Rollup merge of #138570 - folkertdev:naked-function-target-feature-gate, r=Amanieu

add `naked_functions_target_feature` unstable feature

tracking issue: https://github.com/rust-lang/rust/issues/138568

tagging https://github.com/rust-lang/rust/pull/134213 https://github.com/rust-lang/rust/issues/90957

This PR puts `#[target_feature(/* ... */)]` on `#[naked]` functions behind its own feature gate, so that naked functions can be stabilized. It turns out that supporting `target_feature` on naked functions is tricky on some targets, so we're splitting it out to not block stabilization of naked functions themselves. See the tracking issue for more information and workarounds.

Note that at the time of writing, the `target_features` attribute is ignored when generating code for naked functions.

r? ``@Amanieu``

compiler/rustc_feature/src/unstable.rs

@@ -566,6 +566,8 @@ declare_features! (
     (incomplete, mut_ref, "1.79.0", Some(123076)),
     /// Allows using `#[naked]` on functions.
     (unstable, naked_functions, "1.9.0", Some(90957)),
+    /// Allows using `#[target_feature(enable = "...")]` on `#[naked]` on functions.
+    (unstable, naked_functions_target_feature, "1.86.0", Some(138568)),
     /// Allows specifying the as-needed link modifier
     (unstable, native_link_modifiers_as_needed, "1.53.0", Some(81490)),
     /// Allow negative trait implementations.

compiler/rustc_passes/src/check_attr.rs

@@ -600,7 +600,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             sym::repr,
             // code generation
             sym::cold,
-            sym::target_feature,
             // documentation
             sym::doc,
         ];
@@ -626,6 +625,21 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                         _ => {}
                     }
 
+                    if other_attr.has_name(sym::target_feature) {
+                        if !self.tcx.features().naked_functions_target_feature() {
+                            feature_err(
+                                &self.tcx.sess,
+                                sym::naked_functions_target_feature,
+                                other_attr.span(),
+                                "`#[target_feature(/* ... */)]` is currently unstable on `#[naked]` functions",
+                            ).emit();
+
+                            return;
+                        } else {
+                            continue;
+                        }
+                    }
+
                     if !ALLOW_LIST.iter().any(|name| other_attr.has_name(*name)) {
                         self.dcx().emit_err(errors::NakedFunctionIncompatibleAttribute {
                             span: other_attr.span(),

compiler/rustc_span/src/symbol.rs

@@ -1376,6 +1376,7 @@ symbols! {
         naked,
         naked_asm,
         naked_functions,
+        naked_functions_target_feature,
         name,
         names,
         native_link_modifiers,

tests/ui/asm/naked-functions-target-feature.rs

@@ -0,0 +1,21 @@
+//@ build-pass
+//@ needs-asm-support
+
+#![feature(naked_functions, naked_functions_target_feature)]
+#![crate_type = "lib"]
+
+use std::arch::{asm, naked_asm};
+
+#[cfg(target_arch = "x86_64")]
+#[target_feature(enable = "sse2")]
+#[naked]
+pub unsafe extern "C" fn compatible_target_feature() {
+    naked_asm!("");
+}
+
+#[cfg(target_arch = "aarch64")]
+#[target_feature(enable = "neon")]
+#[naked]
+pub unsafe extern "C" fn compatible_target_feature() {
+    naked_asm!("");
+}

tests/ui/asm/naked-functions.rs

@@ -230,13 +230,6 @@ pub unsafe extern "C" fn compatible_codegen_attributes() {
     naked_asm!("", options(raw));
 }
 
-#[cfg(target_arch = "x86_64")]
-#[target_feature(enable = "sse2")]
-#[naked]
-pub unsafe extern "C" fn compatible_target_feature() {
-    naked_asm!("");
-}
-
 #[doc = "foo bar baz"]
 /// a doc comment
 // a normal comment

tests/ui/feature-gates/feature-gate-naked_functions_target_feature.rs

@@ -0,0 +1,15 @@
+//@ needs-asm-support
+//@ only-x86_64
+
+#![feature(naked_functions)]
+
+use std::arch::naked_asm;
+
+#[naked]
+#[target_feature(enable = "avx2")]
+//~^ ERROR: `#[target_feature(/* ... */)]` is currently unstable on `#[naked]` functions
+extern "C" fn naked() {
+    unsafe { naked_asm!("") }
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-naked_functions_target_feature.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `#[target_feature(/* ... */)]` is currently unstable on `#[naked]` functions
+  --> $DIR/feature-gate-naked_functions_target_feature.rs:9:1
+   |
+LL | #[target_feature(enable = "avx2")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #138568 <https://github.com/rust-lang/rust/issues/138568> for more information
+   = help: add `#![feature(naked_functions_target_feature)]` 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`.