Parse qualified paths in type params and provide custom diagnostic

src/libsyntax/parse/parser.rs

@@ -5834,12 +5834,11 @@ impl<'a> Parser<'a> {
         lifetimes
     }
 
-    /// Matches `typaram = IDENT (`?` unbound)? optbounds ( EQ ty )?`.
-    fn parse_ty_param(&mut self,
-                      preceding_attrs: Vec<Attribute>)
-                      -> PResult<'a, GenericParam> {
-        let ident = self.parse_ident()?;
-
+    fn parse_ty_param_inner(
+        &mut self,
+        preceding_attrs: Vec<Attribute>,
+        ident: ast::Ident,
+    ) -> PResult<'a, GenericParam> {
         // Parse optional colon and param bounds.
         let bounds = if self.eat(&token::Colon) {
             self.parse_generic_bounds(Some(self.prev_span))?
@@ -5864,6 +5863,47 @@ impl<'a> Parser<'a> {
         })
     }
 
+    /// Matches `typaram = IDENT (`?` unbound)? optbounds ( EQ ty )?`.
+    fn parse_ty_param(
+        &mut self,
+        preceding_attrs: Vec<Attribute>,
+    ) -> PResult<'a, GenericParam> {
+        let ident = self.parse_ident()?;
+        self.parse_ty_param_inner(preceding_attrs, ident)
+    }
+
+    /// Parse an arbitrary path and suggest moving it to the `where` clause (#26271)
+    fn parse_bad_ty_param(
+        &mut self,
+        preceding_attrs: Vec<Attribute>,
+    ) -> PResult<'a, bool /* break */> {
+        let snapshot = self.clone();
+        Ok(match self.parse_qpath(PathStyle::Type)
+            .and_then(|(_qpath, path)| self.parse_ty_param_inner(
+                preceding_attrs,
+                ast::Ident::from_str(&format!("{}", path)),
+            ).map(|_| path.span.to(self.prev_span)))
+        {   // We discard the generated type param, we only want to confirm that it is otherwise
+            // well-formed. If it isn't, we revert back to normal behavior and emit a non-targeted
+            // parse error. If it *is*, we *don't* add to the type param list to avoid having
+            // spurious "unused type param" errors.
+            Ok(span) => {
+                self.struct_span_err(
+                    span,
+                    "qualified paths are not allowed in generic parameters",
+                )
+                    .note("move this constraint to the `where` clause")
+                    .emit();
+                false
+            }
+            Err(mut err) => {
+                err.cancel();
+                mem::replace(self, snapshot);
+                true
+            }
+        })
+    }
+
     /// Parses the following grammar:
     ///
     ///     TraitItemAssocTy = Ident ["<"...">"] [":" [GenericBounds]] ["where" ...] ["=" Ty]
@@ -5934,13 +5974,18 @@ impl<'a> Parser<'a> {
             } else if self.check_ident() {
                 // Parse type parameter.
                 params.push(self.parse_ty_param(attrs)?);
+            } else if self.token == token::Lt {
+                self.bump() ; // `<`
+                if self.parse_bad_ty_param(attrs)? {
+                    break
+                }
             } else {
                 // Check for trailing attributes and stop parsing.
                 if !attrs.is_empty() {
                     if !params.is_empty() {
                         self.struct_span_err(
                             attrs[0].span,
-                            &format!("trailing attribute after generic parameter"),
+                            "trailing attribute after generic parameter",
                         )
                         .span_label(attrs[0].span, "attributes must go before parameters")
                         .emit();

src/test/ui/parser/qpath-in-type-args.rs

@@ -0,0 +1,11 @@
+trait Tr {
+    type TrSubtype;
+}
+
+struct Bar<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a> {
+//~^ ERROR qualified paths are not allowed in generic parameters
+    item: Item,
+    item_sub: &'a <Item as Tr>::TrSubtype,
+}
+
+fn main() {}

src/test/ui/parser/qpath-in-type-args.stderr

@@ -0,0 +1,10 @@
+error: qualified paths are not allowed in generic parameters
+  --> $DIR/qpath-in-type-args.rs:5:26
+   |
+LL | struct Bar<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a> {
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: move this constraint to the `where` clause
+
+error: aborting due to previous error
+