Implement fmt::Display for enums, flags, and ints (bytecodealliance#18)

* Implement fmt::Display for enums

`wasi_common` relies on `strerror` to nicely format error messages.
`strerror` is autoimplemented in `wig`. I thought it might be useful
to provide a Rust-idiomatic alternative which boils down to autoimplementing
`fmt::Display` for all enums.

* Implement fmt::Display for flags

* Implement fmt::Display for ints

crates/generate/src/types.rs

@@ -72,6 +72,12 @@ fn define_int(names: &Names, name: &witx::Id, i: &witx::IntDatatype) -> TokenStr
             #(#consts;)*
         }
 
+        impl ::std::fmt::Display for #ident {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                write!(f, "{:?}", self)
+            }
+        }
+
         impl ::std::convert::TryFrom<#repr> for #ident {
             type Error = wiggle_runtime::GuestError;
             fn try_from(value: #repr) -> Result<Self, wiggle_runtime::GuestError> {
@@ -153,6 +159,8 @@ fn define_flags(names: &Names, name: &witx::Id, f: &witx::FlagsDatatype) -> Toke
     }
     let all_values_token = Literal::u128_unsuffixed(all_values);
 
+    let ident_str = ident.to_string();
+
     quote! {
         #[repr(transparent)]
         #[derive(Copy, Clone, Debug, ::std::hash::Hash, Eq, PartialEq)]
@@ -167,6 +175,12 @@ fn define_flags(names: &Names, name: &witx::Id, f: &witx::FlagsDatatype) -> Toke
             }
         }
 
+        impl ::std::fmt::Display for #ident {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                write!(f, "{}({:#b})", #ident_str, self.0)
+            }
+        }
+
         impl ::std::ops::BitAnd for #ident {
             type Output = Self;
             fn bitand(self, rhs: Self) -> Self::Output {
@@ -286,15 +300,21 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
         witx::IntRepr::U64 => witx::AtomType::I64,
     });
 
-    let variant_names = e.variants.iter().map(|v| names.enum_variant(&v.name));
-    let tryfrom_repr_cases = e.variants.iter().enumerate().map(|(n, v)| {
-        let variant_name = names.enum_variant(&v.name);
-        quote!(#n => Ok(#ident::#variant_name))
-    });
-    let to_repr_cases = e.variants.iter().enumerate().map(|(n, v)| {
-        let variant_name = names.enum_variant(&v.name);
-        quote!(#ident::#variant_name => #n as #repr)
-    });
+    let mut variant_names = vec![];
+    let mut tryfrom_repr_cases = vec![];
+    let mut to_repr_cases = vec![];
+    let mut to_display = vec![];
+
+    for (n, variant) in e.variants.iter().enumerate() {
+        let variant_name = names.enum_variant(&variant.name);
+        let docs = variant.docs.trim();
+        let ident_str = ident.to_string();
+        let variant_str = variant_name.to_string();
+        tryfrom_repr_cases.push(quote!(#n => Ok(#ident::#variant_name)));
+        to_repr_cases.push(quote!(#ident::#variant_name => #n as #repr));
+        to_display.push(quote!(#ident::#variant_name => format!("{} ({}::{}({}))", #docs, #ident_str, #variant_str, #repr::from(*self))));
+        variant_names.push(variant_name);
+    }
 
     quote! {
         #[repr(#repr)]
@@ -303,6 +323,15 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
             #(#variant_names),*
         }
 
+        impl ::std::fmt::Display for #ident {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                let to_str = match self {
+                    #(#to_display,)*
+                };
+                write!(f, "{}", to_str)
+            }
+        }
+
         impl ::std::convert::TryFrom<#repr> for #ident {
             type Error = wiggle_runtime::GuestError;
             fn try_from(value: #repr) -> Result<#ident, wiggle_runtime::GuestError> {

tests/errno.witx

@@ -1,8 +1,13 @@
 (typename $errno
     (enum u32
+        ;;; Success
         $ok
+        ;;; Invalid argument
         $invalid_arg
+        ;;; I really don't want to
         $dont_want_to
+        ;;; I am physically unable to
         $physically_unable
+        ;;; Well, that's a picket line alright!
         $picket_line))