Rollup merge of #88209 - Amanieu:asm_in_underscore, r=nagisa

Improve error message when _ is used for in/inout asm operands

As suggested by ```@Commeownist``` in https://github.com/rust-lang/rust/issues/72016#issuecomment-903102415.

1 files changed, 12 insertions, 0 deletions

diff --git a/compiler/rustc_builtin_macros/src/asm.rs b/compiler/rustc_builtin_macros/src/asm.rs
index 693114e3786..c032364c008 100644
--- a/compiler/rustc_builtin_macros/src/asm.rs
+++ b/compiler/rustc_builtin_macros/src/asm.rs
@@ -117,6 +117,10 @@ fn parse_args<'a>(
         let mut explicit_reg = false;
         let op = if !is_global_asm && p.eat_keyword(kw::In) {
             let reg = parse_reg(&mut p, &mut explicit_reg)?;
+            if p.eat_keyword(kw::Underscore) {
+                let err = ecx.struct_span_err(p.token.span, "_ cannot be used for input operands");
+                return Err(err);
+            }
             let expr = p.parse_expr()?;
             ast::InlineAsmOperand::In { reg, expr }
         } else if !is_global_asm && p.eat_keyword(sym::out) {
@@ -129,6 +133,10 @@ fn parse_args<'a>(
             ast::InlineAsmOperand::Out { reg, expr, late: true }
         } else if !is_global_asm && p.eat_keyword(sym::inout) {
             let reg = parse_reg(&mut p, &mut explicit_reg)?;
+            if p.eat_keyword(kw::Underscore) {
+                let err = ecx.struct_span_err(p.token.span, "_ cannot be used for input operands");
+                return Err(err);
+            }
             let expr = p.parse_expr()?;
             if p.eat(&token::FatArrow) {
                 let out_expr =
@@ -139,6 +147,10 @@ fn parse_args<'a>(
             }
         } else if !is_global_asm && p.eat_keyword(sym::inlateout) {
             let reg = parse_reg(&mut p, &mut explicit_reg)?;
+            if p.eat_keyword(kw::Underscore) {
+                let err = ecx.struct_span_err(p.token.span, "_ cannot be used for input operands");
+                return Err(err);
+            }
             let expr = p.parse_expr()?;
             if p.eat(&token::FatArrow) {
                 let out_expr =