Rollup merge of rust-lang#77976 - oliviacrain:issue-77915-fix, r=matthewjasper

Mark inout asm! operands as used in liveness pass

Variables used in `inout` operands in inline assembly (that is, they're used as both input and output to some arbitrary assembly instruction) are being marked as read and written, but are not marked as being used in the RWU table during the liveness pass. This can result in such expressions triggering an unused variable lint warning. This is incorrect behavior- reads without uses are currently only used for compound assignments. We conservatively assume that an `inout` operand is being read and used in the context of the assembly instruction.

Closes rust-lang#77915

Author: JohnTitor

compiler/rustc_passes/src/liveness.rs

@@ -1174,7 +1174,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                             }
                         }
                         hir::InlineAsmOperand::InOut { expr, .. } => {
-                            succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE);
+                            succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE | ACC_USE);
                         }
                         hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
                             if let Some(expr) = out_expr {

src/test/ui/liveness/liveness-asm.rs

@@ -0,0 +1,43 @@
+// Ensure inout asm! operands are marked as used by the liveness pass
+
+// check-pass
+
+#![feature(asm)]
+#![allow(dead_code)]
+#![warn(unused_assignments)]
+#![warn(unused_variables)]
+
+// Test the single inout case
+unsafe fn f1(mut src: *const u8) {
+    asm!("/*{0}*/", inout(reg) src); //~ WARN value assigned to `src` is never read
+}
+
+unsafe fn f2(mut src: *const u8) -> *const u8 {
+    asm!("/*{0}*/", inout(reg) src);
+    src
+}
+
+// Test the split inout case
+unsafe fn f3(mut src: *const u8) {
+    asm!("/*{0}*/", inout(reg) src => src); //~ WARN value assigned to `src` is never read
+}
+
+unsafe fn f4(mut src: *const u8) -> *const u8 {
+    asm!("/*{0}*/", inout(reg) src => src);
+    src
+}
+
+// Tests the use of field projections
+struct S {
+    field: *mut u8,
+}
+
+unsafe fn f5(src: &mut S) {
+    asm!("/*{0}*/", inout(reg) src.field);
+}
+
+unsafe fn f6(src: &mut S) {
+    asm!("/*{0}*/", inout(reg) src.field => src.field);
+}
+
+fn main() {}

src/test/ui/liveness/liveness-asm.stderr

@@ -0,0 +1,23 @@
+warning: value assigned to `src` is never read
+  --> $DIR/liveness-asm.rs:12:32
+   |
+LL |     asm!("/*{0}*/", inout(reg) src);
+   |                                ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/liveness-asm.rs:7:9
+   |
+LL | #![warn(unused_assignments)]
+   |         ^^^^^^^^^^^^^^^^^^
+   = help: maybe it is overwritten before being read?
+
+warning: value assigned to `src` is never read
+  --> $DIR/liveness-asm.rs:22:39
+   |
+LL |     asm!("/*{0}*/", inout(reg) src => src);
+   |                                       ^^^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: 2 warnings emitted
+