Auto merge of rust-lang#124776 - fmease:rollup-xu4kudb, r=fmease

Rollup of 5 pull requests

Successful merges:

 - rust-lang#124742 (Add `rustfmt` cfg to well known cfgs list)
 - rust-lang#124747 (Support Result<T, E> across FFI when niche optimization can be used (v2))
 - rust-lang#124753 (Migrate `run-make/rustdoc-error-lines` to new `rmake.rs`)
 - rust-lang#124765 ([rustdoc] Fix bad color for setting cog in ayu theme)
 - rust-lang#124768 ([resubmission] Meta: Enable the brand new triagebot transfer command)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

compiler/rustc_feature/src/unstable.rs

@@ -579,6 +579,9 @@ declare_features! (
     (incomplete, repr128, "1.16.0", Some(56071)),
     /// Allows `repr(simd)` and importing the various simd intrinsics.
     (unstable, repr_simd, "1.4.0", Some(27731)),
+    /// Allows enums like Result<T, E> to be used across FFI, if T's niche value can
+    /// be used to describe E or vise-versa.
+    (unstable, result_ffi_guarantees, "CURRENT_RUSTC_VERSION", Some(110503)),
     /// Allows bounding the return type of AFIT/RPITIT.
     (incomplete, return_type_notation, "1.70.0", Some(109417)),
     /// Allows `extern "rust-cold"`.

compiler/rustc_lint/src/types.rs

@@ -1101,6 +1101,32 @@ fn get_nullable_type<'tcx>(
     })
 }
 
+/// A type is niche-optimization candidate iff:
+/// - Is a zero-sized type with alignment 1 (a “1-ZST”).
+/// - Has no fields.
+/// - Does not have the `#[non_exhaustive]` attribute.
+fn is_niche_optimization_candidate<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+) -> bool {
+    if tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| !layout.is_1zst()) {
+        return false;
+    }
+
+    match ty.kind() {
+        ty::Adt(ty_def, _) => {
+            let non_exhaustive = ty_def.is_variant_list_non_exhaustive();
+            let empty = (ty_def.is_struct() && ty_def.all_fields().next().is_none())
+                || (ty_def.is_enum() && ty_def.variants().is_empty());
+
+            !non_exhaustive && empty
+        }
+        ty::Tuple(tys) => tys.is_empty(),
+        _ => false,
+    }
+}
+
 /// Check if this enum can be safely exported based on the "nullable pointer optimization". If it
 /// can, return the type that `ty` can be safely converted to, otherwise return `None`.
 /// Currently restricted to function pointers, boxes, references, `core::num::NonZero`,
@@ -1117,6 +1143,22 @@ pub(crate) fn repr_nullable_ptr<'tcx>(
         let field_ty = match &ty_def.variants().raw[..] {
             [var_one, var_two] => match (&var_one.fields.raw[..], &var_two.fields.raw[..]) {
                 ([], [field]) | ([field], []) => field.ty(tcx, args),
+                ([field1], [field2]) => {
+                    if !tcx.features().result_ffi_guarantees {
+                        return None;
+                    }
+
+                    let ty1 = field1.ty(tcx, args);
+                    let ty2 = field2.ty(tcx, args);
+
+                    if is_niche_optimization_candidate(tcx, param_env, ty1) {
+                        ty2
+                    } else if is_niche_optimization_candidate(tcx, param_env, ty2) {
+                        ty1
+                    } else {
+                        return None;
+                    }
+                }
                 _ => return None,
             },
             _ => return None,
@@ -1202,7 +1244,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         args: GenericArgsRef<'tcx>,
     ) -> FfiResult<'tcx> {
         use FfiResult::*;
-
         let transparent_with_all_zst_fields = if def.repr().transparent() {
             if let Some(field) = transparent_newtype_field(self.cx.tcx, variant) {
                 // Transparent newtypes have at most one non-ZST field which needs to be checked..
@@ -1329,27 +1370,29 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                             return FfiSafe;
                         }
 
+                        if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
+                            return FfiUnsafe {
+                                ty,
+                                reason: fluent::lint_improper_ctypes_non_exhaustive,
+                                help: None,
+                            };
+                        }
+
                         // Check for a repr() attribute to specify the size of the
                         // discriminant.
                         if !def.repr().c() && !def.repr().transparent() && def.repr().int.is_none()
                         {
-                            // Special-case types like `Option<extern fn()>`.
-                            if repr_nullable_ptr(self.cx.tcx, self.cx.param_env, ty, self.mode)
-                                .is_none()
+                            // Special-case types like `Option<extern fn()>` and `Result<extern fn(), ()>`
+                            if let Some(ty) =
+                                repr_nullable_ptr(self.cx.tcx, self.cx.param_env, ty, self.mode)
                             {
-                                return FfiUnsafe {
-                                    ty,
-                                    reason: fluent::lint_improper_ctypes_enum_repr_reason,
-                                    help: Some(fluent::lint_improper_ctypes_enum_repr_help),
-                                };
+                                return self.check_type_for_ffi(cache, ty);
                             }
-                        }
 
-                        if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
                             return FfiUnsafe {
                                 ty,
-                                reason: fluent::lint_improper_ctypes_non_exhaustive,
-                                help: None,
+                                reason: fluent::lint_improper_ctypes_enum_repr_reason,
+                                help: Some(fluent::lint_improper_ctypes_enum_repr_help),
                             };
                         }
 

compiler/rustc_session/src/config/cfg.rs

@@ -262,13 +262,15 @@ impl CheckCfg {
 
         ins!(sym::debug_assertions, no_values);
 
-        // These four are never set by rustc, but we set them anyway: they
-        // should not trigger a lint because `cargo clippy`, `cargo doc`,
-        // `cargo test` and `cargo miri run` (respectively) can set them.
+        // 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)
+        // can set them.
         ins!(sym::clippy, no_values);
         ins!(sym::doc, no_values);
         ins!(sym::doctest, no_values);
         ins!(sym::miri, no_values);
+        ins!(sym::rustfmt, no_values);
 
         ins!(sym::overflow_checks, no_values);
 

compiler/rustc_span/src/symbol.rs

@@ -1511,6 +1511,7 @@ symbols! {
         require,
         residual,
         result,
+        result_ffi_guarantees,
         resume,
         return_position_impl_trait_in_trait,
         return_type_notation,

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

@@ -71,7 +71,7 @@ Those well known names and values follows the same stability as what they refer
 Well known names and values checking is always enabled as long as at least one
 `--check-cfg` argument is present.
 
-As of `2024-04-06T`, the list of known names is as follows:
+As of `2024-05-06T`, the list of known names is as follows:
 
 <!--- See CheckCfg::fill_well_known in compiler/rustc_session/src/config.rs -->
 
@@ -84,6 +84,7 @@ As of `2024-04-06T`, the list of known names is as follows:
  - `panic`
  - `proc_macro`
  - `relocation_model`
+ - `rustfmt`
  - `sanitize`
  - `sanitizer_cfi_generalize_pointers`
  - `sanitizer_cfi_normalize_integers`

src/doc/unstable-book/src/language-features/result-ffi-guarantees.md

@@ -0,0 +1,14 @@
+# `result_ffi_guarantees`
+
+The tracking issue for this feature is: [#110503]
+
+[#110503]: https://github.com/rust-lang/rust/issues/110503
+
+------------------------
+
+This feature adds the possibility of using `Result<T, E>` in FFI if T's niche
+value can be used to describe E or vise-versa.
+
+See [RFC 3391] for more information.
+
+[RFC 3391]: https://github.com/rust-lang/rfcs/blob/master/text/3391-result_ffi_guarantees.md

src/librustdoc/html/static/css/noscript.css

@@ -85,6 +85,7 @@ nav.sub {
 	--search-tab-button-not-selected-background: #e6e6e6;
 	--search-tab-button-selected-border-top-color: #0089ff;
 	--search-tab-button-selected-background: #fff;
+	--settings-menu-filter: none;
 	--stab-background-color: #fff5d6;
 	--stab-code-color: #000;
 	--code-highlight-kw-color: #8959a8;

src/librustdoc/html/static/css/rustdoc.css

@@ -1625,6 +1625,7 @@ a.tooltip:hover::after {
 	,5.1715698,7.5,6  S6.8284302,7.5,6,7.5z" fill="black"/></svg>');
 	width: 22px;
 	height: 22px;
+	filter: var(--settings-menu-filter);
 }
 
 #sidebar-button > a:before {
@@ -2419,6 +2420,7 @@ by default.
 	--search-tab-button-not-selected-background: #e6e6e6;
 	--search-tab-button-selected-border-top-color: #0089ff;
 	--search-tab-button-selected-background: #fff;
+	--settings-menu-filter: none;
 	--stab-background-color: #fff5d6;
 	--stab-code-color: #000;
 	--code-highlight-kw-color: #8959a8;
@@ -2524,6 +2526,7 @@ by default.
 	--search-tab-button-not-selected-background: #252525;
 	--search-tab-button-selected-border-top-color: #0089ff;
 	--search-tab-button-selected-background: #353535;
+	--settings-menu-filter: none;
 	--stab-background-color: #314559;
 	--stab-code-color: #e6e1cf;
 	--code-highlight-kw-color: #ab8ac1;
@@ -2636,6 +2639,7 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 	--search-tab-button-not-selected-background: transparent !important;
 	--search-tab-button-selected-border-top-color: none;
 	--search-tab-button-selected-background: #141920 !important;
+	--settings-menu-filter: invert(100%);
 	--stab-background-color: #314559;
 	--stab-code-color: #e6e1cf;
 	--code-highlight-kw-color: #ff7733;

src/tools/run-make-support/src/cc.rs

@@ -73,7 +73,7 @@ impl Cc {
     }
 
     /// Get the [`Output`][::std::process::Output] of the finished process.
-    pub fn output(&mut self) -> ::std::process::Output {
+    pub fn command_output(&mut self) -> ::std::process::Output {
         self.cmd.output().expect("failed to get output of finished process")
     }
 }

src/tools/run-make-support/src/clang.rs

@@ -72,7 +72,7 @@ impl Clang {
     }
 
     /// Get the [`Output`][::std::process::Output] of the finished process.
-    pub fn output(&mut self) -> ::std::process::Output {
+    pub fn command_output(&mut self) -> ::std::process::Output {
         self.cmd.output().expect("failed to get output of finished process")
     }
 }