Use fold instead of flat_map

Cargo.lock

@@ -3867,7 +3867,6 @@ version = "0.0.0"
 dependencies = [
  "annotate-snippets 0.10.2",
  "derive_setters",
- "either",
  "rustc_ast",
  "rustc_ast_pretty",
  "rustc_data_structures",

compiler/rustc_errors/Cargo.toml

@@ -7,7 +7,6 @@ edition = "2021"
 # tidy-alphabetical-start
 annotate-snippets = "0.10"
 derive_setters = "0.1.6"
-either = "1.5.0"
 rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_data_structures = { path = "../rustc_data_structures" }

compiler/rustc_errors/src/emitter.rs

@@ -16,7 +16,6 @@ use std::iter;
 use std::path::Path;
 
 use derive_setters::Setters;
-use either::Either;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::sync::{DynSend, IntoDynSyncSend, Lrc};
 use rustc_error_messages::{FluentArgs, SpanLabel};
@@ -2609,16 +2608,21 @@ const OUTPUT_REPLACEMENTS: &[(char, &str)] = &[
     ('\u{2069}', "�"),
 ];
 
-fn normalize_whitespace(str: &str) -> String {
+fn normalize_whitespace(s: &str) -> String {
     // Scan the input string for a character in the ordered table above. If it's present, replace
     // it with it's alternative string (it can be more than 1 char!). Otherwise, retain the input
     // char. At the end, allocate all chars into a string in one operation.
-    str.chars()
-        .flat_map(|c| match OUTPUT_REPLACEMENTS.binary_search_by_key(&c, |(k, _)| *k) {
-            Ok(i) => Either::Left(OUTPUT_REPLACEMENTS[i].1.chars()),
-            _ => Either::Right([c].into_iter()),
-        })
-        .collect()
+    s.chars().fold(String::with_capacity(s.len()), |mut s, c| {
+        match OUTPUT_REPLACEMENTS.binary_search_by_key(&c, |(k, _)| *k) {
+            Ok(i) => {
+                for c in OUTPUT_REPLACEMENTS[i].1.chars() {
+                    s.push(c);
+                }
+            }
+            _ => s.push(c),
+        }
+        s
+    })
 }
 
 fn draw_col_separator(buffer: &mut StyledBuffer, line: usize, col: usize) {