Enable flatten-format-args by default.

compiler/rustc_interface/src/tests.rs

@@ -747,7 +747,7 @@ 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!(flatten_format_args, false);
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));
     tracked!(function_sections, Some(false));

compiler/rustc_session/src/options.rs

@@ -1452,9 +1452,9 @@ 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_format_args: bool = (true, parse_bool, [TRACKED],
         "flatten nested format_args!() and literals into a simplified format_args!() call \
-        (default: no)"),
+        (default: yes)"),
     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],

src/tools/clippy/src/driver.rs

@@ -160,6 +160,9 @@ impl rustc_driver::Callbacks for ClippyCallbacks {
         // MIR passes can be enabled / disabled separately, we should figure out, what passes to
         // use for Clippy.
         config.opts.unstable_opts.mir_opt_level = Some(0);
+
+        // Disable flattening and inlining of format_args!(), so the HIR matches with the AST.
+        config.opts.unstable_opts.flatten_format_args = false;
     }
 }