add .stderr file

Author: nikomatsakis

src/test/ui/issues/issue-57979.stderr

@@ -0,0 +1,17 @@
+error[E0666]: nested `impl Trait` is not allowed
+  --> $DIR/issue-57979.rs:39:61
+   |
+LL | pub fn collect(_: impl IntoIterator<Item = impl Borrow<Data<impl AsRef<[u8]>>>>) {
+   |                                            -----------------^^^^^^^^^^^^^^^^--
+   |                                            |                |
+   |                                            |                nested `impl Trait` here
+   |                                            outer `impl Trait`
+
+error[E0601]: `main` function not found in crate `issue_57979`
+   |
+   = note: consider adding a `main` function to `$DIR/issue-57979.rs`
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0601, E0666.
+For more information about an error, try `rustc --explain E0601`.