Overhaul to_readable_str.

It's a function that prints numbers with underscores inserted for
readability (e.g. "1_234_567"), used by `-Zmeta-stats` and
`-Zinput-stats`. It's the only thing in `rustc_middle::util::common`,
which is a bizarre location for it.

This commit:
- moves it to `rustc_data_structures`, a more logical crate for it;
- puts it in a module `thousands`, like the similar crates.io crate;
- renames it `format_with_underscores`, which is a clearer name;
- rewrites it to be more concise;
- slightly improves the testing.

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;

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
+}

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));
+}