Gate fmt args flattening behind -Zflatten-format-args.

Author: m-ou-se

compiler/rustc_ast_lowering/src/format.rs

@@ -16,8 +16,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // Never call the const constructor of `fmt::Arguments` if the
         // format_args!() had any arguments _before_ flattening/inlining.
         let allow_const = fmt.arguments.all_args().is_empty();
-        let fmt = flatten_format_args(Cow::Borrowed(fmt));
-        let fmt = inline_literals(fmt);
+        let mut fmt = Cow::Borrowed(fmt);
+        if self.tcx.sess.opts.unstable_opts.flatten_format_args {
+            fmt = flatten_format_args(fmt);
+            fmt = inline_literals(fmt);
+        }
         expand_format_args(self, sp, &fmt, allow_const)
     }
 }

compiler/rustc_interface/src/tests.rs

@@ -744,6 +744,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(emit_thin_lto, false);
     tracked!(export_executable_symbols, true);
     tracked!(fewer_names, Some(true));
+    tracked!(flatten_format_args, true);
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));
     tracked!(function_sections, Some(false));

compiler/rustc_session/src/options.rs

@@ -1422,6 +1422,9 @@ options! {
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
         (default: no)"),
+    flatten_format_args: bool = (false, parse_bool, [TRACKED],
+        "flatten nested format_args!() and literals into a simplified format_args!() call \
+        (default: no)"),
     force_unstable_if_unmarked: bool = (false, parse_bool, [TRACKED],
         "force all crates to be `rustc_private` unstable (default: no)"),
     fuel: Option<(String, u64)> = (None, parse_optimization_fuel, [TRACKED],