Do not accept interpolated types as traits in trait impls

src/libsyntax/parse/parser.rs

@@ -5518,6 +5518,7 @@ impl<'a> Parser<'a> {
         };
 
         // Parse both types and traits as a type, then reinterpret if necessary.
+        let ty_first_interpolated = if self.token.is_ty() { Some(self.span) } else { None };
         let ty_first = self.parse_ty()?;
 
         // If `for` is missing we try to recover.
@@ -5547,8 +5548,16 @@ impl<'a> Parser<'a> {
 
                 let ty_first = ty_first.into_inner();
                 let path = match ty_first.node {
-                    // This notably includes paths passed through `ty` macro fragments (#46438).
-                    TyKind::Path(None, path) => path,
+                    TyKind::Path(None, path) => {
+                        if let Some(span) = ty_first_interpolated {
+                            self.diagnostic()
+                                .struct_span_warn(span, "expected a trait, found type")
+                                .note("this warning will become a hard error in a future release")
+                                .span_label(span, "try using `path` instead of `ty` \
+                                                   for this macro fragment").emit();
+                        }
+                        path
+                    }
                     _ => {
                         self.span_err(ty_first.span, "expected a trait, found type");
                         ast::Path::from_ident(ty_first.span, keywords::Invalid.ident())

src/libsyntax/parse/token.rs

@@ -315,6 +315,16 @@ impl Token {
         false
     }
 
+    /// Returns `true` if the token is an interpolated type.
+    pub fn is_ty(&self) -> bool {
+        if let Interpolated(ref nt) = *self {
+            if let NtTy(..) = nt.0 {
+                return true;
+            }
+        }
+        false
+    }
+
     /// Returns a lifetime with the span and a dummy id if it is a lifetime,
     /// or the original lifetime if it is an interpolated lifetime, ignoring
     /// the span.

src/test/compile-fail/issue-46438.rs → src/test/ui/issue-46438.rs

@@ -10,7 +10,7 @@
 
 macro_rules! m {
     ($my_type: ty) => {
-        impl $my_type for u8 {}
+        impl $my_type for u8 {} //~ WARN expected a trait, found type
     }
 }
 

src/test/ui/issue-46438.stderr

@@ -0,0 +1,19 @@
+warning: expected a trait, found type
+  --> $DIR/issue-46438.rs:13:14
+   |
+LL |         impl $my_type for u8 {} //~ WARN expected a trait, found type
+   |              ^^^^^^^^ try using `path` instead of `ty` for this macro fragment
+...
+LL | m!(Tr);
+   | ------- in this macro invocation
+   |
+   = note: this warning will become a hard error in a future release
+
+error: expected a trait, found type
+  --> $DIR/issue-46438.rs:21:4
+   |
+LL | m!(&'static u8); //~ ERROR expected a trait, found type
+   |    ^^^^^^^^^^^
+
+error: aborting due to previous error
+