Remove StructRet arg attr

It is applied exactly when the return value has an indirect pass mode.
Except for InReg on x86 fastcall, arg attrs are now only used for
optimization purposes and thus are fine to ignore.

compiler/rustc_codegen_llvm/src/abi.rs

@@ -36,7 +36,7 @@ fn for_each_kind<F>(&self, mut f: F)
     where
         F: FnMut(llvm::Attribute),
     {
-        for_each_kind!(self, f, NoAlias, NoCapture, NonNull, ReadOnly, StructRet, InReg)
+        for_each_kind!(self, f, NoAlias, NoCapture, NonNull, ReadOnly, InReg)
     }
 }
 
@@ -429,7 +429,8 @@ fn apply_attrs_llfn(&self, cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value) {
             }
             PassMode::Indirect { ref attrs, extra_attrs: _, on_stack } => {
                 assert!(!on_stack);
-                apply(attrs);
+                let i = apply(attrs);
+                llvm::Attribute::StructRet.apply_llfn(llvm::AttributePlace::Argument(i), llfn);
             }
             _ => {}
         }
@@ -484,7 +485,9 @@ fn apply_attrs_callsite(&self, bx: &mut Builder<'a, 'll, 'tcx>, callsite: &'ll V
             }
             PassMode::Indirect { ref attrs, extra_attrs: _, on_stack } => {
                 assert!(!on_stack);
-                apply(attrs);
+                let i = apply(attrs);
+                llvm::Attribute::StructRet
+                    .apply_callsite(llvm::AttributePlace::Argument(i), callsite);
             }
             _ => {}
         }

compiler/rustc_middle/src/ty/layout.rs

@@ -2801,10 +2801,6 @@ fn adjust_for_abi(&mut self, cx: &C, abi: SpecAbi) {
             for arg in &mut self.args {
                 fixup(arg, false);
             }
-            if let PassMode::Indirect { ref mut attrs, extra_attrs: _, on_stack: _ } = self.ret.mode
-            {
-                attrs.set(ArgAttribute::StructRet);
-            }
             return;
         }
 

compiler/rustc_target/src/abi/call/mod.rs

@@ -59,7 +59,6 @@ pub struct ArgAttribute: u16 {
             const NoCapture = 1 << 2;
             const NonNull   = 1 << 3;
             const ReadOnly  = 1 << 4;
-            const StructRet = 1 << 6;
             const InReg     = 1 << 8;
         }
     }
@@ -619,10 +618,6 @@ pub fn adjust_for_cabi<C>(&mut self, cx: &C, abi: spec::abi::Abi) -> Result<(),
             a => return Err(format!("unrecognized arch \"{}\" in target specification", a)),
         }
 
-        if let PassMode::Indirect { ref mut attrs, extra_attrs: _, on_stack: _ } = self.ret.mode {
-            attrs.set(ArgAttribute::StructRet);
-        }
-
         Ok(())
     }
 }