use impl fmt::Write instead of Formatter

crates/flux-middle/src/lib.rs

@@ -1,5 +1,4 @@
 //! This crate contains common type definitions that are used by other crates.
-#![allow(internal_features)] // TODO: needed for the fmt_internals, which is needed for `nested_with_bound_vars`
 #![feature(
     associated_type_defaults,
     box_patterns,
@@ -10,8 +9,7 @@
     min_specialization,
     never_type,
     rustc_private,
-    unwrap_infallible,
-    fmt_internals
+    unwrap_infallible
 )]
 
 extern crate rustc_ast;

crates/flux-middle/src/pretty.rs

@@ -332,7 +332,7 @@ impl<'genv, 'tcx> PrettyCx<'genv, 'tcx> {
         left: &str,
         vars: &[BoundVariableKind],
         right: &str,
-        f: &mut fmt::Formatter<'_>,
+        f: &mut impl fmt::Write,
     ) -> fmt::Result {
         define_scoped!(self, f);
         w!("{left}")?;

crates/flux-middle/src/rty/pretty.rs

@@ -726,9 +726,8 @@ pub fn nested_with_bound_vars(
     f: impl FnOnce(String) -> Result<NestedString, fmt::Error>,
 ) -> Result<NestedString, fmt::Error> {
     let mut buffer = String::new();
-    let mut formatter = fmt::Formatter::new(&mut buffer);
     if !vars.is_empty() {
-        cx.fmt_bound_vars(false, left, vars, ". ", &mut formatter)?;
+        cx.fmt_bound_vars(false, left, vars, ". ", &mut buffer)?;
     }
     f(buffer)
 }