Rollup merge of #47978 - eddyb:iu, r=kennytm

ui tests: diff from old (expected) to new (actual) instead of backwards.

Previously `actual` was "old" and `expected` was "new" which resulted in `+` before `-`.
AFAIK all diff tools put `-` before `+`, which made the previous behavior *very confusing*.

r? @nikomatsakis

src/tools/compiletest/src/runtest.rs

@@ -79,7 +79,7 @@ pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Misma
     let mut results = Vec::new();
     let mut mismatch = Mismatch::new(0);
 
-    for result in diff::lines(actual, expected) {
+    for result in diff::lines(expected, actual) {
         match result {
             diff::Result::Left(str) => {
                 if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
@@ -91,7 +91,8 @@ pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Misma
                     mismatch.lines.push(DiffLine::Context(line.to_owned()));
                 }
 
-                mismatch.lines.push(DiffLine::Resulting(str.to_owned()));
+                mismatch.lines.push(DiffLine::Expected(str.to_owned()));
+                line_number += 1;
                 lines_since_mismatch = 0;
             }
             diff::Result::Right(str) => {
@@ -104,8 +105,7 @@ pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Misma
                     mismatch.lines.push(DiffLine::Context(line.to_owned()));
                 }
 
-                mismatch.lines.push(DiffLine::Expected(str.to_owned()));
-                line_number += 1;
+                mismatch.lines.push(DiffLine::Resulting(str.to_owned()));
                 lines_since_mismatch = 0;
             }
             diff::Result::Both(str, _) => {