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>

Cargo.lock

@@ -4572,6 +4572,7 @@ name = "rustc_span"
 version = "0.0.0"
 dependencies = [
  "indexmap",
+ "itoa",
  "md-5",
  "rustc_arena",
  "rustc_data_structures",

compiler/rustc_span/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 [dependencies]
 # tidy-alphabetical-start
 indexmap = { version = "2.0.0" }
+itoa = "1.0"
 md5 = { package = "md-5", version = "0.10.0" }
 rustc_arena = { path = "../rustc_arena" }
 rustc_data_structures = { path = "../rustc_data_structures" }

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)
     }
 }