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

1 files changed, 35 insertions, 0 deletions

diff --git a/compiler/rustc_mir_build/src/errors.rs b/compiler/rustc_mir_build/src/errors.rs
index 7f9eefd1d52..e7d6239aa9b 100644
--- a/compiler/rustc_mir_build/src/errors.rs
+++ b/compiler/rustc_mir_build/src/errors.rs
@@ -87,6 +87,16 @@ pub(crate) struct UnsafeOpInUnsafeFnInitializingTypeWithRequiresUnsafe {
 }
 
 #[derive(LintDiagnostic)]
+#[diag(mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_target_feature_requires_unsafe, code = E0133)]
+#[note]
+pub(crate) struct UnsafeOpInUnsafeFnInitializingTypeWithTargetFeatureRequiresUnsafe {
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
+}
+
+#[derive(LintDiagnostic)]
 #[diag(mir_build_unsafe_op_in_unsafe_fn_mutable_static_requires_unsafe, code = E0133)]
 #[note]
 pub(crate) struct UnsafeOpInUnsafeFnUseOfMutableStaticRequiresUnsafe {
@@ -251,6 +261,17 @@ pub(crate) struct InitializingTypeWithRequiresUnsafe {
 }
 
 #[derive(Diagnostic)]
+#[diag(mir_build_initializing_type_with_target_feature_requires_unsafe, code = E0133)]
+#[note]
+pub(crate) struct InitializingTypeWithTargetFeatureRequiresUnsafe {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
+#[derive(Diagnostic)]
 #[diag(
     mir_build_initializing_type_with_requires_unsafe_unsafe_op_in_unsafe_fn_allowed,
     code = E0133
@@ -265,6 +286,20 @@ pub(crate) struct InitializingTypeWithRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
 }
 
 #[derive(Diagnostic)]
+#[diag(
+    mir_build_initializing_type_with_target_feature_requires_unsafe_unsafe_op_in_unsafe_fn_allowed,
+    code = E0133
+)]
+#[note]
+pub(crate) struct InitializingTypeWithTargetFeatureRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
+#[derive(Diagnostic)]
 #[diag(mir_build_mutable_static_requires_unsafe, code = E0133)]
 #[note]
 pub(crate) struct UseOfMutableStaticRequiresUnsafe {