hotfix: add help dialog for PermissionExt

Author: chansuke

clippy_lints/src/permissions_set_readonly_false.rs

@@ -1,4 +1,4 @@
-use clippy_utils::diagnostics::span_lint_and_note;
+use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::paths;
 use clippy_utils::ty::match_type;
 use rustc_ast::ast::LitKind;
@@ -36,13 +36,16 @@ impl<'tcx> LateLintPass<'tcx> for PermissionsSetReadonlyFalse {
                 && let ExprKind::Lit(lit) = &arg.kind
                 && LitKind::Bool(false) == lit.node
         {
-                span_lint_and_note(
+                span_lint_and_then(
                     cx,
                     PERMISSIONS_SET_READONLY_FALSE,
                     expr.span,
                     "call to `set_readonly` with argument `false`",
-                    None,
-                    "on Unix platforms this results in the file being world writable",
+                    |diag| {
+                        diag.note("on Unix platforms this results in the file being world writable");
+                        diag.help("`set_readonly(false)` makes the world-writable on Unix. You can avoid this issue with `PermissionsExt`. For more imformation, see\n\
+                            https://doc.rust-lang.org/std/os/unix/fs/trait.PermissionsExt.html");
+                    }
                 );
         }
     }

tests/ui/permissions_set_readonly_false.stderr

@@ -5,6 +5,8 @@ LL |     permissions.set_readonly(false);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: on Unix platforms this results in the file being world writable
+   = help: `set_readonly(false)` makes the world-writable on Unix. You can avoid this issue with `PermissionsExt`. For more imformation, see
+           https://doc.rust-lang.org/std/os/unix/fs/trait.PermissionsExt.html
    = note: `-D clippy::permissions-set-readonly-false` implied by `-D warnings`
 
 error: aborting due to previous error