Auto merge of #127537 - veluca93:struct_tf, r=BoxyUwU

Implement a first version of RFC 3525: struct target features

This PR is an attempt at implementing https://github.com/rust-lang/rfcs/pull/3525, behind a feature gate `struct_target_features`.

There's obviously a few tasks that ought to be done before this is merged; in no particular order:
- add proper error messages
- add tests
- create a tracking issue for the RFC
- properly serialize/deserialize the new target_features field in `rmeta` (assuming I even understood that correctly :-))

That said, as I am definitely not a `rustc` expert, I'd like to get some early feedback on the overall approach before fixing those things (and perhaps some pointers for `rmeta`...), hence this early PR :-)

Here's an example piece of code that I have been using for testing - with the new code, the calls to intrinsics get correctly inlined:
```rust
#![feature(struct_target_features)]

use std::arch::x86_64::*;

/*
// fails to compile
#[target_feature(enable = "avx")]
struct Invalid(u32);
*/

#[target_feature(enable = "avx")]
struct Avx {}

#[target_feature(enable = "sse")]
struct Sse();

/*
// fails to compile
extern "C" fn bad_fun(_: Avx) {}
*/

/*
// fails to compile
#[inline(always)]
fn inline_fun(_: Avx) {}
*/

trait Simd {
    fn do_something(&self);
}

impl Simd for Avx {
    fn do_something(&self) {
        unsafe {
            println!("{:?}", _mm256_setzero_ps());
        }
    }
}

impl Simd for Sse {
    fn do_something(&self) {
        unsafe {
            println!("{:?}", _mm_setzero_ps());
        }
    }
}

struct WithAvx {
    #[allow(dead_code)]
    avx: Avx,
}

impl Simd for WithAvx {
    fn do_something(&self) {
        unsafe {
            println!("{:?}", _mm256_setzero_ps());
        }
    }
}

#[inline(never)]
fn dosomething<S: Simd>(simd: &S) {
    simd.do_something();
}

fn main() {
    /*
    // fails to compile
    Avx {};
    */

    if is_x86_feature_detected!("avx") {
        let avx = unsafe { Avx {} };
        dosomething(&avx);
        dosomething(&WithAvx { avx });
    }
    if is_x86_feature_detected!("sse") {
        dosomething(&unsafe { Sse {} })
    }
}
```

Tracking:

- https://github.com/rust-lang/rust/issues/129107

compiler/rustc_passes/src/check_attr.rs

@@ -747,12 +747,35 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             Target::Field | Target::Arm | Target::MacroDef => {
                 self.inline_attr_str_error_with_macro_def(hir_id, attr, "target_feature");
             }
+            Target::Struct if self.tcx.features().struct_target_features => {
+                let ty = self.tcx.hir_node(hir_id).expect_item();
+                match ty.kind {
+                    ItemKind::Struct(data, _) => {
+                        if data.fields().len() != 0 {
+                            self.dcx().emit_err(errors::AttrShouldBeAppliedToFnOrUnitStruct {
+                                attr_span: attr.span,
+                                defn_span: span,
+                            });
+                        }
+                    }
+                    _ => {
+                        panic!("Target::Struct for a non-struct");
+                    }
+                }
+            }
             _ => {
-                self.dcx().emit_err(errors::AttrShouldBeAppliedToFn {
-                    attr_span: attr.span,
-                    defn_span: span,
-                    on_crate: hir_id == CRATE_HIR_ID,
-                });
+                if self.tcx.features().struct_target_features {
+                    self.dcx().emit_err(errors::AttrShouldBeAppliedToFnOrUnitStruct {
+                        attr_span: attr.span,
+                        defn_span: span,
+                    });
+                } else {
+                    self.dcx().emit_err(errors::AttrShouldBeAppliedToFn {
+                        attr_span: attr.span,
+                        defn_span: span,
+                        on_crate: hir_id == CRATE_HIR_ID,
+                    });
+                }
             }
         }
     }

compiler/rustc_passes/src/errors.rs

@@ -82,6 +82,15 @@ pub struct AttrShouldBeAppliedToFn {
     pub on_crate: bool,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_should_be_applied_to_fn_or_unit_struct)]
+pub struct AttrShouldBeAppliedToFnOrUnitStruct {
+    #[primary_span]
+    pub attr_span: Span,
+    #[label]
+    pub defn_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_should_be_applied_to_fn, code = E0739)]
 pub struct TrackedCallerWrongLocation {