types.

src/args.rs

@@ -59,7 +59,7 @@ macro_rules! traits {
 
             impl<T: $trait, O: FmtOpts> Arguments for $name<T, O> {
                 fn fmt<W: Write, OldOpts: FmtOpts>(&self, f: &mut Formatter<W, OldOpts>) -> Result {
-                    let mut f = f.with_opts(&self.1);
+                    let mut f = f.wrap_with(&self.1);
 
                     <T as $trait>::fmt(&self.0, &mut f)
                 }

src/lib.rs

@@ -79,11 +79,11 @@ impl<W: Write, O: FmtOpts> Formatter<W, O> {
     }
 }
 
-impl<W, O> Formatter<W, O> {
+impl<W, O: FmtOpts> Formatter<W, O> {
-    fn with_opts<'opt, ONew>(&mut self, opts: &'opt ONew) -> Formatter<&mut W, &'opt ONew> {
+    fn wrap_with<'opt, ONew: FmtOpts>(&mut self, opts: &ONew) -> Formatter<&mut W, ONew::ReplaceInnermost<O>> {
         Formatter {
             buf: &mut self.buf,
-            opts,
+            opts: opts.override_other(self.opts),
         }
     }
 }
@@ -155,6 +155,7 @@ mod tests {
     }
 }
 
+// testing
 fn fmt() {
     let a = (
         _private::Str("amount: "),
@@ -164,4 +165,6 @@ fn fmt() {
     let mut str = String::new();
     let mut f = Formatter::new(&mut str);
     Arguments::fmt(&a, &mut f).unwrap();
+
+    println!("{str}");
 }

src/opts.rs

@@ -20,12 +20,24 @@ macro_rules! options {
             }
         )*
     ) => {
-        pub trait FmtOpts {
+        // FIXME: We can get rid of this Copy can't we
+        pub trait FmtOpts: Copy {
             #[doc(hidden)]
             type Inner: FmtOpts;
 
+            /// Replaces the innermost `()` with `I`
+            type ReplaceInnermost<I: FmtOpts>: FmtOpts;
+
             fn inner(&self) -> &Self::Inner;
 
+            /// # Example
+            /// `Self` is `WithAlternate<WithFill<(), ' '>>`
+            /// `Other` is WithMinus<()>
+            ///
+            /// This returns `WithAlternate<WithFille<WithMinus<()>, ' '>>`
+            ///
+            fn override_other<Other: FmtOpts>(self, other: Other) -> Self::ReplaceInnermost<Other>;
+
             $(
                 #[inline]
                 fn $name(&self) -> $ret {
@@ -37,10 +49,16 @@ macro_rules! options {
         impl FmtOpts for () {
             type Inner = Self;
 
+            type ReplaceInnermost<I: FmtOpts> = I;
+
             fn inner(&self) -> &Self::Inner {
                 self
             }
 
+            fn override_other<Other: FmtOpts>(self, other: Other) -> Self::ReplaceInnermost::<Other> {
+                other
+            }
+
             $(
                 #[inline]
                 fn $name(&self) -> $ret {
@@ -52,10 +70,16 @@ macro_rules! options {
         impl<O: FmtOpts> FmtOpts for &'_ O {
             type Inner = O::Inner;
 
-            fn inner(&self) ->  &Self::Inner {
+            type ReplaceInnermost<I: FmtOpts> = O::ReplaceInnermost<I>;
+
+            fn inner(&self) -> &Self::Inner {
                 O::inner(self)
             }
 
+            fn override_other<Other: FmtOpts>(self, other: Other) -> Self::ReplaceInnermost<Other> {
+                O::override_other(*self, other)
+            }
+
             $(
                 #[inline]
                 fn $name(&self) -> $ret {
@@ -75,15 +99,22 @@ macro_rules! options {
         }
 
         $(
+            #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
             pub struct $with_name<I, $($(const $const_gen_name: $with_ty),*)?>(#[doc(hidden)] pub I);
 
             impl<I: FmtOpts, $($(const $const_gen_name: $with_ty),*)?> FmtOpts for $with_name<I, $($($const_gen_name),*)?> {
                 type Inner = I;
 
+                type ReplaceInnermost<Replacement: FmtOpts> = $with_name<I::ReplaceInnermost<Replacement>, $($($const_gen_name),*)?>;
+
                 fn inner(&self) -> &Self::Inner  {
                     &self.0
                 }
 
+                fn override_other<Other: FmtOpts>(self, other: Other) -> Self::ReplaceInnermost<Other> {
+                    $with_name(self.0.override_other(other))
+                }
+
                 fn $name(&self) -> $ret {
                     $($struct_body)*
                 }