Defer allocation for printf suggestions until needed

Author: thaliaarchi

compiler/rustc_builtin_macros/src/format.rs

@@ -711,11 +711,11 @@ fn report_missing_placeholders(
                     };
 
                     let pos = sub.position();
-                    let sub = String::from(sub.as_str());
-                    if explained.contains(&sub) {
+                    let sub = sub.as_str();
+                    if explained.contains(sub.as_ref()) {
                         continue;
                     }
-                    explained.insert(sub);
+                    explained.insert(String::from(sub));
 
                     if !found_foreign {
                         found_foreign = true;

compiler/rustc_builtin_macros/src/format_foreign.rs

@@ -1,4 +1,6 @@
 pub(crate) mod printf {
+    use std::borrow::Cow;
+
     use rustc_span::InnerSpan;
 
     use super::strcursor::StrCursor as Cur;
@@ -13,10 +15,10 @@ pub(crate) mod printf {
     }
 
     impl<'a> Substitution<'a> {
-        pub(crate) fn as_str(&self) -> &str {
+        pub(crate) fn as_str(&self) -> Cow<'_, str> {
             match self {
-                Substitution::Format(fmt) => fmt.span,
-                Substitution::Escape(_) => "%%",
+                Substitution::Format(fmt) => fmt.span.into(),
+                Substitution::Escape(_) => "%%".into(),
             }
         }
 
@@ -616,6 +618,8 @@ pub(crate) mod printf {
 }
 
 pub(crate) mod shell {
+    use std::borrow::Cow;
+
     use rustc_span::InnerSpan;
 
     use super::strcursor::StrCursor as Cur;
@@ -628,10 +632,10 @@ pub(crate) mod shell {
     }
 
     impl Substitution<'_> {
-        pub(crate) fn as_str(&self) -> String {
+        pub(crate) fn as_str(&self) -> Cow<'_, str> {
             match self {
-                Substitution::Ordinal(n, _) => format!("${n}"),
-                Substitution::Name(n, _) => format!("${n}"),
+                Substitution::Ordinal(n, _) => format!("${n}").into(),
+                Substitution::Name(n, _) => format!("${n}").into(),
                 Substitution::Escape(_) => "$$".into(),
             }
         }