have DisplayFn take a Formatter instead of a dyn impl Write

examples/gain/src/lib.rs

@@ -39,9 +39,7 @@ impl Plugin for Gain {
                 default: 1.0,
                 steps: None,
                 parse: Box::new(|s| s.parse().ok()),
-                display: Box::new(|v, w| {
-                    let _ = write!(w, "{:.2}", v);
-                }),
+                display: Box::new(|v, f| write!(f, "{:.2}", v)),
             }],
         }
     }

src/format/clap/instance.rs

@@ -14,7 +14,7 @@ use crate::bus::{BusDir, Format};
 use crate::events::{Data, Event, Events};
 use crate::param::ParamInfo;
 use crate::sync::params::ParamValues;
-use crate::util::{copy_cstring, slice_from_raw_parts_checked};
+use crate::util::{copy_cstring, slice_from_raw_parts_checked, DisplayParam};
 use crate::{Config, Editor, Host, ParamId, ParamValue, Plugin, PluginInfo, Processor};
 
 fn port_type_from_format(format: &Format) -> &'static CStr {
@@ -572,8 +572,7 @@ impl<P: Plugin> Instance<P> {
         if let Some(&index) = instance.param_map.get(&param_id) {
             let param = &instance.info.params[index];
 
-            let mut text = String::new();
-            (param.display)(map_param_in(param, value), &mut text);
+            let text = format!("{}", DisplayParam(param, map_param_in(param, value)));
 
             let dst = slice::from_raw_parts_mut(display, size as usize);
             copy_cstring(&text, dst);

src/format/vst3/component.rs

@@ -12,7 +12,7 @@ use super::view::View;
 use crate::bus::{BusDir, Format, Layout};
 use crate::events::{Data, Event, Events};
 use crate::sync::params::ParamValues;
-use crate::util::slice_from_raw_parts_checked;
+use crate::util::{slice_from_raw_parts_checked, DisplayParam};
 use crate::{Config, Editor, Host, ParamId, Plugin, PluginInfo, Processor};
 
 fn format_to_speaker_arrangement(format: &Format) -> SpeakerArrangement {
@@ -583,8 +583,7 @@ impl<P: Plugin> IEditControllerTrait for Component<P> {
         if let Some(&index) = self.param_map.get(&id) {
             let param = &self.info.params[index];
 
-            let mut display = String::new();
-            (param.display)(valueNormalized, &mut display);
+            let display = format!("{}", DisplayParam(param, valueNormalized));
             copy_wstring(&display, &mut *string);
 
             return kResultOk;

src/param.rs

@@ -1,9 +1,9 @@
-use std::fmt::Write;
+use std::fmt::{self, Formatter};
 
 use crate::{ParamId, ParamValue};
 
 pub type ParseFn = dyn Fn(&str) -> Option<ParamValue> + Send + Sync;
-pub type DisplayFn = dyn Fn(ParamValue, &mut dyn Write) + Send + Sync;
+pub type DisplayFn = dyn Fn(ParamValue, &mut Formatter) -> Result<(), fmt::Error> + Send + Sync;
 
 pub struct ParamInfo {
     pub id: ParamId,

src/util.rs

@@ -1,7 +1,11 @@
 use std::ffi::CString;
+use std::fmt::{self, Display, Formatter};
 use std::os::raw::c_char;
 use std::slice;
 
+use crate::param::ParamInfo;
+use crate::ParamValue;
+
 pub fn copy_cstring(src: &str, dst: &mut [c_char]) {
     let c_string = CString::new(src).unwrap_or_else(|_| CString::default());
     let bytes = c_string.as_bytes_with_nul();
@@ -28,3 +32,11 @@ pub unsafe fn slice_from_raw_parts_checked<'a, T>(ptr: *const T, len: usize) ->
         &[]
     }
 }
+
+pub struct DisplayParam<'a>(pub &'a ParamInfo, pub ParamValue);
+
+impl<'a> Display for DisplayParam<'a> {
+    fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
+        (self.0.display)(self.1, f)
+    }
+}