fix clippy::single_char_pattern perf findings

compiler/rustc_middle/src/lint.rs

@@ -295,7 +295,7 @@ pub fn struct_lint_level<'s, 'd>(
                     Level::Allow => "-A",
                     Level::ForceWarn => "--force-warn",
                 };
-                let hyphen_case_lint_name = name.replace("_", "-");
+                let hyphen_case_lint_name = name.replace('_', "-");
                 if lint_flag_val.as_str() == name {
                     sess.diag_note_once(
                         &mut err,
@@ -306,7 +306,7 @@ pub fn struct_lint_level<'s, 'd>(
                         ),
                     );
                 } else {
-                    let hyphen_case_flag_val = lint_flag_val.as_str().replace("_", "-");
+                    let hyphen_case_flag_val = lint_flag_val.as_str().replace('_', "-");
                     sess.diag_note_once(
                         &mut err,
                         DiagnosticMessageId::from(lint),

compiler/rustc_middle/src/mir/generic_graphviz.rs

@@ -126,7 +126,7 @@ impl<
             write!(
                 w,
                 r#"<tr><td align="left" balign="left">{}</td></tr>"#,
-                dot::escape_html(&section).replace("\n", "<br/>")
+                dot::escape_html(&section).replace('\n', "<br/>")
             )?;
         }
 
@@ -147,7 +147,7 @@ impl<
             let src = self.node(source);
             let trg = self.node(target);
             let escaped_edge_label = if let Some(edge_label) = edge_labels.get(index) {
-                dot::escape_html(edge_label).replace("\n", r#"<br align="left"/>"#)
+                dot::escape_html(edge_label).replace('\n', r#"<br align="left"/>"#)
             } else {
                 "".to_owned()
             };

compiler/rustc_middle/src/mir/spanview.rs

@@ -681,13 +681,13 @@ fn hir_body<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> Option<&'tcx rustc_hir::B
 }
 
 fn escape_html(s: &str) -> String {
-    s.replace("&", "&amp;").replace("<", "&lt;").replace(">", "&gt;")
+    s.replace('&', "&amp;").replace('<', "&lt;").replace('>', "&gt;")
 }
 
 fn escape_attr(s: &str) -> String {
-    s.replace("&", "&amp;")
-        .replace("\"", "&quot;")
-        .replace("'", "&#39;")
-        .replace("<", "&lt;")
-        .replace(">", "&gt;")
+    s.replace('&', "&amp;")
+        .replace('\"', "&quot;")
+        .replace('\'', "&#39;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
 }