diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index cfbf233297c..919badfa4aa 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -1322,6 +1322,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
          epoch). Crates compiled with different epochs can be linked together."),
     run_dsymutil: Option<bool> = (None, parse_opt_bool, [TRACKED],
           "run `dsymutil` and delete intermediate object files"),
+    ui_testing: bool = (false, parse_bool, [UNTRACKED],
+          "format compiler diagnostics in a way that's better suitable for UI testing"),
 }
 
 pub fn default_lib_output() -> CrateType {
diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 9d7a9acc3d5..ed89bf0d806 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -919,11 +919,13 @@ pub fn build_session_with_codemap(sopts: config::Options,
         }
         (config::ErrorOutputType::Json(pretty), None) => {
             Box::new(JsonEmitter::stderr(Some(registry), codemap.clone(),
-                     pretty, sopts.debugging_opts.approximate_suggestions))
+                     pretty, sopts.debugging_opts.approximate_suggestions)
+                     .ui_testing(sopts.debugging_opts.ui_testing))
         }
         (config::ErrorOutputType::Json(pretty), Some(dst)) => {
             Box::new(JsonEmitter::new(dst, Some(registry), codemap.clone(),
-                     pretty, sopts.debugging_opts.approximate_suggestions))
+                     pretty, sopts.debugging_opts.approximate_suggestions)
+                     .ui_testing(sopts.debugging_opts.ui_testing))
         }
         (config::ErrorOutputType::Short(color_config), None) => {
             Box::new(EmitterWriter::stderr(color_config, Some(codemap.clone()), true, false))
diff --git a/src/librustc_errors/emitter.rs b/src/librustc_errors/emitter.rs
index 86e77d404ff..31c1ddca794 100644
--- a/src/librustc_errors/emitter.rs
+++ b/src/librustc_errors/emitter.rs
@@ -25,6 +25,8 @@ use std::collections::{HashMap, HashSet};
 use std::cmp::min;
 use unicode_width;
 
+const ANONYMIZED_LINE_NUM: &str = "LL";
+
 /// Emitter trait for emitting errors.
 pub trait Emitter {
     /// Emit a structured diagnostic.
@@ -108,6 +110,7 @@ pub struct EmitterWriter {
     short_message: bool,
     teach: bool,
     error_codes: HashSet<String>,
+    ui_testing: bool,
 }
 
 struct FileWithAnnotatedLines {
@@ -157,6 +160,7 @@ impl EmitterWriter {
                 short_message,
                 teach,
                 error_codes: HashSet::new(),
+                ui_testing: false,
             }
         } else {
             EmitterWriter {
@@ -165,6 +169,7 @@ impl EmitterWriter {
                 short_message,
                 teach,
                 error_codes: HashSet::new(),
+                ui_testing: false,
             }
         }
     }
@@ -180,9 +185,14 @@ impl EmitterWriter {
             short_message,
             teach,
             error_codes: HashSet::new(),
+            ui_testing: false,
         }
     }
 
+    pub fn ui_testing(self, ui_testing: bool) -> Self {
+        Self { ui_testing, ..self }
+    }
+
     fn preprocess_annotations(&mut self, msp: &MultiSpan) -> Vec<FileWithAnnotatedLines> {
         fn add_annotation_to_file(file_vec: &mut Vec<FileWithAnnotatedLines>,
                                   file: Rc<FileMap>,
@@ -334,9 +344,14 @@ impl EmitterWriter {
 
         // First create the source line we will highlight.
         buffer.puts(line_offset, code_offset, &source_string, Style::Quotation);
+        let line_index = if self.ui_testing {
+            ANONYMIZED_LINE_NUM.to_string()
+        } else {
+            line.line_index.to_string()
+        };
         buffer.puts(line_offset,
                     0,
-                    &(line.line_index.to_string()),
+                    &line_index,
                     Style::LineNumber);
 
         draw_col_separator(buffer, line_offset, width_offset - 2);
@@ -1288,8 +1303,11 @@ impl EmitterWriter {
                              span: &MultiSpan,
                              children: &Vec<SubDiagnostic>,
                              suggestions: &[CodeSuggestion]) {
-        let max_line_num = self.get_max_line_num(span, children);
-        let max_line_num_len = max_line_num.to_string().len();
+        let max_line_num_len = if self.ui_testing {
+            ANONYMIZED_LINE_NUM.len()
+        } else {
+            self.get_max_line_num(span, children).to_string().len()
+        };
 
         match self.emit_message_default(span,
                                         message,
diff --git a/src/libsyntax/json.rs b/src/libsyntax/json.rs
index 98d5fa8f797..57f07ff33f5 100644
--- a/src/libsyntax/json.rs
+++ b/src/libsyntax/json.rs
@@ -40,6 +40,7 @@ pub struct JsonEmitter {
     pretty: bool,
     /// Whether "approximate suggestions" are enabled in the config
     approximate_suggestions: bool,
+    ui_testing: bool,
 }
 
 impl JsonEmitter {
@@ -53,6 +54,7 @@ impl JsonEmitter {
             cm: code_map,
             pretty,
             approximate_suggestions,
+            ui_testing: false,
         }
     }
 
@@ -73,8 +75,13 @@ impl JsonEmitter {
             cm: code_map,
             pretty,
             approximate_suggestions,
+            ui_testing: false,
         }
     }
+
+    pub fn ui_testing(self, ui_testing: bool) -> Self {
+        Self { ui_testing, ..self }
+    }
 }
 
 impl Emitter for JsonEmitter {
@@ -199,7 +206,8 @@ impl Diagnostic {
         }
         let buf = BufWriter::default();
         let output = buf.clone();
-        EmitterWriter::new(Box::new(buf), Some(je.cm.clone()), false, false).emit(db);
+        EmitterWriter::new(Box::new(buf), Some(je.cm.clone()), false, false)
+            .ui_testing(je.ui_testing).emit(db);
         let output = Arc::try_unwrap(output.0).unwrap().into_inner().unwrap();
         let output = String::from_utf8(output).unwrap();
 
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index e5bee56de80..3d751de3dbe 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -1627,15 +1627,12 @@ impl<'test> TestCx<'test> {
                     rustc.args(&["--error-format", "json"]);
                 }
             }
-            Ui => if !self.props
-                .compile_flags
-                .iter()
-                .any(|s| s.starts_with("--error-format"))
-            {
+            Ui => {
                 // In case no "--error-format" has been given in the test, we'll compile
                 // a first time to get the compiler's output then compile with
                 // "--error-format json" to check if all expected errors are actually there
                 // and that no new one appeared.
+                rustc.arg("-Zui-testing");
             }
             MirOpt => {
                 rustc.args(&[