Auto merge of #109999 - m-ou-se:flatten-format-args, r=oli-obk

Enable flatten-format-args by default.

Part of #99012.

This enables the `flatten-format-args` feature that was added by #106824:

> This change inlines string literals, integer literals and nested format_args!() into format_args!() during ast lowering, making all of the following pairs result in equivalent hir:
>
> ```rust
> println!("Hello, {}!", "World");
> println!("Hello, World!");
> ```
>
> ```rust
> println!("[info] {}", format_args!("error"));
> println!("[info] error");
> ```
>
> ```rust
> println!("[{}] {}", status, format_args!("error: {}", msg));
> println!("[{}] error: {}", status, msg);
> ```
>
> ```rust
> println!("{} + {} = {}", 1, 2, 1 + 2);
> println!("1 + 2 = {}", 1 + 2);
> ```
>
> And so on.
>
> This is useful for macros. E.g. a `log::info!()` macro could just pass the tokens from the user directly into a `format_args!()` that gets efficiently flattened/inlined into a `format_args!("info: {}")`.
>
> It also means that `dbg!(x)` will have its file, line, and expression name inlined:
>
> ```rust
> eprintln!("[{}:{}] {} = {:#?}", file!(), line!(), stringify!(x), x); // before
> eprintln!("[example.rs:1] x = {:#?}", x); // after
> ```
>
> Which can be nice in some cases, but also means a lot more unique static strings than before if dbg!() is used a lot.

This is mostly an optimization, except that it will be visible through [`fmt::Arguments::as_str()`](https://doc.rust-lang.org/nightly/std/fmt/struct.Arguments.html#method.as_str).

In #106823, there was already a libs-api FCP about the documentation of `fmt::Arguments::as_str()` to allow it to give `Some` rather than `None` depending on optimizations like this. That was just a documentation update though. This PR is the one that actually makes the user visible change:

```rust
assert_eq!(format_args!("abc").as_str(), Some("abc")); // Unchanged.
assert_eq!(format_args!("ab{}", "c").as_str(), Some("abc")); // Was `None` before!
```

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;
     }
 }