Rollup merge of #129339 - beetrees:make-indirect-from-ignore, r=RalfJung

Make `ArgAbi::make_indirect_force` more specific

As the method is only needed for making ignored ZSTs indirect on some ABIs, rename and add a doc-comment and `self.mode` check to make it harder to accidentally misuse. Addresses review feedback from https://github.com/rust-lang/rust/pull/125854#discussion_r1721047899.

r? ``@RalfJung``

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

@@ -642,7 +642,7 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
     pub fn make_indirect(&mut self) {
         match self.mode {
             PassMode::Direct(_) | PassMode::Pair(_, _) => {
-                self.make_indirect_force();
+                self.mode = Self::indirect_pass_mode(&self.layout);
             }
             PassMode::Indirect { attrs: _, meta_attrs: _, on_stack: false } => {
                 // already indirect
@@ -652,10 +652,20 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
         }
     }
 
-    /// Same as make_indirect, but doesn't check the current `PassMode`.
-    pub fn make_indirect_force(&mut self) {
+    /// Same as `make_indirect`, but for arguments that are ignored. Only needed for ABIs that pass
+    /// ZSTs indirectly.
+    pub fn make_indirect_from_ignore(&mut self) {
+        match self.mode {
+            PassMode::Ignore => {
                 self.mode = Self::indirect_pass_mode(&self.layout);
             }
+            PassMode::Indirect { attrs: _, meta_attrs: _, on_stack: false } => {
+                // already indirect
+                return;
+            }
+            _ => panic!("Tried to make {:?} indirect (expected `PassMode::Ignore`)", self.mode),
+        }
+    }
 
     /// Pass this argument indirectly, by placing it at a fixed stack offset.
     /// This corresponds to the `byval` LLVM argument attribute.

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

@@ -16,7 +16,7 @@ fn classify_arg<Ty>(cx: &impl HasTargetSpec, arg: &mut ArgAbi<'_, Ty>) {
             && matches!(&*cx.target_spec().env, "gnu" | "musl" | "uclibc")
             && arg.layout.is_zst()
         {
-            arg.make_indirect_force();
+            arg.make_indirect_from_ignore();
         }
         return;
     }

compiler/rustc_target/src/abi/call/s390x.rs

@@ -28,7 +28,7 @@ where
             && matches!(&*cx.target_spec().env, "gnu" | "musl" | "uclibc")
             && arg.layout.is_zst()
         {
-            arg.make_indirect_force();
+            arg.make_indirect_from_ignore();
         }
         return;
     }

compiler/rustc_target/src/abi/call/sparc64.rs

@@ -225,7 +225,7 @@ where
                 && matches!(&*cx.target_spec().env, "gnu" | "musl" | "uclibc")
                 && arg.layout.is_zst()
             {
-                arg.make_indirect_force();
+                arg.make_indirect_from_ignore();
             }
             return;
         }

compiler/rustc_target/src/abi/call/x86_win64.rs

@@ -43,7 +43,7 @@ pub fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>)
                 && cx.target_spec().env == "gnu"
                 && arg.layout.is_zst()
             {
-                arg.make_indirect_force();
+                arg.make_indirect_from_ignore();
             }
             continue;
         }