Print to stderr and bless

Author: phansch

src/librustc_errors/annotate_rs_emitter.rs

@@ -204,7 +204,10 @@ impl AnnotateRsEmitterWriter {
         if let Some(snippet) = converter.to_annotation_snippet() {
             let dl = DisplayList::from(snippet);
             let dlf = DisplayListFormatter::new(true);
-            print!("{}", dlf.format(&dl));
+            // FIXME(#59346): Figure out if we can _always_ print to stderr or not.
+            // `emitter.rs` has the `Destination` enum that lists various possible output
+            // destinations.
+            eprint!("{}", dlf.format(&dl));
         };
     }
 }

src/test/ui/annotate-snippet/missing-type.rs

@@ -1,5 +1,5 @@
 // compile-flags: --error-format human-annotate-rs
 
 pub fn main() {
-    let x: Iter;
+    let x: Iter; //~ ERROR cannot find type `Iter` in this scope
 }

src/test/ui/annotate-snippet/missing-type.stderr

@@ -0,0 +1,6 @@
+error[E0412]: cannot find type `Iter` in this scope
+ --> $DIR/missing-type.rs:4:11
+  |
+4 |     let x: Iter;
+  |            ^^^^ not found in this scope
+  |