3 files changed, 31 insertions, 0 deletions
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 65d586124b3..6ef73debadd 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -76,6 +76,7 @@ pub mod sync;
 pub mod tagged_ptr;
 pub mod temp_dir;
 pub mod thinvec;
+pub mod thousands;
 pub mod transitive_relation;
 pub mod unhash;
 pub mod unord;
diff --git a/compiler/rustc_data_structures/src/thousands/mod.rs b/compiler/rustc_data_structures/src/thousands/mod.rs
new file mode 100644
index 00000000000..e7ab7ec2932
--- /dev/null
+++ b/compiler/rustc_data_structures/src/thousands/mod.rs
@@ -0,0 +1,16 @@
+//! This is an extremely bare-bones alternative to the `thousands` crate on
+//! crates.io, for printing large numbers in a readable fashion.
+
+#[cfg(test)]
+mod tests;
+
+// Converts the number to a string, with underscores as the thousands separator.
+pub fn format_with_underscores(n: usize) -> String {
+    let mut s = n.to_string();
+    let mut i = s.len();
+    while i > 3 {
+        i -= 3;
+        s.insert(i, '_');
+    }
+    s
+}
diff --git a/compiler/rustc_data_structures/src/thousands/tests.rs b/compiler/rustc_data_structures/src/thousands/tests.rs
new file mode 100644
index 00000000000..906605d9a93
--- /dev/null
+++ b/compiler/rustc_data_structures/src/thousands/tests.rs
@@ -0,0 +1,14 @@
+use super::*;
+
+#[test]
+fn test_format_with_underscores() {
+    assert_eq!("0", format_with_underscores(0));
+    assert_eq!("1", format_with_underscores(1));
+    assert_eq!("99", format_with_underscores(99));
+    assert_eq!("345", format_with_underscores(345));
+    assert_eq!("1_000", format_with_underscores(1_000));
+    assert_eq!("12_001", format_with_underscores(12_001));
+    assert_eq!("999_999", format_with_underscores(999_999));
+    assert_eq!("1_000_000", format_with_underscores(1_000_000));
+    assert_eq!("12_345_678", format_with_underscores(12_345_678));
+}