WIP: Remove the blanket impl on Display

NEWS.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- **Breaking change:** `Render` no longer has a blanket implementation for `Display`: you can no longer use a string, integer, or some object that implements `Display` as a model directly. You must instead implement `Render` explicitly.
+
+  It seems that the `Display` implementation is often not a very satisfactory progress bar, and the presence of the blanket implementation causes confusing error messages when `Render` is not implemented correctly.
+
 - Change back to `std::sync::Mutex` from `parking_lot`, to keep dependencies smaller.
 
 ## 0.1.4

src/lib.rs

@@ -20,12 +20,11 @@ processed, the amount of data transmitted or received, the currently active task
 
 The Model can be any of these things, from simplest to most powerful:
 
-1. Any type that implements [std::fmt::Display], such as a String or integer.
-2. One of the provided [models].
-3. An application-defined struct (or enum or other type) that implements [Model].
+1. One of the provided [models].
+2. An application-defined struct (or enum or other type) that implements [Model].
 
 The model is responsible for rendering itself into a String, optionally with ANSI styling,
-by implementing [Model::render] (or [std::fmt::Display]).  Applications might
+by implementing [Model::render].  Applications might
 choose to use any of the Rust crates that can render ANSI control codes into a
 string, such as yansi.
 
@@ -211,7 +210,6 @@ is welcome.
 
 #![warn(missing_docs)]
 
-use std::fmt::Display;
 use std::io::{self, Write};
 use std::sync::{Arc, Mutex};
 use std::time::Instant;
@@ -292,29 +290,6 @@ pub trait Model {
     }
 }
 
-/// Blanket implementation of Model for Display.
-///
-/// `self` is converted to a display string without regard for
-/// the terminal width.
-///
-/// This allows direct use of e.g. a String or integer as a model
-/// for very basic progress indications.
-///
-/// ```
-/// use nutmeg::{Options, View};
-///
-/// let view = View::new(0, Options::default());
-/// view.update(|model| *model += 1);
-/// ```
-impl<T> Model for T
-where
-    T: Display,
-{
-    fn render(&mut self, _width: usize) -> String {
-        self.to_string()
-    }
-}
-
 /// A view that draws and coordinates a progress bar on the terminal.
 ///
 /// There should be only one `View` active on a terminal at any time, and