Fix v0 symbol mangling for assoc const bindings

Cargo.lock

@@ -3356,9 +3356,9 @@ dependencies = [
 
 [[package]]
 name = "rustc-demangle"
-version = "0.1.26"
+version = "0.1.27"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "56f7d92ca342cea22a06f2121d944b4fd82af56988c270852495420f961d4ace"
+checksum = "b50b8869d9fc858ce7266cce0194bd74df58b9d0e3f6df3a9fc8eb470d95c09d"
 
 [[package]]
 name = "rustc-hash"

compiler/rustc_symbol_mangling/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2024"
 [dependencies]
 # tidy-alphabetical-start
 punycode = "0.4.0"
-rustc-demangle = "0.1.21"
+rustc-demangle = "0.1.27"
 rustc_abi = { path = "../rustc_abi" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }

compiler/rustc_symbol_mangling/src/v0.rs

@@ -648,7 +648,10 @@ impl<'tcx> Printer<'tcx> for V0SymbolMangler<'tcx> {
                         p.push_ident(name.as_str());
                         match projection.term.kind() {
                             ty::TermKind::Ty(ty) => ty.print(p),
-                            ty::TermKind::Const(c) => c.print(p),
+                            ty::TermKind::Const(c) => {
+                                p.push("K");
+                                c.print(p)
+                            }
                         }?;
                     }
                     ty::ExistentialPredicate::AutoTrait(def_id) => {

src/doc/rustc/src/symbol-mangling/v0.md

@@ -857,7 +857,11 @@ Remaining primitives are encoded as a crate production, e.g. `C4f128`.
   >
   > <span id="dyn-trait">dyn-trait</span> → *[path]* {*[dyn-trait-assoc-binding]*}
   >
-  > <span id="dyn-trait-assoc-binding">dyn-trait-assoc-binding</span> → `p` *[undisambiguated-identifier]* *[type]*
+  > <span id="dyn-trait-assoc-binding">dyn-trait-assoc-binding</span> → `p` *[undisambiguated-identifier]* *[type-or-const]*
+  >
+  > <span id="type-or-const">type-or-const</span> → \
+  > &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *[type]* \
+  > &nbsp;&nbsp; | `K` *[const]*
 
   The tag `D` is followed by a *[dyn-bounds]* which encodes the trait bounds,
   followed by a *[lifetime]* of the trait object lifetime bound.
@@ -867,11 +871,12 @@ Remaining primitives are encoded as a crate production, e.g. `C4f128`.
 
   Each *[dyn-trait]* represents a trait bound, which consists of a *[path]* to the trait followed by zero or more *[dyn-trait-assoc-binding]* which list the associated types.
 
-  Each *[dyn-trait-assoc-binding]* consists of a character `p` followed a *[undisambiguated-identifier]* representing the associated binding name, and finally a *[type]*.
+  Each *[dyn-trait-assoc-binding]* consists of a character `p` followed a *[undisambiguated-identifier]* representing the associated binding name, and finally a *[type-or-const]*.
 
 [dyn-bounds]: #dyn-bounds
 [dyn-trait]: #dyn-trait
 [dyn-trait-assoc-binding]: #dyn-trait-assoc-binding
+[type-or-const]: #type-or-const
 
 
 * A *[path]* to a named type.

tests/ui/const-generics/associated-const-bindings/dyn-compat-symbol-mangling.rs

@@ -0,0 +1,28 @@
+// Ensure that we can successfully mangle & demangle trait object types w/ assoc const bindings.
+
+// FIXME(mgca): Legacy mangling still crashes:
+//              "finding type for [impl], encountered [crate root] with no parent"
+
+//@ build-fail
+//@ revisions: v0
+//\@[legacy] compile-flags: -C symbol-mangling-version=legacy -Z unstable-options
+//@    [v0] compile-flags: -C symbol-mangling-version=v0
+//\@[legacy] normalize-stderr: "h[[:xdigit:]]{16}" -> "h[HASH]"
+//@    [v0] normalize-stderr: "sym\[.*?\]" -> "sym[HASH]"
+
+#![feature(min_generic_const_args, rustc_attrs)]
+#![expect(incomplete_features)]
+#![crate_name = "sym"]
+
+trait Trait {
+    #[type_const]
+    const N: usize;
+}
+
+#[rustc_symbol_name]
+//~^ ERROR symbol-name(_RMCs
+//~| ERROR demangling(<dyn sym[
+//~| ERROR demangling-alt(<dyn sym::Trait<N = 0>>)
+impl dyn Trait<N = 0> {}
+
+fn main() {}

tests/ui/const-generics/associated-const-bindings/dyn-compat-symbol-mangling.v0.stderr

@@ -0,0 +1,20 @@
+error: symbol-name(_RMCsCRATE_HASH_3symDNtB<REF>_5Traitp1NKj0_EL_)
+  --> $DIR/dyn-compat-symbol-mangling.rs:22:1
+   |
+LL | #[rustc_symbol_name]
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(<dyn sym[HASH]::Trait<N = 0usize>>)
+  --> $DIR/dyn-compat-symbol-mangling.rs:22:1
+   |
+LL | #[rustc_symbol_name]
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(<dyn sym::Trait<N = 0>>)
+  --> $DIR/dyn-compat-symbol-mangling.rs:22:1
+   |
+LL | #[rustc_symbol_name]
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+