suggest using double colon when using single colon in struct field type path

compiler/rustc_parse/src/parser/item.rs

@@ -1534,6 +1534,16 @@ impl<'a> Parser<'a> {
         let name = self.parse_field_ident(adt_ty, lo)?;
         self.expect_field_ty_separator()?;
         let ty = self.parse_ty()?;
+        if self.token.kind == token::Colon && self.look_ahead(1, |tok| tok.kind != token::Colon) {
+            self.struct_span_err(self.token.span, "found single colon in a struct field type path")
+                .span_suggestion_verbose(
+                    self.token.span,
+                    "maybe you meant to write a path separator here",
+                    "::".to_string(),
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
+        }
         if self.token.kind == token::Eq {
             self.bump();
             let const_expr = self.parse_anon_const_expr()?;

src/test/ui/suggestions/sturct-field-type-including-single-colon.rs

@@ -0,0 +1,20 @@
+mod foo {
+    struct A;
+    mod bar {
+        struct B;
+    }
+}
+
+struct Foo {
+    a: foo:A,
+    //~^ ERROR found single colon in a struct field type path
+    //~| expected `,`, or `}`, found `:`
+}
+
+struct Bar {
+    b: foo::bar:B,
+    //~^ ERROR found single colon in a struct field type path
+    //~| expected `,`, or `}`, found `:`
+}
+
+fn main() {}

src/test/ui/suggestions/sturct-field-type-including-single-colon.stderr

@@ -0,0 +1,36 @@
+error: found single colon in a struct field type path
+  --> $DIR/sturct-field-type-including-single-colon.rs:9:11
+   |
+LL |     a: foo:A,
+   |           ^
+   |
+help: maybe you meant to write a path separator here
+   |
+LL |     a: foo::A,
+   |           ~~
+
+error: expected `,`, or `}`, found `:`
+  --> $DIR/sturct-field-type-including-single-colon.rs:9:11
+   |
+LL |     a: foo:A,
+   |           ^
+
+error: found single colon in a struct field type path
+  --> $DIR/sturct-field-type-including-single-colon.rs:15:16
+   |
+LL |     b: foo::bar:B,
+   |                ^
+   |
+help: maybe you meant to write a path separator here
+   |
+LL |     b: foo::bar::B,
+   |                ~~
+
+error: expected `,`, or `}`, found `:`
+  --> $DIR/sturct-field-type-including-single-colon.rs:15:16
+   |
+LL |     b: foo::bar:B,
+   |                ^
+
+error: aborting due to 4 previous errors
+