sema: forbid asm output to const locals

src/Sema.zig

@@ -16614,6 +16614,7 @@ fn zirAsm(
     const clobbers_len: u5 = @truncate(extended.small >> 10);
     const is_volatile = @as(u1, @truncate(extended.small >> 15)) != 0;
     const is_global_assembly = sema.func_index == .none;
+    const zir_tags = sema.code.instructions.items(.tag);
 
     const asm_source: []const u8 = if (tmpl_is_expr) blk: {
         const tmpl: Zir.Inst.Ref = @enumFromInt(@intFromEnum(extra.data.asm_source));
@@ -16674,6 +16675,13 @@ fn zirAsm(
         const name = sema.code.nullTerminatedString(output.data.name);
         needed_capacity += (constraint.len + name.len + (2 + 3)) / 4;
 
+        if (output.data.operand.toIndex()) |index| {
+            if (zir_tags[@intFromEnum(index)] == .ref) {
+                // TODO: better error location; it would be even nicer if there were notes that pointed at the output and the variable definition
+                return sema.fail(block, src, "asm cannot output to const local '{s}'", .{name});
+            }
+        }
+
         outputs[out_i] = .{ .c = constraint, .n = name };
     }
 

test/cases/compile_errors/asm_output_to_const.zig

@@ -0,0 +1,14 @@
+export fn foo() void {
+    const f: i64 = 1000;
+
+    asm volatile (
+        \\ movq $10, %[f]
+        : [f] "=r" (f),
+    );
+}
+
+// error
+// backend=llvm
+// target=native
+//
+// :4:5: error: asm cannot output to const local 'f'