Support enum variants that have aliases (#53)

* Support enum variants that have aliases

* Return error instead of panic on enum with 0 variants

Before PR 53:

    thread 'main' panicked at /git/tmp/serde-reflection/serde-reflection/src/de.rs:431:18:
    variant indexes must be a non-empty range 0..variants.len()

First draft of PR 53, debug mode (overflow checks):

    thread 'main' panicked at /git/tmp/serde-reflection/serde-reflection/src/de.rs:435:42:
    attempt to subtract with overflow

First draft of PR 53, release mode:

    Failed to deserialize value: "invalid value: integer `0`, expected variant index 0 <= i < 0"

This commit:

    Not supported: deserialize_enum with 0 variants

serde-reflection/Cargo.toml

@@ -17,9 +17,11 @@ exclude = [
 ]
 
 [dependencies]
-thiserror = "1.0.25"
-serde = { version = "1.0.126", features = ["derive"] }
+erased-discriminant = "1"
 once_cell = "1.7.2"
+serde = { version = "1.0.126", features = ["derive"] }
+thiserror = "1.0.25"
+typeid = "1"
 
 [dev-dependencies]
 bincode = "1.3.3"

serde-reflection/src/de.rs

@@ -4,11 +4,16 @@
 use crate::{
     error::{Error, Result},
     format::{ContainerFormat, ContainerFormatEntry, Format, FormatHolder, Named, VariantFormat},
-    trace::{Samples, Tracer},
+    trace::{EnumProgress, Samples, Tracer, VariantId},
     value::IntoSeqDeserializer,
 };
-use serde::de::{self, DeserializeSeed, IntoDeserializer, Visitor};
-use std::collections::BTreeMap;
+use erased_discriminant::Discriminant;
+use serde::de::{
+    self,
+    value::{BorrowedStrDeserializer, U32Deserializer},
+    DeserializeSeed, IntoDeserializer, Visitor,
+};
+use std::collections::btree_map::{BTreeMap, Entry};
 
 /// Deserialize a single value.
 /// * The lifetime 'a is set by the deserialization call site and the
@@ -391,55 +396,151 @@ impl<'de, 'a> de::Deserializer<'de> for Deserializer<'de, 'a> {
 
     // Assumption: The first variant(s) should be "base cases", i.e. not cause infinite recursion
     // while constructing sample values.
+    #[allow(clippy::map_entry)] // false positive https://github.com/rust-lang/rust-clippy/issues/9470
     fn deserialize_enum<V>(
         self,
-        name: &'static str,
+        enum_name: &'static str,
         variants: &'static [&'static str],
         visitor: V,
     ) -> Result<V::Value>
     where
         V: Visitor<'de>,
     {
-        self.format.unify(Format::TypeName(name.into()))?;
+        if variants.is_empty() {
+            return Err(Error::NotSupported("deserialize_enum with 0 variants"));
+        }
+
+        let enum_type_id = typeid::of::<V::Value>();
+        self.format.unify(Format::TypeName(enum_name.into()))?;
         // Pre-update the registry.
         self.tracer
             .registry
-            .entry(name.to_string())
+            .entry(enum_name.to_string())
             .unify(ContainerFormat::Enum(BTreeMap::new()))?;
-        let known_variants = match self.tracer.registry.get_mut(name) {
+        let known_variants = match self.tracer.registry.get_mut(enum_name) {
             Some(ContainerFormat::Enum(x)) => x,
             _ => unreachable!(),
         };
-        // If we have found all the variants OR if the enum is marked as
-        // incomplete already, pick the first index.
-        let index = if known_variants.len() == variants.len()
-            || self.tracer.incomplete_enums.contains(name)
-        {
-            0
-        } else {
-            let mut index = known_variants.len() as u32;
-            // Scan the range 0..=known_variants.len() downwards to find the next
-            // variant index to explore.
-            while known_variants.contains_key(&index) {
-                index -= 1;
+
+        // If the enum is marked as incomplete, just visit the first index
+        // because we presume it avoids recursion.
+        if self.tracer.incomplete_enums.contains_key(enum_name) {
+            return visitor.visit_enum(EnumDeserializer::new(
+                self.tracer,
+                self.samples,
+                VariantId::Index(0),
+                &mut VariantFormat::unknown(),
+            ));
+        }
+
+        // First visit each of the variants by name according to `variants`.
+        // Later revisit them by u32 index until an index matching each of the
+        // named variants has been determined.
+        let provisional_min = u32::MAX - (variants.len() - 1) as u32;
+        for (i, &variant_name) in variants.iter().enumerate() {
+            if !self
+                .tracer
+                .discriminants
+                .contains_key(&(enum_type_id, VariantId::Name(variant_name)))
+            {
+                // Insert into known_variants with a provisional index.
+                let provisional_index = provisional_min + i as u32;
+                let variant = known_variants
+                    .entry(provisional_index)
+                    .or_insert_with(|| Named {
+                        name: variant_name.to_owned(),
+                        value: VariantFormat::unknown(),
+                    });
+                self.tracer
+                    .incomplete_enums
+                    .insert(enum_name.into(), EnumProgress::NamedVariantsRemaining);
+                // Compute the discriminant and format for this variant.
+                let mut value = variant.value.clone();
+                let enum_value = visitor.visit_enum(EnumDeserializer::new(
+                    self.tracer,
+                    self.samples,
+                    VariantId::Name(variant_name),
+                    &mut value,
+                ))?;
+                let discriminant = Discriminant::of(&enum_value);
+                self.tracer
+                    .discriminants
+                    .insert((enum_type_id, VariantId::Name(variant_name)), discriminant);
+                return Ok(enum_value);
             }
-            index
+        }
+
+        // We know the discriminant for every variant name. Now visit them again
+        // by index to find the u32 id that goes with each name.
+        //
+        // If there are no provisional entries waiting for an index, just go
+        // with index 0.
+        let mut index = 0;
+        if known_variants.range(provisional_min..).next().is_some() {
+            self.tracer
+                .incomplete_enums
+                .insert(enum_name.into(), EnumProgress::IndexedVariantsRemaining);
+            while known_variants.contains_key(&index)
+                && self
+                    .tracer
+                    .discriminants
+                    .contains_key(&(enum_type_id, VariantId::Index(index)))
+            {
+                index += 1;
+            }
+        }
+
+        // Compute the discriminant and format for this variant.
+        let mut value = VariantFormat::unknown();
+        let enum_value = visitor.visit_enum(EnumDeserializer::new(
+            self.tracer,
+            self.samples,
+            VariantId::Index(index),
+            &mut value,
+        ))?;
+        let discriminant = Discriminant::of(&enum_value);
+        self.tracer.discriminants.insert(
+            (enum_type_id, VariantId::Index(index)),
+            discriminant.clone(),
+        );
+        self.tracer.incomplete_enums.remove(enum_name);
+
+        // Rewrite provisional entries for which we now know a u32 index.
+        let known_variants = match self.tracer.registry.get_mut(enum_name) {
+            Some(ContainerFormat::Enum(x)) => x,
+            _ => unreachable!(),
         };
-        let variant = known_variants.entry(index).or_insert_with(|| Named {
-            name: (*variants
-                .get(index as usize)
-                .expect("variant indexes must be a non-empty range 0..variants.len()"))
-            .to_string(),
-            value: VariantFormat::unknown(),
-        });
-        let mut value = variant.value.clone();
-        // Mark the enum as incomplete if this was not the last variant to explore.
-        if known_variants.len() != variants.len() {
-            self.tracer.incomplete_enums.insert(name.into());
+        for provisional_index in provisional_min..=u32::MAX {
+            if let Entry::Occupied(provisional_entry) = known_variants.entry(provisional_index) {
+                if self.tracer.discriminants
+                    [&(enum_type_id, VariantId::Name(&provisional_entry.get().name))]
+                    == discriminant
+                {
+                    let provisional_entry = provisional_entry.remove();
+                    match known_variants.entry(index) {
+                        Entry::Vacant(vacant) => {
+                            vacant.insert(provisional_entry);
+                        }
+                        Entry::Occupied(mut existing_entry) => {
+                            // Discard the provisional entry's name and just
+                            // keep the existing one.
+                            existing_entry
+                                .get_mut()
+                                .value
+                                .unify(provisional_entry.value)?;
+                        }
+                    }
+                } else {
+                    self.tracer
+                        .incomplete_enums
+                        .insert(enum_name.into(), EnumProgress::IndexedVariantsRemaining);
+                }
+            }
+        }
+        if let Some(existing_entry) = known_variants.get_mut(&index) {
+            existing_entry.value.unify(value)?;
         }
-        // Compute the format for this variant.
-        let inner = EnumDeserializer::new(self.tracer, self.samples, index, &mut value);
-        visitor.visit_enum(inner)
+        Ok(enum_value)
     }
 
     fn deserialize_identifier<V>(self, _visitor: V) -> Result<V::Value>
@@ -539,21 +640,21 @@ where
 struct EnumDeserializer<'de, 'a> {
     tracer: &'a mut Tracer,
     samples: &'de Samples,
-    index: u32,
+    variant_id: VariantId<'static>,
     format: &'a mut VariantFormat,
 }
 
 impl<'de, 'a> EnumDeserializer<'de, 'a> {
     fn new(
         tracer: &'a mut Tracer,
         samples: &'de Samples,
-        index: u32,
+        variant_id: VariantId<'static>,
         format: &'a mut VariantFormat,
     ) -> Self {
         Self {
             tracer,
             samples,
-            index,
+            variant_id,
             format,
         }
     }
@@ -567,8 +668,10 @@ impl<'de, 'a> de::EnumAccess<'de> for EnumDeserializer<'de, 'a> {
     where
         V: DeserializeSeed<'de>,
     {
-        let index = self.index;
-        let value = seed.deserialize(index.into_deserializer())?;
+        let value = match self.variant_id {
+            VariantId::Index(index) => seed.deserialize(U32Deserializer::new(index)),
+            VariantId::Name(name) => seed.deserialize(BorrowedStrDeserializer::new(name)),
+        }?;
         Ok((value, self))
     }
 }