rust-lang · rust-bors · Mar 23, 2026 · Mar 22, 2026
diff --git a/compiler/rustc_ast_lowering/src/asm.rs b/compiler/rustc_ast_lowering/src/asm.rs
@@ -1,5 +1,4 @@
 use std::collections::hash_map::Entry;
-use std::fmt::Write;
 
 use rustc_ast::*;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
@@ -124,13 +123,9 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
                         self.dcx().emit_err(ClobberAbiNotSupported { abi_span: *abi_span });
                     }
                     Err(supported_abis) => {
-                        let mut abis = format!("`{}`", supported_abis[0]);
-                        for m in &supported_abis[1..] {
-                            let _ = write!(abis, ", `{m}`");
-                        }
                         self.dcx().emit_err(InvalidAbiClobberAbi {
                             abi_span: *abi_span,
-                            supported_abis: abis,
+                            supported_abis: supported_abis.to_vec().into(),
                         });
                     }
                 }
@@ -164,15 +159,12 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
                         asm::InlineAsmRegOrRegClass::RegClass(if let Some(asm_arch) = asm_arch {
                             asm::InlineAsmRegClass::parse(asm_arch, reg_class).unwrap_or_else(
                                 |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,
-                                        supported_register_classes: register_classes,
+                                        supported_register_classes: supported_register_classes
+                                            .to_vec()
+                                            .into(),
                                     });
                                     asm::InlineAsmRegClass::Err
                                 },
@@ -272,23 +264,20 @@ impl<'hir, R: ResolverAstLoweringExt<'hir>> LoweringContext<'_, 'hir, R> {
                         }
                         let valid_modifiers = class.valid_modifiers(asm_arch.unwrap());
                         if !valid_modifiers.contains(&modifier) {
-                            let sub = if !valid_modifiers.is_empty() {
-                                let mut mods = format!("`{}`", valid_modifiers[0]);
-                                for m in &valid_modifiers[1..] {
-                                    let _ = write!(mods, ", `{m}`");
-                                }
-                                InvalidAsmTemplateModifierRegClassSub::SupportModifier {
+                            let sub = if valid_modifiers.is_empty() {
+                                InvalidAsmTemplateModifierRegClassSub::DoesNotSupportModifier {
                                     class_name: class.name(),
-                                    modifiers: mods,
                                 }
                             } else {
-                                InvalidAsmTemplateModifierRegClassSub::DoesNotSupportModifier {
+                                InvalidAsmTemplateModifierRegClassSub::SupportModifier {
                                     class_name: class.name(),
+                                    modifiers: valid_modifiers.to_vec().into(),
                                 }
                             };
                             self.dcx().emit_err(InvalidAsmTemplateModifierRegClass {
                                 placeholder_span,
                                 op_span: op_sp,
+                                modifier: modifier.to_string(),
                                 sub,
                             });
                         }

diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
@@ -1,5 +1,5 @@
-use rustc_errors::DiagArgFromDisplay;
 use rustc_errors::codes::*;
+use rustc_errors::{DiagArgFromDisplay, DiagSymbolList};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{Ident, Span, Symbol};
 
@@ -191,10 +191,10 @@ pub(crate) struct ClobberAbiNotSupported {
 #[derive(Diagnostic)]
 #[note("the following ABIs are supported on this target: {$supported_abis}")]
 #[diag("invalid ABI for `clobber_abi`")]
-pub(crate) struct InvalidAbiClobberAbi {
+pub(crate) struct InvalidAbiClobberAbi<'a> {
     #[primary_span]
     pub abi_span: Span,
-    pub supported_abis: String,
+    pub supported_abis: DiagSymbolList<&'a str>,
 }
 
 #[derive(Diagnostic)]
@@ -215,17 +215,18 @@ pub(crate) struct InvalidRegisterClass {
     #[primary_span]
     pub op_span: Span,
     pub reg_class: Symbol,
-    pub supported_register_classes: String,
+    pub supported_register_classes: DiagSymbolList<Symbol>,
 }
 
 #[derive(Diagnostic)]
-#[diag("invalid asm template modifier for this register class")]
+#[diag("invalid asm template modifier `{$modifier}` for this register class")]
 pub(crate) struct InvalidAsmTemplateModifierRegClass {
     #[primary_span]
     #[label("template modifier")]
     pub placeholder_span: Span,
     #[label("argument")]
     pub op_span: Span,
+    pub modifier: String,
     #[subdiagnostic]
     pub sub: InvalidAsmTemplateModifierRegClassSub,
 }
@@ -235,7 +236,7 @@ pub(crate) enum InvalidAsmTemplateModifierRegClassSub {
     #[note(
         "the `{$class_name}` register class supports the following template modifiers: {$modifiers}"
     )]
-    SupportModifier { class_name: Symbol, modifiers: String },
+    SupportModifier { class_name: Symbol, modifiers: DiagSymbolList<char> },
     #[note("the `{$class_name}` register class does not support template modifiers")]
     DoesNotSupportModifier { class_name: Symbol },
 }

diff --git a/tests/ui/asm/aarch64/bad-options.stderr b/tests/ui/asm/aarch64/bad-options.stderr
@@ -78,7 +78,7 @@ error: invalid ABI for `clobber_abi`
 LL |         asm!("", clobber_abi("foo"));
    |                  ^^^^^^^^^^^^^^^^^^
    |
-   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`
+   = note: the following ABIs are supported on this target: `C`, `system`, and `efiapi`
 
 error: aborting due to 13 previous errors
 
diff --git a/tests/ui/asm/aarch64/bad-reg.rs b/tests/ui/asm/aarch64/bad-reg.rs
@@ -1,7 +1,12 @@
+//@ add-minicore
 //@ only-aarch64
 //@ compile-flags: -C target-feature=+neon
+#![crate_type = "lib"]
+#![feature(no_core)]
+#![no_core]
 
-use std::arch::asm;
+extern crate minicore;
+use minicore::*;
 
 fn main() {
     let mut foo = 0;
@@ -14,11 +19,11 @@ fn main() {
         asm!("", in("foo") foo);
         //~^ ERROR invalid register `foo`: unknown register
         asm!("{:z}", in(reg) foo);
-        //~^ ERROR invalid asm template modifier for this register class
+        //~^ ERROR invalid asm template modifier `z` for this register class
         asm!("{:r}", in(vreg) foo);
-        //~^ ERROR invalid asm template modifier for this register class
+        //~^ ERROR invalid asm template modifier `r` for this register class
         asm!("{:r}", in(vreg_low16) foo);
-        //~^ ERROR invalid asm template modifier for this register class
+        //~^ ERROR invalid asm template modifier `r` for this register class
         asm!("{:a}", const 0);
         //~^ ERROR asm template modifiers are not allowed for `const` arguments
         asm!("{:a}", sym main);

diff --git a/tests/ui/asm/aarch64/bad-reg.stderr b/tests/ui/asm/aarch64/bad-reg.stderr
@@ -1,159 +1,159 @@
 error: invalid register class `foo`: unknown register class
-  --> $DIR/bad-reg.rs:12:20
+  --> $DIR/bad-reg.rs:17:20
    |
 LL |         asm!("{}", in(foo) foo);
    |                    ^^^^^^^^^^^
    |
-   = note: the following register classes are supported on this target: `reg`, `vreg`, `vreg_low16`, `preg`
+   = note: the following register classes are supported on this target: `reg`, `vreg`, `vreg_low16`, and `preg`
 
 error: invalid register `foo`: unknown register
-  --> $DIR/bad-reg.rs:14:18
+  --> $DIR/bad-reg.rs:19:18
    |
 LL |         asm!("", in("foo") foo);
    |                  ^^^^^^^^^^^^^
 
-error: invalid asm template modifier for this register class
-  --> $DIR/bad-reg.rs:16:15
+error: invalid asm template modifier `z` for this register class
+  --> $DIR/bad-reg.rs:21:15
    |
 LL |         asm!("{:z}", in(reg) foo);
    |               ^^^^   ----------- argument
    |               |
    |               template modifier
    |
-   = note: the `reg` register class supports the following template modifiers: `w`, `x`
+   = note: the `reg` register class supports the following template modifiers: `w` and `x`
 
-error: invalid asm template modifier for this register class
-  --> $DIR/bad-reg.rs:18:15
+error: invalid asm template modifier `r` for this register class
+  --> $DIR/bad-reg.rs:23:15
    |
 LL |         asm!("{:r}", in(vreg) foo);
    |               ^^^^   ------------ argument
    |               |
    |               template modifier
    |
-   = note: the `vreg` register class supports the following template modifiers: `b`, `h`, `s`, `d`, `q`, `v`
+   = note: the `vreg` register class supports the following template modifiers: `b`, `h`, `s`, `d`, `q`, and `v`
 
-error: invalid asm template modifier for this register class
-  --> $DIR/bad-reg.rs:20:15
+error: invalid asm template modifier `r` for this register class
+  --> $DIR/bad-reg.rs:25:15
    |
 LL |         asm!("{:r}", in(vreg_low16) foo);
    |               ^^^^   ------------------ argument
    |               |
    |               template modifier
    |
-   = note: the `vreg_low16` register class supports the following template modifiers: `b`, `h`, `s`, `d`, `q`, `v`
+   = note: the `vreg_low16` register class supports the following template modifiers: `b`, `h`, `s`, `d`, `q`, and `v`
 
 error: asm template modifiers are not allowed for `const` arguments
-  --> $DIR/bad-reg.rs:22:15
+  --> $DIR/bad-reg.rs:27:15
    |
 LL |         asm!("{:a}", const 0);
    |               ^^^^   ------- argument
    |               |
    |               template modifier
 
 error: asm template modifiers are not allowed for `sym` arguments
-  --> $DIR/bad-reg.rs:24:15
+  --> $DIR/bad-reg.rs:29:15
    |
 LL |         asm!("{:a}", sym main);
    |               ^^^^   -------- argument
    |               |
    |               template modifier
 
 error: invalid register `x29`: the frame pointer cannot be used as an operand for inline asm
-  --> $DIR/bad-reg.rs:26:18
+  --> $DIR/bad-reg.rs:31:18
    |
 LL |         asm!("", in("x29") foo);
    |                  ^^^^^^^^^^^^^
 
 error: invalid register `sp`: the stack pointer cannot be used as an operand for inline asm
-  --> $DIR/bad-reg.rs:28:18
+  --> $DIR/bad-reg.rs:33:18
    |
 LL |         asm!("", in("sp") foo);
    |                  ^^^^^^^^^^^^
 
 error: invalid register `xzr`: the zero register cannot be used as an operand for inline asm
-  --> $DIR/bad-reg.rs:30:18
+  --> $DIR/bad-reg.rs:35:18
    |
 LL |         asm!("", in("xzr") foo);
    |                  ^^^^^^^^^^^^^
 
 error: invalid register `x19`: x19 is used internally by LLVM and cannot be used as an operand for inline asm
-  --> $DIR/bad-reg.rs:32:18
+  --> $DIR/bad-reg.rs:37:18
    |
 LL |         asm!("", in("x19") foo);
    |                  ^^^^^^^^^^^^^
 
 error: register class `preg` can only be used as a clobber, not as an input or output
-  --> $DIR/bad-reg.rs:35:18
+  --> $DIR/bad-reg.rs:40:18
    |
 LL |         asm!("", in("p0") foo);
    |                  ^^^^^^^^^^^^
 
 error: register class `preg` can only be used as a clobber, not as an input or output
-  --> $DIR/bad-reg.rs:39:20
+  --> $DIR/bad-reg.rs:44:20
    |
 LL |         asm!("{}", in(preg) foo);
    |                    ^^^^^^^^^^^^
 
 error: register class `preg` can only be used as a clobber, not as an input or output
-  --> $DIR/bad-reg.rs:42:20
+  --> $DIR/bad-reg.rs:47:20
    |
 LL |         asm!("{}", out(preg) _);
    |                    ^^^^^^^^^^^
 
 error: register `w0` conflicts with register `x0`
-  --> $DIR/bad-reg.rs:48:32
+  --> $DIR/bad-reg.rs:53:32
    |
 LL |         asm!("", in("x0") foo, in("w0") bar);
    |                  ------------  ^^^^^^^^^^^^ register `w0`
    |                  |
    |                  register `x0`
 
 error: register `x0` conflicts with register `x0`
-  --> $DIR/bad-reg.rs:50:32
+  --> $DIR/bad-reg.rs:55:32
    |
 LL |         asm!("", in("x0") foo, out("x0") bar);
    |                  ------------  ^^^^^^^^^^^^^ register `x0`
    |                  |
    |                  register `x0`
    |
 help: use `lateout` instead of `out` to avoid conflict
-  --> $DIR/bad-reg.rs:50:18
+  --> $DIR/bad-reg.rs:55:18
    |
 LL |         asm!("", in("x0") foo, out("x0") bar);
    |                  ^^^^^^^^^^^^
 
 error: register `q0` conflicts with register `v0`
-  --> $DIR/bad-reg.rs:53:32
+  --> $DIR/bad-reg.rs:58:32
    |
 LL |         asm!("", in("v0") foo, in("q0") bar);
    |                  ------------  ^^^^^^^^^^^^ register `q0`
    |                  |
    |                  register `v0`
 
 error: register `q0` conflicts with register `v0`
-  --> $DIR/bad-reg.rs:55:32
+  --> $DIR/bad-reg.rs:60:32
    |
 LL |         asm!("", in("v0") foo, out("q0") bar);
    |                  ------------  ^^^^^^^^^^^^^ register `q0`
    |                  |
    |                  register `v0`
    |
 help: use `lateout` instead of `out` to avoid conflict
-  --> $DIR/bad-reg.rs:55:18
+  --> $DIR/bad-reg.rs:60:18
    |
 LL |         asm!("", in("v0") foo, out("q0") bar);
    |                  ^^^^^^^^^^^^
 
 error: type `i32` cannot be used with this register class
-  --> $DIR/bad-reg.rs:35:27
+  --> $DIR/bad-reg.rs:40:27
    |
 LL |         asm!("", in("p0") foo);
    |                           ^^^
    |
    = note: register class `preg` supports these types: 
 
 error: type `i32` cannot be used with this register class
-  --> $DIR/bad-reg.rs:39:29
+  --> $DIR/bad-reg.rs:44:29
    |
 LL |         asm!("{}", in(preg) foo);
    |                             ^^^

diff --git a/tests/ui/asm/x86_64/bad-clobber-abi.stderr b/tests/ui/asm/x86_64/bad-clobber-abi.stderr
@@ -4,15 +4,15 @@ error: invalid ABI for `clobber_abi`
 LL |         asm!("", clobber_abi("foo"));
    |                  ^^^^^^^^^^^^^^^^^^
    |
-   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, `sysv64`
+   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, and `sysv64`
 
 error: invalid ABI for `clobber_abi`
   --> $DIR/bad-clobber-abi.rs:13:35
    |
 LL |         asm!("", clobber_abi("C", "foo"));
    |                                   ^^^^^
    |
-   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, `sysv64`
+   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, and `sysv64`
 
 error: `C` ABI specified multiple times
   --> $DIR/bad-clobber-abi.rs:15:35
@@ -38,7 +38,7 @@ error: invalid ABI for `clobber_abi`
 LL |         asm!("", clobber_abi("C", "foo", "C"));
    |                                   ^^^^^
    |
-   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, `sysv64`
+   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, and `sysv64`
 
 error: `C` ABI specified multiple times
   --> $DIR/bad-clobber-abi.rs:20:42
@@ -54,7 +54,7 @@ error: invalid ABI for `clobber_abi`
 LL |         asm!("", clobber_abi("win64", "foo", "efiapi"));
    |                                       ^^^^^
    |
-   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, `sysv64`
+   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, and `sysv64`
 
 error: `win64` ABI specified multiple times
   --> $DIR/bad-clobber-abi.rs:23:46

diff --git a/tests/ui/asm/x86_64/bad-options.stderr b/tests/ui/asm/x86_64/bad-options.stderr
@@ -93,7 +93,7 @@ error: invalid ABI for `clobber_abi`
 LL |         asm!("", clobber_abi("foo"));
    |                  ^^^^^^^^^^^^^^^^^^
    |
-   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, `sysv64`
+   = note: the following ABIs are supported on this target: `C`, `system`, `efiapi`, `win64`, and `sysv64`
 
 error: `C` ABI specified multiple times
   --> $DIR/bad-options.rs:28:52