Always format to internal String in FmtPrinter

This avoids monomorphizing for different parameters, decreasing generic code
instantiated downstream from rustc_middle.

compiler/rustc_middle/src/ty/error.rs

@@ -983,10 +983,9 @@ fn foo(&self) -> Self::T { String::new() }
     }
 
     fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
-        let mut item_args = String::new();
-        FmtPrinter::new(self, &mut item_args, hir::def::Namespace::TypeNS)
+        FmtPrinter::new(self, hir::def::Namespace::TypeNS)
             .path_generic_args(Ok, args)
-            .expect("could not write to `String`.");
-        item_args
+            .expect("could not write to `String`.")
+            .into_buffer()
     }
 }