Auto merge of #122102 - Urgau:optimize-symbol-integer, r=cjgillot

Optimize `Symbol::integer` by utilizing in-place formatting

This PR optimize `Symbol::integer` by utilizing `itoa` in-place formatting instead of going through a dynamically allocated `String` and the format machinery.

<details>

For some context: I was profiling `rustc --check-cfg` with callgrind and due to the way we currently setup all the targets and we end-up calling `Symbol::integer` multiple times for all the targets. Using `itoa` reduced the number of instructions.

</details>

1 files changed, 4 insertions, 2 deletions

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 9dadd471247..80c4dcb6b18 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -2327,13 +2327,15 @@ pub mod sym {
     ///
     /// The first few non-negative integers each have a static symbol and therefore
     /// are fast.
-    pub fn integer<N: TryInto<usize> + Copy + ToString>(n: N) -> Symbol {
+    pub fn integer<N: TryInto<usize> + Copy + itoa::Integer>(n: N) -> Symbol {
         if let Result::Ok(idx) = n.try_into() {
             if idx < 10 {
                 return Symbol::new(super::SYMBOL_DIGITS_BASE + idx as u32);
             }
         }
-        Symbol::intern(&n.to_string())
+        let mut buffer = itoa::Buffer::new();
+        let printed = buffer.format(n);
+        Symbol::intern(printed)
     }
 }