diff --git a/Cargo.lock b/Cargo.lock
index 7e03474565d85..365d14c579bdb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -431,6 +431,7 @@ dependencies = [
  "serde 1.0.81 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 1.0.81 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_json 1.0.33 (registry+https://github.com/rust-lang/crates.io-index)",
+ "walkdir 2.2.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "winapi 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
diff --git a/src/tools/compiletest/Cargo.toml b/src/tools/compiletest/Cargo.toml
index 0ee3143935897..edf4aeab1c70f 100644
--- a/src/tools/compiletest/Cargo.toml
+++ b/src/tools/compiletest/Cargo.toml
@@ -15,6 +15,7 @@ serde_json = "1.0"
 serde_derive = "1.0"
 rustfix = "0.4.1"
 lazy_static = "1.0"
+walkdir = "2"
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"
diff --git a/src/tools/compiletest/src/main.rs b/src/tools/compiletest/src/main.rs
index 9aefd15765df6..a532e900ea3e0 100644
--- a/src/tools/compiletest/src/main.rs
+++ b/src/tools/compiletest/src/main.rs
@@ -28,6 +28,7 @@ extern crate serde_derive;
 extern crate serde_json;
 extern crate test;
 extern crate rustfix;
+extern crate walkdir;
 
 use common::CompareMode;
 use common::{expected_output_path, output_base_dir, output_relative_path, UI_EXTENSIONS};
@@ -43,6 +44,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 use test::ColorConfig;
 use util::logv;
+use walkdir::WalkDir;
 
 use self::header::{EarlyProps, Ignore};
 
@@ -682,6 +684,15 @@ fn stamp(config: &Config, testpaths: &TestPaths, revision: Option<&str>) -> Path
     output_base_dir(config, testpaths, revision).join("stamp")
 }
 
+/// Return an iterator over timestamps of files in the directory at `path`.
+fn collect_timestamps(path: &PathBuf) -> impl Iterator<Item=FileTime> {
+    WalkDir::new(path)
+        .into_iter()
+        .map(|entry| entry.unwrap())
+        .filter(|entry| entry.metadata().unwrap().is_file())
+        .map(|entry| mtime(entry.path()))
+}
+
 fn up_to_date(
     config: &Config,
     testpaths: &TestPaths,
@@ -725,16 +736,7 @@ fn up_to_date(
     for pretty_printer_file in &pretty_printer_files {
         inputs.push(mtime(&rust_src_dir.join(pretty_printer_file)));
     }
-    let mut entries = config.run_lib_path.read_dir().unwrap().collect::<Vec<_>>();
-    while let Some(entry) = entries.pop() {
-        let entry = entry.unwrap();
-        let path = entry.path();
-        if entry.metadata().unwrap().is_file() {
-            inputs.push(mtime(&path));
-        } else {
-            entries.extend(path.read_dir().unwrap());
-        }
-    }
+    inputs.extend(collect_timestamps(&config.run_lib_path));
     if let Some(ref rustdoc_path) = config.rustdoc_path {
         inputs.push(mtime(&rustdoc_path));
         inputs.push(mtime(&rust_src_dir.join("src/etc/htmldocck.py")));
@@ -746,6 +748,9 @@ fn up_to_date(
         inputs.push(mtime(path));
     }
 
+    // Compiletest itself.
+    inputs.extend(collect_timestamps(&rust_src_dir.join("src/tools/compiletest/")));
+
     inputs.iter().any(|input| *input > stamp)
 }