Slim rustc_parse_format dependencies down

`rustc_index` is only used for its size assertion macro, so demote it to a dev-dependency gated under testing instead

compiler/rustc_parse_format/Cargo.toml

@@ -8,5 +8,7 @@ edition = "2024"
 rustc_lexer = { path = "../rustc_lexer" }
 # tidy-alphabetical-end
 
-[target.'cfg(target_pointer_width = "64")'.dependencies]
+[target.'cfg(target_pointer_width = "64")'.dev-dependencies]
+# tidy-alphabetical-start
 rustc_index = { path = "../rustc_index", default-features = false }
+# tidy-alphabetical-end

compiler/rustc_parse_format/src/lib.rs

@@ -1105,7 +1105,7 @@ fn unescape_string(string: &str) -> Option<String> {
 }
 
 // Assert a reasonable size for `Piece`
-#[cfg(target_pointer_width = "64")]
+#[cfg(all(test, target_pointer_width = "64"))]
 rustc_index::static_assert_size!(Piece<'_>, 16);
 
 #[cfg(test)]