Support phantom types inside a method body

src/lib.rs

@@ -251,7 +251,7 @@ pub fn phantom(args: TokenStream, input: TokenStream) -> TokenStream {
     let ident = &input.ident;
     let call_site = Span::call_site();
     let void_namespace = Ident::new(&format!("__void_{}", ident), call_site);
-    let value_namespace = Ident::new(&format!("__value_{}", ident), call_site);
+    let fake_ident = Ident::new(&format!("{}__Phantom", ident), ident.span());
 
     let vis = &input.vis;
     let vis_super = visibility::vis_super(vis);
@@ -296,7 +296,11 @@ pub fn phantom(args: TokenStream, input: TokenStream) -> TokenStream {
                 ty_generics.push(quote!(#lifetime));
                 phantoms.push(variance::apply(param, elem, &type_param));
             }
-            GenericParam::Const(_) => {}
+            GenericParam::Const(param) => {
+                let ident = &param.ident;
+                impl_generics.push(quote!(#param));
+                ty_generics.push(quote!(#ident));
+            }
         }
     }
 
@@ -355,19 +359,19 @@ pub fn phantom(args: TokenStream, input: TokenStream) -> TokenStream {
             }
         }
 
-        mod #value_namespace {
-            #[doc(hidden)]
-            #vis_super use super::#ident::#ident;
-        }
-
+        #[doc(hidden)]
+        #[allow(non_camel_case_types)]
         #(#attrs)*
-        #vis #enum_token #ident #generics #where_clause {
+        #vis #enum_token #fake_ident #generics #where_clause {
             __Phantom(#void_namespace::#ident <#(#ty_generics),*>),
             #ident,
         }
 
         #[doc(hidden)]
-        #vis use self::#value_namespace::*;
+        #vis use #fake_ident::*;
+
+        #[allow(unused_imports, type_alias_bounds)]
+        #vis type #ident #generics #where_clause = #fake_ident <#(#ty_generics),*>;
 
         #derives
     })

tests/function-body.rs

@@ -0,0 +1,6 @@
+use ghost::phantom;
+
+const fn main() {
+    #[phantom]
+    struct MyPhantom<T: ?Sized>;
+}

tests/ui/autotraits.stderr

@@ -15,7 +15,7 @@ note: required because it appears within the type `__void_MyPhantom::MyPhantom<*
    |
  4 | struct MyPhantom<T: ?Sized>;
    |        ^^^^^^^^^
-note: required because it appears within the type `MyPhantom<*const u8>`
+note: required because it appears within the type `MyPhantom__Phantom<*const u8>`
   --> tests/ui/autotraits.rs:4:8
    |
  4 | struct MyPhantom<T: ?Sized>;
@@ -44,7 +44,7 @@ note: required because it appears within the type `__void_MyPhantom::MyPhantom<*
    |
  4 | struct MyPhantom<T: ?Sized>;
    |        ^^^^^^^^^
-note: required because it appears within the type `MyPhantom<*const u8>`
+note: required because it appears within the type `MyPhantom__Phantom<*const u8>`
   --> tests/ui/autotraits.rs:4:8
    |
  4 | struct MyPhantom<T: ?Sized>;

tests/ui/ffi.stderr

@@ -1,4 +1,4 @@
-error: `extern` fn uses type `MyPhantom<i32>`, which is not FFI-safe
+error: `extern` fn uses type `MyPhantom__Phantom<i32>`, which is not FFI-safe
  --> tests/ui/ffi.rs:8:39
   |
 8 | pub extern "C" fn extern_fn(_phantom: MyPhantom<i32>) {}

tests/ui/invariant.stderr

@@ -6,8 +6,8 @@ error: lifetime may not live long enough
 7 |     phantom
   |     ^^^^^^^ returning this value requires that `'a` must outlive `'static`
   |
-  = note: requirement occurs because of the type `InvariantPhantom<&str>`, which makes the generic argument `&str` invariant
-  = note: the enum `InvariantPhantom<T>` is invariant over the parameter `T`
+  = note: requirement occurs because of the type `InvariantPhantom__Phantom<&str>`, which makes the generic argument `&str` invariant
+  = note: the enum `InvariantPhantom__Phantom<T>` is invariant over the parameter `T`
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: lifetime may not live long enough
@@ -18,6 +18,6 @@ error: lifetime may not live long enough
 11 |     phantom
    |     ^^^^^^^ returning this value requires that `'a` must outlive `'static`
    |
-   = note: requirement occurs because of the type `InvariantPhantom<&str>`, which makes the generic argument `&str` invariant
-   = note: the enum `InvariantPhantom<T>` is invariant over the parameter `T`
+   = note: requirement occurs because of the type `InvariantPhantom__Phantom<&str>`, which makes the generic argument `&str` invariant
+   = note: the enum `InvariantPhantom__Phantom<T>` is invariant over the parameter `T`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

tests/ui/pattern-match.stderr

@@ -1,39 +1,39 @@
-error[E0004]: non-exhaustive patterns: `MyPhantom::__Phantom(_)` not covered
+error[E0004]: non-exhaustive patterns: `MyPhantom__Phantom::__Phantom(_)` not covered
  --> tests/ui/pattern-match.rs:9:11
   |
 9 |     match phantom {
-  |           ^^^^^^^ pattern `MyPhantom::__Phantom(_)` not covered
+  |           ^^^^^^^ pattern `MyPhantom__Phantom::__Phantom(_)` not covered
   |
-note: `MyPhantom<u8>` defined here
+note: `MyPhantom__Phantom<u8>` defined here
  --> tests/ui/pattern-match.rs:4:8
   |
 3 | #[phantom]
   | ---------- not covered
 4 | struct MyPhantom<T: ?Sized>;
   |        ^^^^^^^^^
-  = note: the matched value is of type `MyPhantom<u8>`
+  = note: the matched value is of type `MyPhantom__Phantom<u8>`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
   |
 10~         MyPhantom => {},
-11+         MyPhantom::__Phantom(_) => todo!()
+11+         MyPhantom__Phantom::__Phantom(_) => todo!()
   |
 
-error[E0004]: non-exhaustive patterns: `MyPhantom::__Phantom(_)` not covered
+error[E0004]: non-exhaustive patterns: `MyPhantom__Phantom::__Phantom(_)` not covered
   --> tests/ui/pattern-match.rs:13:11
    |
 13 |     match phantom {
-   |           ^^^^^^^ pattern `MyPhantom::__Phantom(_)` not covered
+   |           ^^^^^^^ pattern `MyPhantom__Phantom::__Phantom(_)` not covered
    |
-note: `MyPhantom<u8>` defined here
+note: `MyPhantom__Phantom<u8>` defined here
   --> tests/ui/pattern-match.rs:4:8
    |
  3 | #[phantom]
    | ---------- not covered
  4 | struct MyPhantom<T: ?Sized>;
    |        ^^^^^^^^^
-   = note: the matched value is of type `MyPhantom<u8>`
+   = note: the matched value is of type `MyPhantom__Phantom<u8>`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 14 ~         MyPhantom::<u8> => {},
-15 +         MyPhantom::__Phantom(_) => todo!()
+15 +         MyPhantom__Phantom::__Phantom(_) => todo!()
    |