wiggle: new error configuration for generating a "trappable error" (#5276)

* Add a new "trappable" mode for wiggle to make an error type

start refactoring how errors are generated and configured

put a pin in this - you can now configure a generated error

but i need to go fix Names

Names is no longer a struct, rt is hardcoded to wiggle

rest of fixes to pass tests

its called a trappable error now

don't generate UserErrorConversion trait if empty

mention in macro docs

* undo omitting the user error conversion trait when empty

crates/wiggle/generate/src/config.rs

@@ -27,6 +27,7 @@ mod kw {
     syn::custom_keyword!(wasmtime);
     syn::custom_keyword!(tracing);
     syn::custom_keyword!(disable_for);
+    syn::custom_keyword!(trappable);
 }
 
 #[derive(Debug, Clone)]
@@ -274,14 +275,14 @@ impl Parse for ErrorConf {
             content.parse_terminated(Parse::parse)?;
         let mut m = HashMap::new();
         for i in items {
-            match m.insert(i.abi_error.clone(), i.clone()) {
+            match m.insert(i.abi_error().clone(), i.clone()) {
                 None => {}
                 Some(prev_def) => {
                     return Err(Error::new(
-                        i.err_loc,
+                        *i.err_loc(),
                         format!(
                         "duplicate definition of rich error type for {:?}: previously defined at {:?}",
-                        i.abi_error, prev_def.err_loc,
+                        i.abi_error(), prev_def.err_loc(),
                     ),
                     ))
                 }
@@ -291,20 +292,23 @@ impl Parse for ErrorConf {
     }
 }
 
-#[derive(Clone)]
-pub struct ErrorConfField {
-    pub abi_error: Ident,
-    pub rich_error: syn::Path,
-    pub err_loc: Span,
+#[derive(Debug, Clone)]
+pub enum ErrorConfField {
+    Trappable(TrappableErrorConfField),
+    User(UserErrorConfField),
 }
-
-impl std::fmt::Debug for ErrorConfField {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.debug_struct("ErrorConfField")
-            .field("abi_error", &self.abi_error)
-            .field("rich_error", &"(...)")
-            .field("err_loc", &self.err_loc)
-            .finish()
+impl ErrorConfField {
+    pub fn abi_error(&self) -> &Ident {
+        match self {
+            Self::Trappable(t) => &t.abi_error,
+            Self::User(u) => &u.abi_error,
+        }
+    }
+    pub fn err_loc(&self) -> &Span {
+        match self {
+            Self::Trappable(t) => &t.err_loc,
+            Self::User(u) => &u.err_loc,
+        }
     }
 }
 
@@ -313,12 +317,48 @@ impl Parse for ErrorConfField {
         let err_loc = input.span();
         let abi_error = input.parse::<Ident>()?;
         let _arrow: Token![=>] = input.parse()?;
-        let rich_error = input.parse::<syn::Path>()?;
-        Ok(ErrorConfField {
-            abi_error,
-            rich_error,
-            err_loc,
-        })
+
+        let lookahead = input.lookahead1();
+        if lookahead.peek(kw::trappable) {
+            let _ = input.parse::<kw::trappable>()?;
+            let rich_error = input.parse()?;
+            Ok(ErrorConfField::Trappable(TrappableErrorConfField {
+                abi_error,
+                rich_error,
+                err_loc,
+            }))
+        } else {
+            let rich_error = input.parse::<syn::Path>()?;
+            Ok(ErrorConfField::User(UserErrorConfField {
+                abi_error,
+                rich_error,
+                err_loc,
+            }))
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct TrappableErrorConfField {
+    pub abi_error: Ident,
+    pub rich_error: Ident,
+    pub err_loc: Span,
+}
+
+#[derive(Clone)]
+pub struct UserErrorConfField {
+    pub abi_error: Ident,
+    pub rich_error: syn::Path,
+    pub err_loc: Span,
+}
+
+impl std::fmt::Debug for UserErrorConfField {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("ErrorConfField")
+            .field("abi_error", &self.abi_error)
+            .field("rich_error", &"(...)")
+            .field("err_loc", &self.err_loc)
+            .finish()
     }
 }