Remove the second lifetime from `DiagnosticArg`.

Because it's always static.

I'm surprised the compiler allowed this unused lifetime without any
complaint.

compiler/rustc_errors/src/diagnostic.rs

@@ -23,7 +23,7 @@ pub struct SuggestionsDisabled;
 /// Simplified version of `FluentArg` that can implement `Encodable` and `Decodable`. Collection of
 /// `DiagnosticArg` are converted to `FluentArgs` (consuming the collection) at the start of
 /// diagnostic emission.
-pub type DiagnosticArg<'iter, 'source> = (&'iter DiagnosticArgName, &'iter DiagnosticArgValue);
+pub type DiagnosticArg<'iter> = (&'iter DiagnosticArgName, &'iter DiagnosticArgValue);
 
 /// Name of a diagnostic argument.
 pub type DiagnosticArgName = Cow<'static, str>;
@@ -909,7 +909,7 @@ impl Diagnostic {
     // Exact iteration order of diagnostic arguments shouldn't make a difference to output because
     // they're only used in interpolation.
     #[allow(rustc::potential_query_instability)]
-    pub fn args(&self) -> impl Iterator<Item = DiagnosticArg<'_, 'static>> {
+    pub fn args(&self) -> impl Iterator<Item = DiagnosticArg<'_>> {
         self.args.iter()
     }
 

compiler/rustc_errors/src/lib.rs

@@ -628,7 +628,7 @@ impl DiagCtxt {
     pub fn eagerly_translate<'a>(
         &self,
         message: DiagnosticMessage,
-        args: impl Iterator<Item = DiagnosticArg<'a, 'static>>,
+        args: impl Iterator<Item = DiagnosticArg<'a>>,
     ) -> SubdiagnosticMessage {
         SubdiagnosticMessage::Eager(Cow::from(self.eagerly_translate_to_string(message, args)))
     }
@@ -637,7 +637,7 @@ impl DiagCtxt {
     pub fn eagerly_translate_to_string<'a>(
         &self,
         message: DiagnosticMessage,
-        args: impl Iterator<Item = DiagnosticArg<'a, 'static>>,
+        args: impl Iterator<Item = DiagnosticArg<'a>>,
     ) -> String {
         let inner = self.inner.borrow();
         let args = crate::translation::to_fluent_args(args);

compiler/rustc_errors/src/translation.rs

@@ -13,7 +13,7 @@ use std::error::Report;
 /// Typically performed once for each diagnostic at the start of `emit_diagnostic` and then
 /// passed around as a reference thereafter.
 pub fn to_fluent_args<'iter>(
-    iter: impl Iterator<Item = DiagnosticArg<'iter, 'static>>,
+    iter: impl Iterator<Item = DiagnosticArg<'iter>>,
 ) -> FluentArgs<'static> {
     let mut args = if let Some(size) = iter.size_hint().1 {
         FluentArgs::with_capacity(size)