remove the unused :: between trait and type to give user correct diag…

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2263,13 +2263,22 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                                     trait_impls.non_blanket_impls().len()
                                 )
                             };
-
+                            let mut suggestions = vec![(
+                                trait_path_segment.ident.span.shrink_to_lo(),
+                                format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())
+                            )];
+                            if let Some(generic_arg) = trait_path_segment.args {
+                                let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
+                                // get rid of :: between Trait and <type>
+                                // must be '::' between them, otherwise the parser won't accept the code
+                                suggestions.push((between_span, "".to_string(),));
+                                suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
+                            } else {
+                                suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
+                            }
                             err.multipart_suggestion(
                                 message,
-                                vec![
-                                    (trait_path_segment.ident.span.shrink_to_lo(), format!("<{} as ", self.tcx.def_path(impl_def_id).to_string_no_crate_verbose())),
-                                    (trait_path_segment.ident.span.shrink_to_hi(), format!(">"))
-                                ],
+                                suggestions,
                                 Applicability::MaybeIncorrect
                             );
                         }

src/test/ui/type/issue-101866.rs

@@ -0,0 +1,15 @@
+trait TraitA<T> {
+    fn func();
+}
+
+struct StructA {}
+
+impl TraitA<i32> for StructA {
+    fn func() {}
+}
+
+fn main() {
+    TraitA::<i32>::func();
+    //~^ ERROR: cannot call associated function on trait without specifying the corresponding `impl` type [E0790]
+    //~| help: use the fully-qualified path to the only available implementation
+}

src/test/ui/type/issue-101866.stderr

@@ -0,0 +1,18 @@
+error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
+  --> $DIR/issue-101866.rs:12:5
+   |
+LL |     fn func();
+   |     ---------- `TraitA::func` defined here
+...
+LL |     TraitA::<i32>::func();
+   |     ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
+   |
+help: use the fully-qualified path to the only available implementation
+   |
+LL -     TraitA::<i32>::func();
+LL +     <::StructA as TraitA<i32>>::func();
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0790`.