rustc: Forbid #[inline(always)] with #[target_feature]

Once a target feature is enabled for a function that means that it in general
can't be inlined into other functions which don't have that target feature
enabled. This can cause both safety and LLVM issues if we were to actually
inline it, so `#[inline(always)]` both can't be respected and would be an error
if we did so!

Today LLVM doesn't inline functions with different `#[target_feature]`
annotations, but it turns out that if one is tagged with `#[inline(always)]`
it'll override this and cause scary LLVM error to arise!

This commit fixes this issue by forbidding these two attributes to be used in
conjunction with one another.

cc rust-lang-nursery/stdsimd#404

src/librustc_typeck/collect.rs

@@ -1769,6 +1769,7 @@ fn trans_fn_attrs<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefId) -> TransFnAt
 
     let whitelist = tcx.target_features_whitelist(LOCAL_CRATE);
 
+    let mut inline_span = None;
     for attr in attrs.iter() {
         if attr.check_name("cold") {
             trans_fn_attrs.flags |= TransFnAttrFlags::COLD;
@@ -1800,6 +1801,7 @@ fn trans_fn_attrs<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefId) -> TransFnAt
                     }
                     MetaItemKind::List(ref items) => {
                         mark_used(attr);
+                        inline_span = Some(attr.span);
                         if items.len() != 1 {
                             span_err!(tcx.sess.diagnostic(), attr.span, E0534,
                                         "expected one argument");
@@ -1855,5 +1857,18 @@ fn trans_fn_attrs<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefId) -> TransFnAt
         }
     }
 
+    // If a function uses #[target_feature] it can't be inlined into general
+    // purpose functions as they wouldn't have the right target features
+    // enabled. For that reason we also forbid #[inline(always)] as it can't be
+    // respected.
+    if trans_fn_attrs.target_features.len() > 0 {
+        if trans_fn_attrs.inline == InlineAttr::Always {
+            if let Some(span) = inline_span {
+                tcx.sess.span_err(span, "cannot use #[inline(always)] with \
+                                         #[target_feature]");
+            }
+        }
+    }
+
     trans_fn_attrs
 }

src/test/ui/target-feature-wrong.rs

@@ -33,6 +33,11 @@ fn bar() {}
 //~^ ERROR: should be applied to a function
 mod another {}
 
+#[inline(always)]
+//~^ ERROR: cannot use #[inline(always)]
+#[target_feature(enable = "sse2")]
+unsafe fn test() {}
+
 fn main() {
     unsafe {
         foo();

src/test/ui/target-feature-wrong.stderr

@@ -37,5 +37,11 @@ LL | //~^ ERROR: should be applied to a function
 LL | mod another {}
    | -------------- not a function
 
-error: aborting due to 5 previous errors
+error: cannot use #[inline(always)] with #[target_feature]
+  --> $DIR/target-feature-wrong.rs:36:1
+   |
+LL | #[inline(always)]
+   | ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors