show supported register classes in error message

compiler/rustc_ast_lowering/messages.ftl

@@ -112,7 +112,8 @@ ast_lowering_invalid_register =
     invalid register `{$reg}`: {$error}
 
 ast_lowering_invalid_register_class =
-    invalid register class `{$reg_class}`: {$error}
+    invalid register class `{$reg_class}`: unknown register class
+    .note = the following register classes are supported on this target: {$supported_register_classes}
 
 ast_lowering_match_arm_with_no_body =
     `match` arm with no body

compiler/rustc_ast_lowering/src/asm.rs

@@ -152,11 +152,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     InlineAsmRegOrRegClass::RegClass(reg_class) => {
                         asm::InlineAsmRegOrRegClass::RegClass(if let Some(asm_arch) = asm_arch {
                             asm::InlineAsmRegClass::parse(asm_arch, reg_class).unwrap_or_else(
-                                |error| {
+                                |supported_register_classes| {
+                                    let mut register_classes =
+                                        format!("`{}`", supported_register_classes[0]);
+                                    for m in &supported_register_classes[1..] {
+                                        let _ = write!(register_classes, ", `{m}`");
+                                    }
                                     self.dcx().emit_err(InvalidRegisterClass {
                                         op_span: *op_sp,
                                         reg_class,
-                                        error,
+                                        supported_register_classes: register_classes,
                                     });
                                     asm::InlineAsmRegClass::Err
                                 },

compiler/rustc_ast_lowering/src/errors.rs

@@ -212,12 +212,13 @@ pub(crate) struct InvalidRegister<'a> {
 }
 
 #[derive(Diagnostic)]
+#[note]
 #[diag(ast_lowering_invalid_register_class)]
-pub(crate) struct InvalidRegisterClass<'a> {
+pub(crate) struct InvalidRegisterClass {
     #[primary_span]
     pub op_span: Span,
     pub reg_class: Symbol,
-    pub error: &'a str,
+    pub supported_register_classes: String,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_target/src/asm/mod.rs

@@ -39,12 +39,12 @@ macro_rules! def_reg_class {
                 }
             }
 
-            pub fn parse(name: rustc_span::Symbol) -> Result<Self, &'static str> {
+            pub fn parse(name: rustc_span::Symbol) -> Result<Self, &'static [rustc_span::Symbol]> {
                 match name {
                     $(
                         rustc_span::sym::$class => Ok(Self::$class),
                     )*
-                    _ => Err("unknown register class"),
+                    _ => Err(&[$(rustc_span::sym::$class),*]),
                 }
             }
         }
@@ -635,7 +635,7 @@ impl InlineAsmRegClass {
         }
     }
 
-    pub fn parse(arch: InlineAsmArch, name: Symbol) -> Result<Self, &'static str> {
+    pub fn parse(arch: InlineAsmArch, name: Symbol) -> Result<Self, &'static [rustc_span::Symbol]> {
         Ok(match arch {
             InlineAsmArch::X86 | InlineAsmArch::X86_64 => {
                 Self::X86(X86InlineAsmRegClass::parse(name)?)

tests/ui/asm/aarch64/bad-reg.stderr

@@ -3,6 +3,8 @@ error: invalid register class `foo`: unknown register class
    |
 LL |         asm!("{}", in(foo) foo);
    |                    ^^^^^^^^^^^
+   |
+   = note: the following register classes are supported on this target: `reg`, `vreg`, `vreg_low16`, `preg`
 
 error: invalid register `foo`: unknown register
   --> $DIR/bad-reg.rs:14:18

tests/ui/asm/x86_64/bad-reg.stderr

@@ -3,6 +3,8 @@ error: invalid register class `foo`: unknown register class
    |
 LL |         asm!("{}", in(foo) foo);
    |                    ^^^^^^^^^^^
+   |
+   = note: the following register classes are supported on this target: `reg`, `reg_abcd`, `reg_byte`, `xmm_reg`, `ymm_reg`, `zmm_reg`, `kreg`, `kreg0`, `mmx_reg`, `x87_reg`, `tmm_reg`
 
 error: invalid register `foo`: unknown register
   --> $DIR/bad-reg.rs:14:18

tests/ui/asm/x86_64/issue-82869.stderr

@@ -3,12 +3,16 @@ error: invalid register class `vreg`: unknown register class
    |
 LL |     asm!("add {:d}, {:d}, d0", out(vreg) c, in(vreg) a, in("d0") {
    |                                ^^^^^^^^^^^
+   |
+   = note: the following register classes are supported on this target: `reg`, `reg_abcd`, `reg_byte`, `xmm_reg`, `ymm_reg`, `zmm_reg`, `kreg`, `kreg0`, `mmx_reg`, `x87_reg`, `tmm_reg`
 
 error: invalid register class `vreg`: unknown register class
   --> $DIR/issue-82869.rs:11:45
    |
 LL |     asm!("add {:d}, {:d}, d0", out(vreg) c, in(vreg) a, in("d0") {
    |                                             ^^^^^^^^^^
+   |
+   = note: the following register classes are supported on this target: `reg`, `reg_abcd`, `reg_byte`, `xmm_reg`, `ymm_reg`, `zmm_reg`, `kreg`, `kreg0`, `mmx_reg`, `x87_reg`, `tmm_reg`
 
 error: invalid register `d0`: unknown register
   --> $DIR/issue-82869.rs:11:57