use String::from() instead of format!() macro to construct Strings.

src/librustc/hir/map/mod.rs

@@ -1254,7 +1254,7 @@ fn node_id_to_string(map: &Map<'_>, id: NodeId, include_id: bool) -> String {
         Some(Node::MacroDef(_)) => {
             format!("macro {}{}",  path_str(), id_str)
         }
-        Some(Node::Crate) => format!("root_crate"),
+        Some(Node::Crate) => String::from("root_crate"),
         None => format!("unknown node{}", id_str),
     }
 }

src/librustc_mir/borrow_check/error_reporting.rs

@@ -788,7 +788,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
 
         let what_was_dropped = match self.describe_place(place) {
             Some(name) => format!("`{}`", name.as_str()),
-            None => format!("temporary value"),
+            None => String::from("temporary value"),
         };
 
         let label = match self.describe_place(&borrow.borrowed_place) {
@@ -1028,7 +1028,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
 
         match category {
             ConstraintCategory::Return => {
-                err.span_note(constraint_span, &format!("closure is returned here"));
+                err.span_note(constraint_span, "closure is returned here");
             }
             ConstraintCategory::CallArgument => {
                 fr_name.highlight_region_name(&mut err);

src/librustc_mir/interpret/validity.rs

@@ -408,7 +408,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                             ),
                         _ =>
                             return validation_failure!(
-                                format!("non-integer enum discriminant"), path
+                                String::from("non-integer enum discriminant"), path
                             ),
                     }
                 };

src/librustc_mir/util/borrowck_errors.rs

@@ -575,7 +575,7 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
             OGN = o
         );
         err.span_label(mutate_span, format!("cannot {}", action));
-        err.span_label(match_span, format!("value is immutable in match guard"));
+        err.span_label(match_span, String::from("value is immutable in match guard"));
 
         self.cancel_if_wrong_origin(err, o)
     }

src/libsyntax/ext/expand.rs

@@ -687,7 +687,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                         "proc_macro_hygiene",
                         self.span,
                         GateIssue::Language,
-                        &format!("procedural macros cannot expand to macro definitions"),
+                        "procedural macros cannot expand to macro definitions",
                     );
                 }
                 visit::walk_item(self, i);

src/tools/compiletest/src/main.rs

@@ -766,7 +766,7 @@ fn make_test_name(
         .join(&testpaths.file.file_name().unwrap());
     let mode_suffix = match config.compare_mode {
         Some(ref mode) => format!(" ({})", mode.to_str()),
-        None => format!(""),
+        None => String::new(),
     };
     test::DynTestName(format!(
         "[{}{}] {}{}",