diff --git a/Cargo.lock b/Cargo.lock
index f8fa2971b49..e058d0b2127 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -997,6 +997,16 @@ dependencies = [
  "yaml-rust 0.4.4",
 ]
 
+[[package]]
+name = "expect-test"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a3e383741ea1982866572109d1a8c807bd36aad91fca701489fdca56ef92b3b8"
+dependencies = [
+ "difference",
+ "once_cell",
+]
+
 [[package]]
 name = "failure"
 version = "0.1.8"
@@ -3642,6 +3652,7 @@ dependencies = [
 name = "rustc_lexer"
 version = "0.1.0"
 dependencies = [
+ "expect-test",
  "unicode-xid",
 ]
 
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index ac833a55d4c..afa72b5d58c 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -1754,6 +1754,11 @@ impl Step for Crate {
             cargo.arg("--quiet");
         }
 
+        if builder.config.cmd.bless() {
+            // Bless `expect!` tests.
+            cargo.env("UPDATE_EXPECT", "1");
+        }
+
         if target.contains("emscripten") {
             cargo.env(
                 format!("CARGO_TARGET_{}_RUNNER", envify(&target.triple)),
diff --git a/src/librustc_lexer/Cargo.toml b/src/librustc_lexer/Cargo.toml
index 950771f0a69..28b56f6fef4 100644
--- a/src/librustc_lexer/Cargo.toml
+++ b/src/librustc_lexer/Cargo.toml
@@ -19,3 +19,6 @@ name = "rustc_lexer"
 # Note that this crate purposefully does not depend on other rustc crates
 [dependencies]
 unicode-xid = "0.2.0"
+
+[dev-dependencies]
+expect-test = "0.1"
diff --git a/src/librustc_lexer/src/lib.rs b/src/librustc_lexer/src/lib.rs
index 2d80ca5a4de..b7d6194cd77 100644
--- a/src/librustc_lexer/src/lib.rs
+++ b/src/librustc_lexer/src/lib.rs
@@ -35,6 +35,7 @@ use std::convert::TryFrom;
 /// Parsed token.
 /// It doesn't contain information about data that has been parsed,
 /// only the type of the token and its size.
+#[derive(Debug)]
 pub struct Token {
     pub kind: TokenKind,
     pub len: usize,
diff --git a/src/librustc_lexer/src/tests.rs b/src/librustc_lexer/src/tests.rs
index b0209ac2899..a1ea5ceb1f6 100644
--- a/src/librustc_lexer/src/tests.rs
+++ b/src/librustc_lexer/src/tests.rs
@@ -1,5 +1,7 @@
 use super::*;
 
+use expect_test::{expect, Expect};
+
 fn check_raw_str(s: &str, expected_hashes: u16, expected_err: Option<RawStrError>) {
     let s = &format!("r{}", s);
     let mut cursor = Cursor::new(s);
@@ -120,3 +122,46 @@ fn test_shebang_followed_by_attrib() {
     let input = "#!/bin/rust-scripts\n#![allow_unused(true)]";
     assert_eq!(strip_shebang(input), Some(19));
 }
+
+fn check_lexing(src: &str, expect: Expect) {
+    let actual: String = tokenize(src).map(|token| format!("{:?}\n", token)).collect();
+    expect.assert_eq(&actual)
+}
+
+#[test]
+fn comment_flavors() {
+    check_lexing(
+        r"
+// line
+//// line as well
+/// outer doc line
+//! inner doc line
+/* block */
+/**/
+/*** also block */
+/** outer doc block */
+/*! inner doc block */
+",
+        expect![[r#"
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: LineComment { doc_style: None }, len: 7 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: LineComment { doc_style: None }, len: 17 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: LineComment { doc_style: Some(Outer) }, len: 18 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: LineComment { doc_style: Some(Inner) }, len: 18 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: BlockComment { doc_style: None, terminated: true }, len: 11 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: BlockComment { doc_style: None, terminated: true }, len: 4 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: BlockComment { doc_style: None, terminated: true }, len: 18 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: BlockComment { doc_style: Some(Outer), terminated: true }, len: 22 }
+                Token { kind: Whitespace, len: 1 }
+                Token { kind: BlockComment { doc_style: Some(Inner), terminated: true }, len: 22 }
+                Token { kind: Whitespace, len: 1 }
+            "#]],
+    )
+}
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index 4f98944e4c8..af3fb403703 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -85,11 +85,13 @@ const PERMITTED_DEPENDENCIES: &[&str] = &[
     "crossbeam-queue",
     "crossbeam-utils",
     "datafrog",
+    "difference",
     "digest",
     "dlmalloc",
     "either",
     "ena",
     "env_logger",
+    "expect-test",
     "fake-simd",
     "filetime",
     "flate2",