diff --git a/compiler/rustc_codegen_ssa/src/back/write.rs b/compiler/rustc_codegen_ssa/src/back/write.rs
index eeb251320a78..4211f875dd01 100644
--- a/compiler/rustc_codegen_ssa/src/back/write.rs
+++ b/compiler/rustc_codegen_ssa/src/back/write.rs
@@ -39,7 +39,6 @@ use rustc_target::spec::{MergeFunctions, SanitizerSet};
 
 use crate::errors::ErrorCreatingRemarkDir;
 use std::any::Any;
-use std::borrow::Cow;
 use std::fs;
 use std::io;
 use std::marker::PhantomData;
@@ -1812,7 +1811,7 @@ impl Translate for SharedEmitter {
 
 impl Emitter for SharedEmitter {
     fn emit_diagnostic(&mut self, diag: &rustc_errors::Diagnostic) {
-        let args: FxHashMap<Cow<'_, str>, DiagnosticArgValue> =
+        let args: FxHashMap<DiagnosticArgName, DiagnosticArgValue> =
             diag.args().map(|(name, arg)| (name.clone(), arg.clone())).collect();
         drop(self.sender.send(SharedEmitterMessage::Diagnostic(Diagnostic {
             msgs: diag.messages.clone(),
diff --git a/compiler/rustc_const_eval/src/const_eval/error.rs b/compiler/rustc_const_eval/src/const_eval/error.rs
index dabf78ef90e5..1ebc82ad6d4b 100644
--- a/compiler/rustc_const_eval/src/const_eval/error.rs
+++ b/compiler/rustc_const_eval/src/const_eval/error.rs
@@ -1,6 +1,8 @@
 use std::mem;
 
-use rustc_errors::{DiagnosticArgValue, DiagnosticMessage, IntoDiagnostic, IntoDiagnosticArg};
+use rustc_errors::{
+    DiagnosticArgName, DiagnosticArgValue, DiagnosticMessage, IntoDiagnostic, IntoDiagnosticArg,
+};
 use rustc_hir::CRATE_HIR_ID;
 use rustc_middle::mir::AssertKind;
 use rustc_middle::query::TyCtxtAt;
@@ -32,10 +34,7 @@ impl MachineStopType for ConstEvalErrKind {
             AssertFailure(x) => x.diagnostic_message(),
         }
     }
-    fn add_args(
-        self: Box<Self>,
-        adder: &mut dyn FnMut(std::borrow::Cow<'static, str>, DiagnosticArgValue),
-    ) {
+    fn add_args(self: Box<Self>, adder: &mut dyn FnMut(DiagnosticArgName, DiagnosticArgValue)) {
         use ConstEvalErrKind::*;
         match *self {
             ConstAccessesStatic | ModifiedGlobal => {}
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 789889973e2d..5579b50e85a9 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -900,7 +900,7 @@ impl Diagnostic {
 
     pub fn arg(
         &mut self,
-        name: impl Into<Cow<'static, str>>,
+        name: impl Into<DiagnosticArgName>,
         arg: impl IntoDiagnosticArg,
     ) -> &mut Self {
         self.args.insert(name.into(), arg.into_diagnostic_arg());
diff --git a/compiler/rustc_middle/src/error.rs b/compiler/rustc_middle/src/error.rs
index 211da8002029..3ebbcd650302 100644
--- a/compiler/rustc_middle/src/error.rs
+++ b/compiler/rustc_middle/src/error.rs
@@ -1,7 +1,6 @@
-use std::borrow::Cow;
 use std::fmt;
 
-use rustc_errors::{codes::*, DiagnosticArgValue, DiagnosticMessage};
+use rustc_errors::{codes::*, DiagnosticArgName, DiagnosticArgValue, DiagnosticMessage};
 use rustc_macros::Diagnostic;
 use rustc_span::{Span, Symbol};
 
@@ -95,14 +94,14 @@ pub(super) struct ConstNotUsedTraitAlias {
 
 pub struct CustomSubdiagnostic<'a> {
     pub msg: fn() -> DiagnosticMessage,
-    pub add_args: Box<dyn FnOnce(&mut dyn FnMut(Cow<'static, str>, DiagnosticArgValue)) + 'a>,
+    pub add_args: Box<dyn FnOnce(&mut dyn FnMut(DiagnosticArgName, DiagnosticArgValue)) + 'a>,
 }
 
 impl<'a> CustomSubdiagnostic<'a> {
     pub fn label(x: fn() -> DiagnosticMessage) -> Self {
         Self::label_and_then(x, |_| {})
     }
-    pub fn label_and_then<F: FnOnce(&mut dyn FnMut(Cow<'static, str>, DiagnosticArgValue)) + 'a>(
+    pub fn label_and_then<F: FnOnce(&mut dyn FnMut(DiagnosticArgName, DiagnosticArgValue)) + 'a>(
         msg: fn() -> DiagnosticMessage,
         f: F,
     ) -> Self {
diff --git a/compiler/rustc_middle/src/mir/interpret/error.rs b/compiler/rustc_middle/src/mir/interpret/error.rs
index 95574aee499f..0f69ab93452f 100644
--- a/compiler/rustc_middle/src/mir/interpret/error.rs
+++ b/compiler/rustc_middle/src/mir/interpret/error.rs
@@ -5,7 +5,9 @@ use crate::mir::{ConstAlloc, ConstValue};
 use crate::ty::{layout, tls, Ty, TyCtxt, ValTree};
 
 use rustc_data_structures::sync::Lock;
-use rustc_errors::{DiagnosticArgValue, DiagnosticMessage, ErrorGuaranteed, IntoDiagnosticArg};
+use rustc_errors::{
+    DiagnosticArgName, DiagnosticArgValue, DiagnosticMessage, ErrorGuaranteed, IntoDiagnosticArg,
+};
 use rustc_macros::HashStable;
 use rustc_session::CtfeBacktrace;
 use rustc_span::{def_id::DefId, Span, DUMMY_SP};
@@ -485,7 +487,7 @@ pub trait MachineStopType: Any + fmt::Debug + Send {
     fn diagnostic_message(&self) -> DiagnosticMessage;
     /// Add diagnostic arguments by passing name and value pairs to `adder`, which are passed to
     /// fluent for formatting the translated diagnostic message.
-    fn add_args(self: Box<Self>, adder: &mut dyn FnMut(Cow<'static, str>, DiagnosticArgValue));
+    fn add_args(self: Box<Self>, adder: &mut dyn FnMut(DiagnosticArgName, DiagnosticArgValue));
 }
 
 impl dyn MachineStopType {
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 37c5bba46a7d..c9e69253701c 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -14,7 +14,9 @@ use crate::ty::{AdtDef, InstanceDef, UserTypeAnnotationIndex};
 use crate::ty::{GenericArg, GenericArgsRef};
 
 use rustc_data_structures::captures::Captures;
-use rustc_errors::{DiagnosticArgValue, DiagnosticMessage, ErrorGuaranteed, IntoDiagnosticArg};
+use rustc_errors::{
+    DiagnosticArgName, DiagnosticArgValue, DiagnosticMessage, ErrorGuaranteed, IntoDiagnosticArg,
+};
 use rustc_hir::def::{CtorKind, Namespace};
 use rustc_hir::def_id::{DefId, CRATE_DEF_ID};
 use rustc_hir::{self, CoroutineDesugaring, CoroutineKind, ImplicitSelfKind};
diff --git a/compiler/rustc_middle/src/mir/terminator.rs b/compiler/rustc_middle/src/mir/terminator.rs
index 0fe33e441f43..91b7952bec5e 100644
--- a/compiler/rustc_middle/src/mir/terminator.rs
+++ b/compiler/rustc_middle/src/mir/terminator.rs
@@ -292,7 +292,7 @@ impl<O> AssertKind<O> {
         }
     }
 
-    pub fn add_args(self, adder: &mut dyn FnMut(Cow<'static, str>, DiagnosticArgValue))
+    pub fn add_args(self, adder: &mut dyn FnMut(DiagnosticArgName, DiagnosticArgValue))
     where
         O: fmt::Debug,
     {
diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index c49f92f4f859..eba62aae60f7 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -34,7 +34,7 @@ pub(crate) macro throw_machine_stop_str($($tt:tt)*) {{
 
         fn add_args(
             self: Box<Self>,
-            _: &mut dyn FnMut(std::borrow::Cow<'static, str>, rustc_errors::DiagnosticArgValue),
+            _: &mut dyn FnMut(rustc_errors::DiagnosticArgName, rustc_errors::DiagnosticArgValue),
         ) {}
     }
     throw_machine_stop!(Zst)