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_ast_lowering/src/format.rs

@@ -4,9 +4,10 @@
 use rustc_ast::visit::Visitor;
 use rustc_ast::*;
 use rustc_data_structures::fx::FxIndexMap;
+use rustc_hir as hir;
+use rustc_session::config::FmtDebug;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{sym, Span, Symbol};
-use {rustc_ast as ast, rustc_hir as hir};
 
 use super::LoweringContext;
 
@@ -243,7 +244,10 @@ fn make_argument<'hir>(
         hir::LangItem::FormatArgument,
         match ty {
             Format(Display) => sym::new_display,
-            Format(Debug) => sym::new_debug,
+            Format(Debug) => match ctx.tcx.sess.opts.unstable_opts.fmt_debug {
+                FmtDebug::Full | FmtDebug::Shallow => sym::new_debug,
+                FmtDebug::None => sym::new_debug_noop,
+            },
             Format(LowerExp) => sym::new_lower_exp,
             Format(UpperExp) => sym::new_upper_exp,
             Format(Octal) => sym::new_octal,

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -1,5 +1,6 @@
 use rustc_ast::{self as ast, EnumDef, MetaItem};
 use rustc_expand::base::{Annotatable, ExtCtxt};
+use rustc_session::config::FmtDebug;
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
 use thin_vec::{thin_vec, ThinVec};
@@ -49,6 +50,11 @@ fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) ->
     // We want to make sure we have the ctxt set so that we can use unstable methods
     let span = cx.with_def_site_ctxt(span);
 
+    let fmt_detail = cx.sess.opts.unstable_opts.fmt_debug;
+    if fmt_detail == FmtDebug::None {
+        return BlockOrExpr::new_expr(cx.expr_ok(span, cx.expr_tuple(span, ThinVec::new())));
+    }
+
     let (ident, vdata, fields) = match substr.fields {
         Struct(vdata, fields) => (substr.type_ident, *vdata, fields),
         EnumMatching(_, v, fields) => (v.ident, &v.data, fields),
@@ -61,6 +67,13 @@ fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) ->
     let name = cx.expr_str(span, ident.name);
     let fmt = substr.nonselflike_args[0].clone();
 
+    // Fieldless enums have been special-cased earlier
+    if fmt_detail == FmtDebug::Shallow {
+        let fn_path_write_str = cx.std_path(&[sym::fmt, sym::Formatter, sym::write_str]);
+        let expr = cx.expr_call_global(span, fn_path_write_str, thin_vec![fmt, name]);
+        return BlockOrExpr::new_expr(expr);
+    }
+
     // Struct and tuples are similar enough that we use the same code for both,
     // with some extra pieces for structs due to the field names.
     let (is_struct, args_per_field) = match vdata {

compiler/rustc_feature/src/builtin_attrs.rs

@@ -37,6 +37,8 @@ macro_rules! cfg_fn {
     (sym::relocation_model, sym::cfg_relocation_model, cfg_fn!(cfg_relocation_model)),
     (sym::sanitizer_cfi_generalize_pointers, sym::cfg_sanitizer_cfi, cfg_fn!(cfg_sanitizer_cfi)),
     (sym::sanitizer_cfi_normalize_integers, sym::cfg_sanitizer_cfi, cfg_fn!(cfg_sanitizer_cfi)),
+    // this is consistent with naming of the compiler flag it's for
+    (sym::fmt_debug, sym::fmt_debug, cfg_fn!(fmt_debug)),
 ];
 
 /// Find a gated cfg determined by the `pred`icate which is given the cfg's name.

compiler/rustc_feature/src/unstable.rs

@@ -471,6 +471,8 @@ pub fn internal(&self, feature: Symbol) -> bool {
     (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.

compiler/rustc_interface/src/tests.rs

@@ -10,11 +10,11 @@
 use rustc_session::config::{
     build_configuration, build_session_options, rustc_optgroups, BranchProtection, CFGuard, Cfg,
     CollapseMacroDebuginfo, CoverageLevel, CoverageOptions, DebugInfo, DumpMonoStatsFormat,
-    ErrorOutputType, ExternEntry, ExternLocation, Externs, FunctionReturn, InliningThreshold,
-    Input, InstrumentCoverage, InstrumentXRay, LinkSelfContained, LinkerPluginLto, LocationDetail,
-    LtoCli, NextSolverConfig, OomStrategy, Options, OutFileName, OutputType, OutputTypes, PAuthKey,
-    PacRet, Passes, PatchableFunctionEntry, Polonius, ProcMacroExecutionStrategy, Strip,
-    SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
+    ErrorOutputType, ExternEntry, ExternLocation, Externs, FmtDebug, FunctionReturn,
+    InliningThreshold, Input, InstrumentCoverage, InstrumentXRay, LinkSelfContained,
+    LinkerPluginLto, LocationDetail, LtoCli, NextSolverConfig, OomStrategy, Options, OutFileName,
+    OutputType, OutputTypes, PAuthKey, PacRet, Passes, PatchableFunctionEntry, Polonius,
+    ProcMacroExecutionStrategy, Strip, SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
 };
 use rustc_session::lint::Level;
 use rustc_session::search_paths::SearchPath;
@@ -780,6 +780,7 @@ macro_rules! tracked {
     tracked!(fewer_names, Some(true));
     tracked!(fixed_x18, true);
     tracked!(flatten_format_args, false);
+    tracked!(fmt_debug, FmtDebug::Shallow);
     tracked!(force_unstable_if_unmarked, true);
     tracked!(fuel, Some(("abc".to_string(), 99)));
     tracked!(function_return, FunctionReturn::ThunkExtern);

compiler/rustc_session/src/config.rs

@@ -22,7 +22,9 @@
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_span::edition::{Edition, DEFAULT_EDITION, EDITION_NAME_LIST, LATEST_STABLE_EDITION};
 use rustc_span::source_map::FilePathMapping;
-use rustc_span::{FileName, FileNameDisplayPreference, RealFileName, SourceFileHashAlgorithm};
+use rustc_span::{
+    sym, FileName, FileNameDisplayPreference, RealFileName, SourceFileHashAlgorithm, Symbol,
+};
 use rustc_target::spec::{
     FramePointer, LinkSelfContainedComponents, LinkerFeatures, SplitDebuginfo, Target, TargetTriple,
 };
@@ -402,6 +404,23 @@ pub(crate) fn all() -> Self {
     }
 }
 
+/// Values for the `-Z fmt-debug` flag.
+#[derive(Copy, Clone, PartialEq, Hash, Debug)]
+pub enum FmtDebug {
+    /// Derive fully-featured implementation
+    Full,
+    /// Print only type name, without fields
+    Shallow,
+    /// `#[derive(Debug)]` and `{:?}` are no-ops
+    None,
+}
+
+impl FmtDebug {
+    pub(crate) fn all() -> [Symbol; 3] {
+        [sym::full, sym::none, sym::shallow]
+    }
+}
+
 #[derive(Clone, PartialEq, Hash, Debug)]
 pub enum SwitchWithOptPath {
     Enabled(Option<PathBuf>),
@@ -2994,7 +3013,7 @@ pub(crate) mod dep_tracking {
 
     use super::{
         BranchProtection, CFGuard, CFProtection, CollapseMacroDebuginfo, CoverageOptions,
-        CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FunctionReturn,
+        CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FmtDebug, FunctionReturn,
         InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
         LtoCli, NextSolverConfig, OomStrategy, OptLevel, OutFileName, OutputType, OutputTypes,
         PatchableFunctionEntry, Polonius, RemapPathScopeComponents, ResolveDocLinks,
@@ -3088,6 +3107,7 @@ fn hash(
         OutputType,
         RealFileName,
         LocationDetail,
+        FmtDebug,
         BranchProtection,
         OomStrategy,
         LanguageIdentifier,

compiler/rustc_session/src/config/cfg.rs

@@ -31,7 +31,7 @@
 use rustc_target::abi::Align;
 use rustc_target::spec::{PanicStrategy, RelocModel, SanitizerSet, Target, TargetTriple, TARGETS};
 
-use crate::config::CrateType;
+use crate::config::{CrateType, FmtDebug};
 use crate::Session;
 
 /// The parsed `--cfg` options that define the compilation environment of the
@@ -142,6 +142,7 @@ pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {
             | (sym::target_has_atomic_equal_alignment, Some(_))
             | (sym::target_has_atomic_load_store, Some(_))
             | (sym::target_thread_local, None) => disallow(cfg, "--target"),
+            (sym::fmt_debug, None | Some(_)) => disallow(cfg, "-Z fmt-debug"),
             _ => {}
         }
     }
@@ -179,6 +180,20 @@ macro_rules! ins_sym {
         ins_none!(sym::debug_assertions);
     }
 
+    if sess.is_nightly_build() {
+        match sess.opts.unstable_opts.fmt_debug {
+            FmtDebug::Full => {
+                ins_sym!(sym::fmt_debug, sym::full);
+            }
+            FmtDebug::Shallow => {
+                ins_sym!(sym::fmt_debug, sym::shallow);
+            }
+            FmtDebug::None => {
+                ins_sym!(sym::fmt_debug, sym::none);
+            }
+        }
+    }
+
     if sess.overflow_checks() {
         ins_none!(sym::overflow_checks);
     }
@@ -326,6 +341,8 @@ macro_rules! ins {
 
         ins!(sym::debug_assertions, no_values);
 
+        ins!(sym::fmt_debug, empty_values).extend(FmtDebug::all());
+
         // These four are never set by rustc, but we set them anyway; they
         // should not trigger the lint because `cargo clippy`, `cargo doc`,
         // `cargo test`, `cargo miri run` and `cargo fmt` (respectively)

compiler/rustc_session/src/options.rs

@@ -408,6 +408,7 @@ mod desc {
     pub const parse_linker_plugin_lto: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or the path to the linker plugin";
     pub const parse_location_detail: &str = "either `none`, or a comma separated list of location details to track: `file`, `line`, or `column`";
+    pub const parse_fmt_debug: &str = "either `full`, `shallow`, or `none`";
     pub const parse_switch_with_opt_path: &str =
         "an optional path to the profiling data output directory";
     pub const parse_merge_functions: &str = "one of: `disabled`, `trampolines`, or `aliases`";
@@ -589,6 +590,16 @@ pub(crate) fn parse_list_with_polarity(
         }
     }
 
+    pub(crate) fn parse_fmt_debug(opt: &mut FmtDebug, v: Option<&str>) -> bool {
+        *opt = match v {
+            Some("full") => FmtDebug::Full,
+            Some("shallow") => FmtDebug::Shallow,
+            Some("none") => FmtDebug::None,
+            _ => return false,
+        };
+        true
+    }
+
     pub(crate) fn parse_location_detail(ld: &mut LocationDetail, v: Option<&str>) -> bool {
         if let Some(v) = v {
             ld.line = false;
@@ -1724,6 +1735,9 @@ pub(crate) fn parse_wasm_c_abi(slot: &mut WasmCAbi, v: Option<&str>) -> bool {
     flatten_format_args: bool = (true, parse_bool, [TRACKED],
         "flatten nested format_args!() and literals into a simplified format_args!() call \
         (default: yes)"),
+    fmt_debug: FmtDebug = (FmtDebug::Full, parse_fmt_debug, [TRACKED],
+        "how detailed `#[derive(Debug)]` should be. `full` prints types recursively, \
+        `shallow` prints only type names, `none` prints nothing and disables `{:?}`. (default: `full`)"),
     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],

compiler/rustc_span/src/symbol.rs

@@ -536,6 +536,7 @@
         cfg_attr_multi,
         cfg_doctest,
         cfg_eval,
+        cfg_fmt_debug,
         cfg_hide,
         cfg_overflow_checks,
         cfg_panic,
@@ -895,6 +896,7 @@
         fmaf32,
         fmaf64,
         fmt,
+        fmt_debug,
         fmul_algebraic,
         fmul_fast,
         fn_align,
@@ -938,6 +940,7 @@
         fs_create_dir,
         fsub_algebraic,
         fsub_fast,
+        full,
         fundamental,
         fused_iterator,
         future,
@@ -1281,6 +1284,7 @@
         new_binary,
         new_const,
         new_debug,
+        new_debug_noop,
         new_display,
         new_lower_exp,
         new_lower_hex,
@@ -1715,6 +1719,7 @@
         semitransparent,
         sha512_sm_x86,
         shadow_call_stack,
+        shallow,
         shl,
         shl_assign,
         shorter_tail_lifetimes,

library/core/src/fmt/rt.rs

@@ -118,6 +118,10 @@ pub fn new_debug<'b, T: Debug>(x: &'b T) -> Argument<'_> {
         Self::new(x, Debug::fmt)
     }
     #[inline(always)]
+    pub fn new_debug_noop<'b, T: Debug>(x: &'b T) -> Argument<'_> {
+        Self::new(x, |_, _| Ok(()))
+    }
+    #[inline(always)]
     pub fn new_octal<'b, T: Octal>(x: &'b T) -> Argument<'_> {
         Self::new(x, Octal::fmt)
     }

src/doc/rustc/src/check-cfg.md

@@ -99,7 +99,7 @@ the need to specify them manually.
 Well known names and values are implicitly added as long as at least one `--check-cfg` argument
 is present.
 
-As of `2024-05-06T`, the list of known names is as follows:
+As of `2024-08-20T`, the list of known names is as follows:
 
 <!--- See CheckCfg::fill_well_known in compiler/rustc_session/src/config.rs -->
 
@@ -107,6 +107,7 @@ As of `2024-05-06T`, the list of known names is as follows:
  - `debug_assertions`
  - `doc`
  - `doctest`
+ - `fmt_debug`
  - `miri`
  - `overflow_checks`
  - `panic`

src/doc/unstable-book/src/compiler-flags/fmt-debug.md

@@ -0,0 +1,15 @@
+# `fmt-debug`
+
+The tracking issue for this feature is: [#129709](https://github.com/rust-lang/rust/issues/129709).
+
+------------------------
+
+Option `-Z fmt-debug=val` controls verbosity of derived `Debug` implementations
+and debug formatting in format strings (`{:?}`).
+
+* `full` — `#[derive(Debug)]` prints types recursively. This is the default behavior.
+
+* `shallow` — `#[derive(Debug)]` prints only the type name, or name of a variant of a fieldless enums. Details of the `Debug` implementation are not stable and may change in the future. Behavior of custom `fmt::Debug` implementations is not affected.
+
+* `none` — `#[derive(Debug)]` does not print anything at all. `{:?}` in formatting strings has no effect.
+  This option may reduce size of binaries, and remove occurrences of type names in the binary that are not removed by striping symbols. However, it may also cause `panic!` and `assert!` messages to be incomplete.

tests/ui/cfg/disallowed-cli-cfgs.fmt_debug_.stderr

@@ -0,0 +1,8 @@
+error: unexpected `--cfg fmt_debug="shallow"` flag
+   |
+   = note: config `fmt_debug` is only supposed to be controlled by `-Z fmt-debug`
+   = note: manually setting a built-in cfg can and does create incoherent behaviors
+   = note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
+
+error: aborting due to 1 previous error
+

tests/ui/cfg/disallowed-cli-cfgs.rs

@@ -6,6 +6,7 @@
 //@ revisions: target_pointer_width_ target_vendor_ target_has_atomic_
 //@ revisions: target_has_atomic_equal_alignment_ target_has_atomic_load_store_
 //@ revisions: target_thread_local_ relocation_model_
+//@ revisions: fmt_debug_
 
 //@ [overflow_checks_]compile-flags: --cfg overflow_checks
 //@ [debug_assertions_]compile-flags: --cfg debug_assertions
@@ -31,5 +32,6 @@
 //@ [target_has_atomic_load_store_]compile-flags: --cfg target_has_atomic_load_store="32"
 //@ [target_thread_local_]compile-flags: --cfg target_thread_local
 //@ [relocation_model_]compile-flags: --cfg relocation_model="a"
+//@ [fmt_debug_]compile-flags: --cfg fmt_debug="shallow"
 
 fn main() {}

tests/ui/check-cfg/allow-same-level.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `FALSE`
 LL | #[cfg(FALSE)]
    |       ^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(FALSE)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/cargo-build-script.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `has_foo`
 LL | #[cfg(has_foo)]
    |       ^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `has_bar`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `has_bar`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]

tests/ui/check-cfg/cargo-feature.none.stderr

@@ -25,7 +25,7 @@ warning: unexpected `cfg` condition name: `tokio_unstable`
 LL | #[cfg(tokio_unstable)]
    |       ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]

tests/ui/check-cfg/cargo-feature.some.stderr

@@ -25,7 +25,7 @@ warning: unexpected `cfg` condition name: `tokio_unstable`
 LL | #[cfg(tokio_unstable)]
    |       ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `CONFIG_NVME`, `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `CONFIG_NVME`, `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]

tests/ui/check-cfg/cfg-value-for-cfg-name-duplicate.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `value`
 LL | #[cfg(value)]
    |       ^^^^^
    |
-   = help: expected names are: `bar`, `bee`, `clippy`, `cow`, `debug_assertions`, `doc`, `doctest`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `bar`, `bee`, `clippy`, `cow`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(value)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/cfg-value-for-cfg-name-multiple.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `my_value`
 LL | #[cfg(my_value)]
    |       ^^^^^^^^
    |
-   = help: expected names are: `bar`, `clippy`, `debug_assertions`, `doc`, `doctest`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `bar`, `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(my_value)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/cfg-value-for-cfg-name.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `linux`
 LL | #[cfg(linux)]
    |       ^^^^^ help: found config with similar value: `target_os = "linux"`
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(linux)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/compact-names.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `target_architecture`
 LL | #[cfg(target(os = "linux", architecture = "arm"))]
    |                            ^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(target_architecture, values("arm"))`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/exhaustive-names-values.empty_cfg.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/exhaustive-names-values.feature.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/exhaustive-names-values.full.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/exhaustive-names.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/mix.stderr

@@ -44,7 +44,7 @@ warning: unexpected `cfg` condition name: `uu`
 LL | #[cfg_attr(uu, test)]
    |            ^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(uu)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 

tests/ui/check-cfg/stmt-no-ice.stderr

@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `crossbeam_loom`
 LL |     #[cfg(crossbeam_loom)]
    |           ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(crossbeam_loom)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

tests/ui/check-cfg/well-known-names.stderr

@@ -18,7 +18,7 @@ warning: unexpected `cfg` condition name: `features`
 LL | #[cfg(features = "foo")]
    |       ^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: to expect this configuration use `--check-cfg=cfg(features, values("foo"))`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 

tests/ui/check-cfg/well-known-values.rs

@@ -16,6 +16,7 @@
 #![feature(cfg_target_has_atomic_equal_alignment)]
 #![feature(cfg_target_thread_local)]
 #![feature(cfg_ub_checks)]
+#![feature(fmt_debug)]
 
 // This part makes sure that none of the well known names are
 // unexpected.
@@ -33,6 +34,8 @@
     //~^ WARN unexpected `cfg` condition value
     doctest = "_UNEXPECTED_VALUE",
     //~^ WARN unexpected `cfg` condition value
+    fmt_debug = "_UNEXPECTED_VALUE",
+    //~^ WARN unexpected `cfg` condition value
     miri = "_UNEXPECTED_VALUE",
     //~^ WARN unexpected `cfg` condition value
     overflow_checks = "_UNEXPECTED_VALUE",

tests/ui/check-cfg/well-known-values.stderr

@@ -1,5 +1,5 @@
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:28:5
+  --> $DIR/well-known-values.rs:29:5
    |
 LL |     clippy = "_UNEXPECTED_VALUE",
    |     ^^^^^^----------------------
@@ -11,7 +11,7 @@ LL |     clippy = "_UNEXPECTED_VALUE",
    = note: `#[warn(unexpected_cfgs)]` on by default
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:30:5
+  --> $DIR/well-known-values.rs:31:5
    |
 LL |     debug_assertions = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^----------------------
@@ -22,7 +22,7 @@ LL |     debug_assertions = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:32:5
+  --> $DIR/well-known-values.rs:33:5
    |
 LL |     doc = "_UNEXPECTED_VALUE",
    |     ^^^----------------------
@@ -33,7 +33,7 @@ LL |     doc = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:34:5
+  --> $DIR/well-known-values.rs:35:5
    |
 LL |     doctest = "_UNEXPECTED_VALUE",
    |     ^^^^^^^----------------------
@@ -44,7 +44,16 @@ LL |     doctest = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:36:5
+  --> $DIR/well-known-values.rs:37:5
+   |
+LL |     fmt_debug = "_UNEXPECTED_VALUE",
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: expected values for `fmt_debug` are: `full`, `none`, and `shallow`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+
+warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
+  --> $DIR/well-known-values.rs:39:5
    |
 LL |     miri = "_UNEXPECTED_VALUE",
    |     ^^^^----------------------
@@ -55,7 +64,7 @@ LL |     miri = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:38:5
+  --> $DIR/well-known-values.rs:41:5
    |
 LL |     overflow_checks = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^----------------------
@@ -66,7 +75,7 @@ LL |     overflow_checks = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:40:5
+  --> $DIR/well-known-values.rs:43:5
    |
 LL |     panic = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -75,7 +84,7 @@ LL |     panic = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:42:5
+  --> $DIR/well-known-values.rs:45:5
    |
 LL |     proc_macro = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^----------------------
@@ -86,7 +95,7 @@ LL |     proc_macro = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:44:5
+  --> $DIR/well-known-values.rs:47:5
    |
 LL |     relocation_model = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -95,7 +104,7 @@ LL |     relocation_model = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:46:5
+  --> $DIR/well-known-values.rs:49:5
    |
 LL |     rustfmt = "_UNEXPECTED_VALUE",
    |     ^^^^^^^----------------------
@@ -106,7 +115,7 @@ LL |     rustfmt = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:48:5
+  --> $DIR/well-known-values.rs:51:5
    |
 LL |     sanitize = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -115,7 +124,7 @@ LL |     sanitize = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:50:5
+  --> $DIR/well-known-values.rs:53:5
    |
 LL |     target_abi = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -124,7 +133,7 @@ LL |     target_abi = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:52:5
+  --> $DIR/well-known-values.rs:55:5
    |
 LL |     target_arch = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -133,7 +142,7 @@ LL |     target_arch = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:54:5
+  --> $DIR/well-known-values.rs:57:5
    |
 LL |     target_endian = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -142,7 +151,7 @@ LL |     target_endian = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:56:5
+  --> $DIR/well-known-values.rs:59:5
    |
 LL |     target_env = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -151,7 +160,7 @@ LL |     target_env = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:58:5
+  --> $DIR/well-known-values.rs:61:5
    |
 LL |     target_family = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -160,7 +169,7 @@ LL |     target_family = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:60:5
+  --> $DIR/well-known-values.rs:63:5
    |
 LL |     target_feature = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -169,7 +178,7 @@ LL |     target_feature = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:62:5
+  --> $DIR/well-known-values.rs:65:5
    |
 LL |     target_has_atomic = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -178,7 +187,7 @@ LL |     target_has_atomic = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:64:5
+  --> $DIR/well-known-values.rs:67:5
    |
 LL |     target_has_atomic_equal_alignment = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -187,7 +196,7 @@ LL |     target_has_atomic_equal_alignment = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:66:5
+  --> $DIR/well-known-values.rs:69:5
    |
 LL |     target_has_atomic_load_store = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -196,7 +205,7 @@ LL |     target_has_atomic_load_store = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:68:5
+  --> $DIR/well-known-values.rs:71:5
    |
 LL |     target_os = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -205,7 +214,7 @@ LL |     target_os = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:70:5
+  --> $DIR/well-known-values.rs:73:5
    |
 LL |     target_pointer_width = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -214,7 +223,7 @@ LL |     target_pointer_width = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:72:5
+  --> $DIR/well-known-values.rs:75:5
    |
 LL |     target_thread_local = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^----------------------
@@ -225,7 +234,7 @@ LL |     target_thread_local = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:74:5
+  --> $DIR/well-known-values.rs:77:5
    |
 LL |     target_vendor = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -234,7 +243,7 @@ LL |     target_vendor = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:76:5
+  --> $DIR/well-known-values.rs:79:5
    |
 LL |     test = "_UNEXPECTED_VALUE",
    |     ^^^^----------------------
@@ -245,7 +254,7 @@ LL |     test = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:78:5
+  --> $DIR/well-known-values.rs:81:5
    |
 LL |     ub_checks = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^----------------------
@@ -256,7 +265,7 @@ LL |     ub_checks = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:80:5
+  --> $DIR/well-known-values.rs:83:5
    |
 LL |     unix = "_UNEXPECTED_VALUE",
    |     ^^^^----------------------
@@ -267,7 +276,7 @@ LL |     unix = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:82:5
+  --> $DIR/well-known-values.rs:85:5
    |
 LL |     windows = "_UNEXPECTED_VALUE",
    |     ^^^^^^^----------------------
@@ -278,7 +287,7 @@ LL |     windows = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `linuz`
-  --> $DIR/well-known-values.rs:88:7
+  --> $DIR/well-known-values.rs:91:7
    |
 LL | #[cfg(target_os = "linuz")] // testing that we suggest `linux`
    |       ^^^^^^^^^^^^-------
@@ -288,5 +297,5 @@ LL | #[cfg(target_os = "linuz")] // testing that we suggest `linux`
    = note: expected values for `target_os` are: `aix`, `android`, `cuda`, `dragonfly`, `emscripten`, `espidf`, `freebsd`, `fuchsia`, `haiku`, `hermit`, `horizon`, `hurd`, `illumos`, `ios`, `l4re`, `linux`, `macos`, `netbsd`, `none`, `nto`, `nuttx`, `openbsd`, `psp`, `redox`, `solaris`, `solid_asp3`, `teeos`, `trusty`, `tvos`, `uefi`, `unknown`, `visionos`, `vita`, `vxworks`, `wasi`, `watchos`, `windows`, `xous`, and `zkvm`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
-warning: 29 warnings emitted
+warning: 30 warnings emitted
 

tests/ui/feature-gates/feature-gate-fmt-debug.rs

@@ -0,0 +1,5 @@
+#[cfg(fmt_debug = "full")]
+//~^ ERROR is experimental
+fn main() {
+
+}

tests/ui/feature-gates/feature-gate-fmt-debug.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `cfg(fmt_debug)` is experimental and subject to change
+  --> $DIR/feature-gate-fmt-debug.rs:1:7
+   |
+LL | #[cfg(fmt_debug = "full")]
+   |       ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #129709 <https://github.com/rust-lang/rust/issues/129709> for more information
+   = help: add `#![feature(fmt_debug)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/fmt/fmt_debug/full.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: -Zfmt-debug=full
+//@ run-pass
+#![feature(fmt_debug)]
+#![allow(dead_code)]
+#![allow(unused)]
+
+#[derive(Debug)]
+struct Foo {
+    bar: u32,
+}
+
+fn main() {
+    let s = format!("Still works: {:?} '{:?}'", cfg!(fmt_debug = "full"), Foo { bar: 1 });
+    assert_eq!("Still works: true 'Foo { bar: 1 }'", s);
+}

tests/ui/fmt/fmt_debug/invalid.rs

@@ -0,0 +1,4 @@
+//@ compile-flags: -Zfmt-debug=invalid-value
+//@ failure-status: 1
+fn main() {
+}

tests/ui/fmt/fmt_debug/invalid.stderr

@@ -0,0 +1,2 @@
+error: incorrect value `invalid-value` for unstable option `fmt-debug` - either `full`, `shallow`, or `none` was expected
+

tests/ui/fmt/fmt_debug/none.rs

@@ -0,0 +1,37 @@
+//@ compile-flags: -Zfmt-debug=none
+//@ run-pass
+#![feature(fmt_debug)]
+#![allow(dead_code)]
+#![allow(unused)]
+
+#[derive(Debug)]
+struct Foo {
+    bar: u32,
+}
+
+#[derive(Debug)]
+enum Baz {
+    Quz,
+}
+
+#[cfg(fmt_debug = "full")]
+compile_error!("nope");
+
+#[cfg(fmt_debug = "none")]
+struct Custom;
+
+impl std::fmt::Debug for Custom {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("custom_fmt")
+    }
+}
+
+fn main() {
+    let c = Custom;
+    let s = format!("Debug is '{:?}', '{:#?}', and '{c:?}'", Foo { bar: 1 }, Baz::Quz);
+    assert_eq!("Debug is '', '', and ''", s);
+
+    let f = 3.0;
+    let s = format_args!("{:?}x{:#?}y{f:?}", 1234, "can't debug this").to_string();
+    assert_eq!("xy", s);
+}

tests/ui/fmt/fmt_debug/shallow.rs

@@ -0,0 +1,33 @@
+//@ compile-flags: -Zfmt-debug=shallow
+//@ run-pass
+#![feature(fmt_debug)]
+#![allow(dead_code)]
+#![allow(unused)]
+
+#[derive(Debug)]
+struct Foo {
+    bar: u32,
+    bomb: Bomb,
+}
+
+#[derive(Debug)]
+enum Baz {
+    Quz,
+}
+
+struct Bomb;
+
+impl std::fmt::Debug for Bomb {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        panic!()
+    }
+}
+
+fn main() {
+    let s = format!("Debug is '{:?}' and '{:#?}'", Foo { bar: 1, bomb: Bomb }, Baz::Quz);
+    assert_eq!("Debug is 'Foo' and 'Quz'", s);
+
+    let f = 3.0;
+    let s = format_args!("{:?}{:#?}{f:?}", 1234, cfg!(fmt_debug = "shallow")).to_string();
+    assert_eq!("1234true3.0", s);
+}