Relax ordering rules for asm! operands

The `asm!` and `global_asm!` macros require their operands to appear
strictly in the following order:
- Template strings
- Positional operands
- Named operands
- Explicit register operands
- `clobber_abi`
- `options`

This is overly strict and can be inconvienent when building complex
`asm!` statements with macros. This PR relaxes the ordering requirements
as follows:
- Template strings must still come before all other operands.
- Positional operands must still come before named and explicit register
operands.
- Named and explicit register operands can be freely mixed.
- `options` and `clobber_abi` can appear in any position.

compiler/rustc_builtin_macros/src/asm.rs

@@ -203,17 +203,6 @@ pub fn parse_asm_args<'a>(
         // Validate the order of named, positional & explicit register operands and
         // clobber_abi/options. We do this at the end once we have the full span
         // of the argument available.
-        if !args.options_spans.is_empty() {
-            diag.struct_span_err(span, "arguments are not allowed after options")
-                .span_labels(args.options_spans.clone(), "previous options")
-                .span_label(span, "argument")
-                .emit();
-        } else if let Some((_, abi_span)) = args.clobber_abis.last() {
-            diag.struct_span_err(span, "arguments are not allowed after clobber_abi")
-                .span_label(*abi_span, "clobber_abi")
-                .span_label(span, "argument")
-                .emit();
-        }
         if explicit_reg {
             if name.is_some() {
                 diag.struct_span_err(span, "explicit register arguments cannot have names").emit();
@@ -227,17 +216,6 @@ pub fn parse_asm_args<'a>(
                     .emit();
                 continue;
             }
-            if !args.reg_args.is_empty() {
-                let mut err = diag.struct_span_err(
-                    span,
-                    "named arguments cannot follow explicit register arguments",
-                );
-                err.span_label(span, "named argument");
-                for pos in &args.reg_args {
-                    err.span_label(args.operands[*pos].1, "explicit register argument");
-                }
-                err.emit();
-            }
             args.named_args.insert(name, slot);
         } else {
             if !args.named_args.is_empty() || !args.reg_args.is_empty() {
@@ -478,15 +456,6 @@ fn parse_clobber_abi<'a>(p: &mut Parser<'a>, args: &mut AsmArgs) -> PResult<'a,
 
     let full_span = span_start.to(p.prev_token.span);
 
-    if !args.options_spans.is_empty() {
-        let mut err = p
-            .sess
-            .span_diagnostic
-            .struct_span_err(full_span, "clobber_abi is not allowed after options");
-        err.span_labels(args.options_spans.clone(), "options");
-        return Err(err);
-    }
-
     match &new_abis[..] {
         // should have errored above during parsing
         [] => unreachable!(),
@@ -699,6 +668,10 @@ fn expand_preparsed_asm(ecx: &mut ExtCtxt<'_>, args: AsmArgs) -> Option<ast::Inl
                                         args.operands[idx].1,
                                         "explicit register arguments cannot be used in the asm template",
                                     );
+                                    err.span_help(
+                                        args.operands[idx].1,
+                                        "use the register name directly in the assembly code",
+                                    );
                                 }
                                 err.emit();
                                 None

tests/ui/asm/aarch64/parse-error.rs

@@ -37,21 +37,18 @@ fn main() {
         asm!("", options(nomem, foo));
         //~^ ERROR expected one of
         asm!("{}", options(), const foo);
-        //~^ ERROR arguments are not allowed after options
-        //~^^ ERROR attempt to use a non-constant value in a constant
+        //~^ ERROR attempt to use a non-constant value in a constant
         asm!("", clobber_abi(foo));
         //~^ ERROR expected string literal
         asm!("", clobber_abi("C" foo));
         //~^ ERROR expected one of `)` or `,`, found `foo`
         asm!("", clobber_abi("C", foo));
         //~^ ERROR expected string literal
         asm!("{}", clobber_abi("C"), const foo);
-        //~^ ERROR arguments are not allowed after clobber_abi
-        //~^^ ERROR attempt to use a non-constant value in a constant
+        //~^ ERROR attempt to use a non-constant value in a constant
         asm!("", options(), clobber_abi("C"));
-        //~^ ERROR clobber_abi is not allowed after options
         asm!("{}", options(), clobber_abi("C"), const foo);
-        //~^ ERROR clobber_abi is not allowed after options
+        //~^ ERROR attempt to use a non-constant value in a constant
         asm!("{a}", a = const foo, a = const bar);
         //~^ ERROR duplicate argument named `a`
         //~^^ ERROR argument never used
@@ -60,11 +57,9 @@ fn main() {
         asm!("", a = in("x0") foo);
         //~^ ERROR explicit register arguments cannot have names
         asm!("{a}", in("x0") foo, a = const bar);
-        //~^ ERROR named arguments cannot follow explicit register arguments
-        //~^^ ERROR attempt to use a non-constant value in a constant
+        //~^ ERROR attempt to use a non-constant value in a constant
         asm!("{a}", in("x0") foo, a = const bar);
-        //~^ ERROR named arguments cannot follow explicit register arguments
-        //~^^ ERROR attempt to use a non-constant value in a constant
+        //~^ ERROR attempt to use a non-constant value in a constant
         asm!("{1}", in("x0") foo, const bar);
         //~^ ERROR positional arguments cannot follow named arguments or explicit register arguments
         //~^^ ERROR attempt to use a non-constant value in a constant
@@ -106,20 +101,18 @@ global_asm!("", options(nomem FOO));
 global_asm!("", options(nomem, FOO));
 //~^ ERROR expected one of
 global_asm!("{}", options(), const FOO);
-//~^ ERROR arguments are not allowed after options
 global_asm!("", clobber_abi(FOO));
 //~^ ERROR expected string literal
 global_asm!("", clobber_abi("C" FOO));
 //~^ ERROR expected one of `)` or `,`, found `FOO`
 global_asm!("", clobber_abi("C", FOO));
 //~^ ERROR expected string literal
 global_asm!("{}", clobber_abi("C"), const FOO);
-//~^ ERROR arguments are not allowed after clobber_abi
-//~^^ ERROR `clobber_abi` cannot be used with `global_asm!`
+//~^ ERROR `clobber_abi` cannot be used with `global_asm!`
 global_asm!("", options(), clobber_abi("C"));
-//~^ ERROR clobber_abi is not allowed after options
+//~^ ERROR `clobber_abi` cannot be used with `global_asm!`
 global_asm!("{}", options(), clobber_abi("C"), const FOO);
-//~^ ERROR clobber_abi is not allowed after options
+//~^ ERROR `clobber_abi` cannot be used with `global_asm!`
 global_asm!("{a}", a = const FOO, a = const BAR);
 //~^ ERROR duplicate argument named `a`
 //~^^ ERROR argument never used