Rollup merge of #123940 - kornelski:remove-derived-debug, r=Urgau

debug-fmt-detail option

I'd like to propose a new option that makes `#[derive(Debug)]` generate no-op implementations that don't print anything, and makes `{:?}` in format strings a no-op.

There are a couple of motivations for this:

1. A more thorough stripping of debug symbols. Binaries stripped of debug symbols still retain some of them through `Debug` implementations. It's hard to avoid that without compiler's help, because debug formatting can be used in many places, including dependencies, and their loggers, asserts, panics, etc.
   * In my testing it gives about 2% binary size reduction on top of all other binary-minimizing best practices (including `panic_immediate_abort`). There are targets like Web WASM or embedded where users pay attention to binary sizes.
   * Users distributing closed-source binaries may not want to "leak" any symbol names as a matter of principle.
2. Adds ability to test whether code depends on specifics of the `Debug` format implementation in unwise ways (e.g. trying to get data unavailable via public interface, or using it as a serialization format). Because current Rust's debug implementation doesn't change, there's a risk of it becoming a fragile de-facto API that [won't be possible to change in the future](https://www.hyrumslaw.com/). An option that "breaks" it can act as a [grease](https://www.rfc-editor.org/rfc/rfc8701.html).

This implementation is a `-Z fmt-debug=opt` flag that takes:

* `full` — the default, current state.
* `none` — makes derived `Debug` and `{:?}` no-ops. Explicit `impl Debug for T` implementations are left unharmed, but `{:?}` format won't use them, so they may get dead-code eliminated if they aren't invoked directly.
* `shallow` — makes derived `Debug` print only the type's name, without recursing into fields. Fieldless enums print their variant names. `{:?}` works.

The `shallow` option is a compromise between minimizing the `Debug` code, and compatibility. There are popular proc-macro crates that use `Debug::fmt` as a way to convert enum values into their Rust source code.

There's a corresponding `cfg` flag: `#[cfg(fmt_debug = "none")]` that can be used in user code to react to this setting to minimize custom `Debug` implementations or remove unnecessary formatting helper functions.

compiler/rustc_feature/src/unstable.rs

@@ -471,6 +471,8 @@ declare_features! (
     (unstable, ffi_const, "1.45.0", Some(58328)),
     /// Allows the use of `#[ffi_pure]` on foreign functions.
     (unstable, ffi_pure, "1.45.0", Some(58329)),
+    /// Controlling the behavior of fmt::Debug
+    (unstable, fmt_debug, "CURRENT_RUSTC_VERSION", Some(129709)),
     /// Allows using `#[repr(align(...))]` on function items
     (unstable, fn_align, "1.53.0", Some(82232)),
     /// Support delegating implementation of functions to other already implemented functions.