diff --git a/src/tools/compiletest/src/read2.rs b/src/tools/compiletest/src/read2.rs
index 897b9dd400793..d26678ecbeb0d 100644
--- a/src/tools/compiletest/src/read2.rs
+++ b/src/tools/compiletest/src/read2.rs
@@ -5,7 +5,7 @@ pub use self::imp::read2;
 use std::io;
 use std::process::{Child, Output};
 
-pub fn read2_abbreviated(mut child: Child) -> io::Result<Output> {
+pub fn read2_abbreviated(mut child: Child, allow_stderr_truncation: bool) -> io::Result<Output> {
     use io::Write;
     use std::mem::replace;
 
@@ -45,6 +45,17 @@ pub fn read2_abbreviated(mut child: Child) -> io::Result<Output> {
             *self = new_self;
         }
 
+        fn extend_no_truncate(&mut self, data: &[u8]) {
+            match *self {
+                ProcOutput::Full(ref mut bytes) => {
+                    bytes.extend_from_slice(data);
+                }
+                ProcOutput::Abbreviated { .. } => {
+                    panic!("must not use extend_no_truncate on abbreviated output")
+                }
+            };
+        }
+
         fn into_bytes(self) -> Vec<u8> {
             match self {
                 ProcOutput::Full(bytes) => bytes,
@@ -65,7 +76,15 @@ pub fn read2_abbreviated(mut child: Child) -> io::Result<Output> {
         child.stdout.take().unwrap(),
         child.stderr.take().unwrap(),
         &mut |is_stdout, data, _| {
-            if is_stdout { &mut stdout } else { &mut stderr }.extend(data);
+            if is_stdout {
+                stdout.extend(data);
+            } else {
+                if allow_stderr_truncation {
+                    stderr.extend(data);
+                } else {
+                    stderr.extend_no_truncate(data);
+                }
+            }
             data.clear();
         },
     )?;
diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index a59a0584d5e99..aebe43ec0b3c7 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -1769,8 +1769,10 @@ impl<'test> TestCx<'test> {
             child.stdin.as_mut().unwrap().write_all(input.as_bytes()).unwrap();
         }
 
+        // For Ui tests we parse the stderr as json, which breaks if the output gets truncated.
+        let allow_truncation = self.config.mode != Ui;
         let Output { status, stdout, stderr } =
-            read2_abbreviated(child).expect("failed to read output");
+            read2_abbreviated(child, allow_truncation).expect("failed to read output");
 
         let result = ProcRes {
             status,
@@ -2959,7 +2961,8 @@ impl<'test> TestCx<'test> {
             }
         }
 
-        let output = cmd.spawn().and_then(read2_abbreviated).expect("failed to spawn `make`");
+        let output =
+            cmd.spawn().and_then(|c| read2_abbreviated(c, true)).expect("failed to spawn `make`");
         if !output.status.success() {
             let res = ProcRes {
                 status: output.status,