Avoid &format("...") calls in error message code.

Error message all end up passing into a function as an `impl
Into<{D,Subd}iagnosticMessage>`. If an error message is creatd as
`&format("...")` that means we allocate a string (in the `format!`
call), then take a reference, and then clone (allocating again) the
reference to produce the `{D,Subd}iagnosticMessage`, which is silly.

This commit removes the leading `&` from a lot of these cases. This
means the original `String` is moved into the
`{D,Subd}iagnosticMessage`, avoiding the double allocations. This
requires changing some function argument types from `&str` to `String`
(when all arguments are `String`) or `impl
Into<{D,Subd}iagnosticMessage>` (when some arguments are `String` and
some are `&str`).

compiler/rustc_borrowck/src/nll.rs

@@ -430,7 +430,7 @@ pub(super) fn dump_annotation<'tcx>(
 fn for_each_region_constraint<'tcx>(
     tcx: TyCtxt<'tcx>,
     closure_region_requirements: &ClosureRegionRequirements<'tcx>,
-    with_msg: &mut dyn FnMut(&str) -> io::Result<()>,
+    with_msg: &mut dyn FnMut(String) -> io::Result<()>,
 ) -> io::Result<()> {
     for req in &closure_region_requirements.outlives_requirements {
         let subject = match req.subject {
@@ -439,7 +439,7 @@ fn for_each_region_constraint<'tcx>(
                 format!("{:?}", ty.instantiate(tcx, |vid| tcx.mk_re_var(vid)))
             }
         };
-        with_msg(&format!("where {}: {:?}", subject, req.outlived_free_region,))?;
+        with_msg(format!("where {}: {:?}", subject, req.outlived_free_region,))?;
     }
     Ok(())
 }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -71,7 +71,7 @@ macro_rules! span_mirbug {
         $crate::type_check::mirbug(
             $context.tcx(),
             $context.last_span,
-            &format!(
+            format!(
                 "broken MIR in {:?} ({:?}): {}",
                 $context.body().source.def_id(),
                 $elem,
@@ -274,7 +274,7 @@ fn translate_outlives_facts(typeck: &mut TypeChecker<'_, '_>) {
 }
 
 #[track_caller]
-fn mirbug(tcx: TyCtxt<'_>, span: Span, msg: &str) {
+fn mirbug(tcx: TyCtxt<'_>, span: Span, msg: String) {
     // We sometimes see MIR failures (notably predicate failures) due to
     // the fact that we check rvalue sized predicates here. So use `delay_span_bug`
     // to avoid reporting bugs in those cases.