Coerce safe-to-call target_feature functions to fn pointers.

compiler/rustc_hir_typeck/src/coercion.rs

@@ -920,7 +920,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         match b.kind() {
             ty::FnPtr(_, b_hdr) => {
-                let a_sig = a.fn_sig(self.tcx);
+                let mut a_sig = a.fn_sig(self.tcx);
                 if let ty::FnDef(def_id, _) = *a.kind() {
                     // Intrinsics are not coercible to function pointers
                     if self.tcx.intrinsic(def_id).is_some() {
@@ -932,19 +932,26 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                         return Err(TypeError::ForceInlineCast);
                     }
 
-                    let fn_attrs = self.tcx.codegen_fn_attrs(def_id);
-                    if matches!(fn_attrs.inline, InlineAttr::Force { .. }) {
-                        return Err(TypeError::ForceInlineCast);
-                    }
-
-                    // FIXME(target_feature): Safe `#[target_feature]` functions could be cast to safe fn pointers (RFC 2396),
-                    // as you can already write that "cast" in user code by wrapping a target_feature fn call in a closure,
-                    // which is safe. This is sound because you already need to be executing code that is satisfying the target
-                    // feature constraints..
                     if b_hdr.safety.is_safe()
                         && self.tcx.codegen_fn_attrs(def_id).safe_target_features
                     {
-                        return Err(TypeError::TargetFeatureCast(def_id));
+                        // Allow the coercion if the current function has all the features that would be
+                        // needed to call the coercee safely.
+                        let coercee_features = &self.tcx.codegen_fn_attrs(def_id).target_features;
+                        let body_features =
+                            &self.tcx.codegen_fn_attrs(self.fcx.body_id).target_features;
+                        if !self.tcx.sess.target.options.is_like_wasm
+                            && !coercee_features
+                                .iter()
+                                .all(|feature| body_features.iter().any(|f| f.name == feature.name))
+                        {
+                            return Err(TypeError::TargetFeatureCast(def_id));
+                        } else {
+                            // The coercee behaves like a safe function, since it is a target_feature
+                            // function that would be callable safely in this context.
+                            a_sig = a_sig
+                                .map_bound(|sig| ty::FnSig { safety: hir::Safety::Safe, ..sig })
+                        }
                     }
                 }
 

tests/ui/rfcs/rfc-2396-target_feature-11/fn-ptr.rs

@@ -2,9 +2,23 @@
 
 #![feature(target_feature_11)]
 
+#[target_feature(enable = "avx")]
+fn foo_avx() {}
+
 #[target_feature(enable = "sse2")]
 fn foo() {}
 
+#[target_feature(enable = "sse2")]
+fn bar() {
+    let foo: fn() = foo; // this is OK, as we have the necessary target features.
+    let foo: fn() = foo_avx; //~ ERROR mismatched types
+}
+
 fn main() {
+    if std::is_x86_feature_detected!("sse2") {
+        unsafe {
+            bar();
+        }
+    }
     let foo: fn() = foo; //~ ERROR mismatched types
 }

tests/ui/rfcs/rfc-2396-target_feature-11/fn-ptr.stderr

@@ -1,5 +1,20 @@
 error[E0308]: mismatched types
-  --> $DIR/fn-ptr.rs:9:21
+  --> $DIR/fn-ptr.rs:14:21
+   |
+LL | #[target_feature(enable = "avx")]
+   | --------------------------------- `#[target_feature]` added here
+...
+LL |     let foo: fn() = foo_avx;
+   |              ----   ^^^^^^^ cannot coerce functions with `#[target_feature]` to safe function pointers
+   |              |
+   |              expected due to this
+   |
+   = note: expected fn pointer `fn()`
+                 found fn item `#[target_features] fn() {foo_avx}`
+   = note: functions with `#[target_feature]` can only be coerced to `unsafe` function pointers
+
+error[E0308]: mismatched types
+  --> $DIR/fn-ptr.rs:23:21
    |
 LL | #[target_feature(enable = "sse2")]
    | ---------------------------------- `#[target_feature]` added here
@@ -13,6 +28,6 @@ LL |     let foo: fn() = foo;
                  found fn item `#[target_features] fn() {foo}`
    = note: functions with `#[target_feature]` can only be coerced to `unsafe` function pointers
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

tests/ui/rfcs/rfc-2396-target_feature-11/return-fn-ptr.rs

@@ -0,0 +1,22 @@
+//@ only-x86_64
+//@ run-pass
+
+#![feature(target_feature_11)]
+
+#[target_feature(enable = "sse2")]
+fn foo() -> bool {
+    true
+}
+
+#[target_feature(enable = "sse2")]
+fn bar() -> fn() -> bool {
+    foo
+}
+
+fn main() {
+    if !std::is_x86_feature_detected!("sse2") {
+        return;
+    }
+    let f = unsafe { bar() };
+    assert!(f());
+}