-            arms.push(cx.arm_unreachable(trait_span));
+            let expr_unreachable = cx.expr_macro_call(
+                trait_span,
+                cx.macro_call(
+                    trait_span,
+                    cx.path_global(
+                        trait_span,
+                        [sym::std, sym::unreachable]
+                            .map(|s| Ident::new(s, trait_span))
+                            .to_vec(),
+                    ),
+                    ast::token::Delimiter::Parenthesis,
+                    ast::tokenstream::TokenStream::default(),
+                ),
+            );
+            let arm_unreachable = cx.arm(trait_span, cx.pat_wild(trait_span), expr_unreachable);
+            arms.push(arm_unreachable);
 
             let result = cx.expr_ok(
                 trait_span,

compiler/rustc_expand/src/build.rs

@@ -48,6 +48,23 @@ impl<'a> ExtCtxt<'a> {
         ast::Path { span, segments, tokens: None }
     }
 
+    pub fn macro_call(
+        &self,
+        span: Span,
+        path: ast::Path,
+        delim: ast::token::Delimiter,
+        tokens: ast::tokenstream::TokenStream,
+    ) -> P<ast::MacCall> {
+        P(ast::MacCall {
+            path,
+            args: P(ast::DelimArgs {
+                dspan: ast::tokenstream::DelimSpan { open: span, close: span },
+                delim,
+                tokens,
+            }),
+        })
+    }
+
     pub fn ty_mt(&self, ty: P<ast::Ty>, mutbl: ast::Mutability) -> ast::MutTy {
         ast::MutTy { ty, mutbl }
     }
@@ -265,6 +282,10 @@ impl<'a> ExtCtxt<'a> {
         self.expr(span, ast::ExprKind::Field(expr, field))
     }
 
+    pub fn expr_macro_call(&self, span: Span, call: P<ast::MacCall>) -> P<ast::Expr> {
+        self.expr(span, ast::ExprKind::MacCall(call))
+    }
+
     pub fn expr_binary(
         &self,
         sp: Span,
@@ -410,18 +431,6 @@ impl<'a> ExtCtxt<'a> {
         self.expr(sp, ast::ExprKind::Tup(exprs))
     }
 
-    pub fn expr_fail(&self, span: Span, msg: Symbol) -> P<ast::Expr> {
-        self.expr_call_global(
-            span,
-            [sym::std, sym::rt, sym::begin_panic].iter().map(|s| Ident::new(*s, span)).collect(),
-            thin_vec![self.expr_str(span, msg)],
-        )
-    }
-
-    pub fn expr_unreachable(&self, span: Span) -> P<ast::Expr> {
-        self.expr_fail(span, Symbol::intern("internal error: entered unreachable code"))
-    }
-
     pub fn expr_ok(&self, sp: Span, expr: P<ast::Expr>) -> P<ast::Expr> {
         let ok = self.std_path(&[sym::result, sym::Result, sym::Ok]);
         self.expr_call_global(sp, ok, thin_vec![expr])
@@ -519,10 +528,6 @@ impl<'a> ExtCtxt<'a> {
         }
     }
 
-    pub fn arm_unreachable(&self, span: Span) -> ast::Arm {
-        self.arm(span, self.pat_wild(span), self.expr_unreachable(span))
-    }
-
     pub fn expr_match(&self, span: Span, arg: P<ast::Expr>, arms: ThinVec<ast::Arm>) -> P<Expr> {
         self.expr(span, ast::ExprKind::Match(arg, arms, MatchKind::Prefix))
     }

tests/ui/derives/auxiliary/rustc-serialize.rs

@@ -0,0 +1,16 @@
+#![crate_type = "lib"]
+
+pub trait Decoder {
+    type Error;
+
+    fn read_enum<T, F>(&mut self, name: &str, f: F) -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+    fn read_enum_variant<T, F>(&mut self, names: &[&str], f: F)
+                               -> Result<T, Self::Error>
+        where F: FnMut(&mut Self, usize) -> Result<T, Self::Error>;
+
+}
+
+pub trait Decodable: Sized {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Self, D::Error>;
+}

tests/ui/derives/issue-123156.rs

@@ -0,0 +1,11 @@
+//@ check-pass
+//@ edition:2021
+//@ aux-build:rustc-serialize.rs
+
+#![crate_type = "lib"]
+#![allow(deprecated, soft_unstable)]
+
+extern crate rustc_serialize;
+
+#[derive(RustcDecodable)]
+pub enum Foo {}

tests/ui/derives/issue-123156.stderr

@@ -0,0 +1,10 @@
+Future incompatibility report: Future breakage diagnostic:
+warning: use of unstable library feature 'rustc_encodable_decodable': derive macro for `rustc-serialize`; should not be used in new code
+  --> $DIR/issue-123156.rs:10:10
+   |
+LL | #[derive(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>
+