diff --git a/src/tools/tidy/src/ui_tests.rs b/src/tools/tidy/src/ui_tests.rs
index f7fec2e667ab..bda58bc09f77 100644
--- a/src/tools/tidy/src/ui_tests.rs
+++ b/src/tools/tidy/src/ui_tests.rs
@@ -12,14 +12,41 @@
 
 use std::path::Path;
 
+// See rust-lang/rust#48879: In addition to the mapping from `foo.rs`
+// to `foo.stderr`/`foo.stdout`, we also can optionally have
+// `foo.$mode.stderr`, where $mode is one of the strings on this list,
+// as an alternative to use when running under that mode.
+static COMPARE_MODE_NAMES: [&'static str; 1] = ["nll"];
+
 pub fn check(path: &Path, bad: &mut bool) {
     super::walk_many(&[&path.join("test/ui"), &path.join("test/ui-fulldeps")],
                      &mut |_| false,
                      &mut |file_path| {
         if let Some(ext) = file_path.extension() {
             if (ext == "stderr" || ext == "stdout") && !file_path.with_extension("rs").exists() {
-                println!("Stray file with UI testing output: {:?}", file_path);
-                *bad = true;
+
+                // rust-lang/rust#48879: this fn used to be beautful
+                // because Path API special-cases replacing
+                // extensions. That works great for ".stderr" but not
+                // so well for ".nll.stderr". To support the latter,
+                // we explicitly search backwards for mode's starting
+                // point and build corresponding source name.
+                let filename = file_path.file_name().expect("need filename")
+                    .to_str().expect("need UTF-8 filename");
+                let found_matching_prefix = COMPARE_MODE_NAMES.iter().any(|mode| {
+                    if let Some(r_idx) = filename.rfind(&format!(".{}", mode)) {
+                        let source_name = format!("{}.rs", &filename[0..r_idx]);
+                        let source_path = file_path.with_file_name(source_name);
+                        source_path.exists()
+                    } else {
+                        false
+                    }
+                });
+
+                if !found_matching_prefix {
+                    println!("Stray file with UI testing output: {:?}", file_path);
+                    *bad = true;
+                }
             }
         }
     });