feat: preliminary support for WIT templates

This is a minimum viable implementation of WIT templates per
WebAssembly/component-model#172.  It
supports interfaces with at most one wildcard function, which may be
expanded (i.e. monomorphized) with a set of substitutions provided by
the application developer when generating guest bindings.  I will be
posting separate PRs to the `wit-bindgen` and `wasmtime` repos to
implement and test guest and host binding generation, respectively.

Signed-off-by: Joel Dice <[email protected]>

crates/wit-component/src/decoding.rs

@@ -607,6 +607,7 @@ impl WitPackageDecoder<'_> {
                     types: IndexMap::default(),
                     functions: IndexMap::new(),
                     document: doc,
+                    wildcard: None,
                 })
             });
         Ok(interface)
@@ -624,6 +625,7 @@ impl WitPackageDecoder<'_> {
             types: IndexMap::default(),
             functions: IndexMap::new(),
             document: doc,
+            wildcard: None,
         };
 
         for (name, export_url, ty) in ty.exports(self.info.types.as_ref()) {

crates/wit-parser/src/ast.rs

@@ -8,6 +8,9 @@ use std::path::{Path, PathBuf};
 
 pub mod lex;
 
+pub use expand::expand;
+mod expand;
+
 pub use resolve::Resolver;
 mod resolve;
 pub mod toposort;
@@ -464,10 +467,31 @@ struct Stream<'a> {
 
 pub struct Value<'a> {
     docs: Docs<'a>,
-    name: Id<'a>,
+    name: VariableId<'a>,
     kind: ValueKind<'a>,
 }
 
+impl<'a> Value<'a> {
+    pub(crate) fn name(&self) -> &'a str {
+        match &self.name {
+            VariableId::Id(id) => id.name,
+            VariableId::Wildcard(_) => "*",
+        }
+    }
+
+    pub(crate) fn span(&self) -> Span {
+        match self.name {
+            VariableId::Id(Id { span, .. }) => span,
+            VariableId::Wildcard(span) => span,
+        }
+    }
+}
+
+pub enum VariableId<'a> {
+    Wildcard(Span), // `*`
+    Id(Id<'a>),
+}
+
 struct Union<'a> {
     span: Span,
     cases: Vec<UnionCase<'a>>,
@@ -552,7 +576,7 @@ impl<'a> InterfaceItem<'a> {
             Some((_span, Token::Union)) => {
                 TypeDef::parse_union(tokens, docs).map(InterfaceItem::TypeDef)
             }
-            Some((_span, Token::Id)) | Some((_span, Token::ExplicitId)) => {
+            Some((_span, Token::Star | Token::Id | Token::ExplicitId)) => {
                 Value::parse(tokens, docs).map(InterfaceItem::Value)
             }
             Some((_span, Token::Use)) => Use::parse(tokens).map(InterfaceItem::Use),
@@ -670,13 +694,24 @@ impl<'a> TypeDef<'a> {
 
 impl<'a> Value<'a> {
     fn parse(tokens: &mut Tokenizer<'a>, docs: Docs<'a>) -> Result<Self> {
-        let name = parse_id(tokens)?;
+        let name = parse_variable_id(tokens)?;
         tokens.expect(Token::Colon)?;
         let kind = ValueKind::parse(tokens)?;
         Ok(Value { docs, name, kind })
     }
 }
 
+fn parse_variable_id<'a>(tokens: &mut Tokenizer<'a>) -> Result<VariableId<'a>> {
+    match tokens.clone().next()? {
+        Some((_span, Token::Id | Token::ExplicitId)) => parse_id(tokens).map(VariableId::Id),
+        Some((span, Token::Star)) => {
+            tokens.expect(Token::Star)?;
+            Ok(VariableId::Wildcard(span))
+        }
+        other => Err(err_expected(tokens, "an identifier or `*`", other).into()),
+    }
+}
+
 fn parse_id<'a>(tokens: &mut Tokenizer<'a>) -> Result<Id<'a>> {
     match tokens.next()? {
         Some((span, Token::Id)) => Ok(Id {

crates/wit-parser/src/ast/expand.rs

@@ -0,0 +1,90 @@
+use crate::{Interface, InterfaceId, Resolve, WorldItem};
+use anyhow::{anyhow, Result};
+use id_arena::Arena;
+use indexmap::IndexMap;
+use std::collections::{HashMap, HashSet};
+
+pub type Substitutions = HashMap<String, HashMap<String, HashSet<String>>>;
+
+pub fn expand(resolve: &mut Resolve, mut substitutions: Substitutions) -> Result<()> {
+    let mut new_interfaces = resolve.interfaces.clone();
+    for (_, world) in &mut resolve.worlds {
+        let mut subs = substitutions.remove(&world.name).unwrap_or_default();
+        expand_interfaces(
+            &world.name,
+            "imports",
+            &mut world.imports,
+            &mut new_interfaces,
+            &mut subs,
+        )?;
+        expand_interfaces(
+            &world.name,
+            "exports",
+            &mut world.exports,
+            &mut new_interfaces,
+            &mut subs,
+        )?;
+    }
+
+    if !substitutions.is_empty() {
+        log::warn!("unused substitutions were provided: {substitutions:?}",);
+    }
+
+    resolve.interfaces = new_interfaces;
+
+    Ok(())
+}
+
+fn expand_interfaces(
+    world_name: &str,
+    desc: &str,
+    items: &mut IndexMap<String, WorldItem>,
+    new_interfaces: &mut Arena<Interface>,
+    substitutions: &mut HashMap<String, HashSet<String>>,
+) -> Result<()> {
+    for (name, item) in items {
+        if let WorldItem::Interface(interface) = item {
+            if new_interfaces[*interface].wildcard.is_some() {
+                let new_interface = expand_interface(
+                    *interface,
+                    new_interfaces,
+                    substitutions.remove(name).ok_or_else(|| {
+                        anyhow!(
+                            "world {world_name} {desc} item {name} contains wildcards \
+                             but no substitutions were provided",
+                        )
+                    })?,
+                );
+                *interface = new_interfaces.alloc(new_interface);
+            }
+        }
+    }
+
+    if !substitutions.is_empty() {
+        log::warn!("unused substitutions were provided for world {world_name}: {substitutions:?}",);
+    }
+
+    Ok(())
+}
+
+fn expand_interface(
+    interface: InterfaceId,
+    new_interfaces: &Arena<Interface>,
+    substitutions: HashSet<String>,
+) -> Interface {
+    let mut new_interface = new_interfaces[interface].clone();
+    // Make the expanded interface anonymous; otherwise the generated component type will fail to validate due to
+    // the existence of multiple interfaces with the same name.
+    //
+    // TODO: implement something like
+    // https://github.com/WebAssembly/component-model/issues/172#issuecomment-1466939890, which may entail changes
+    // to how interfaces are modeled in WIT, `Resolve`, and the component model.
+    new_interface.name = None;
+    let function = new_interface.wildcard.take().unwrap();
+    for var_name in substitutions {
+        let mut new_func = function.clone();
+        new_func.name = var_name.clone();
+        assert!(new_interface.functions.insert(var_name, new_func).is_none());
+    }
+    new_interface
+}

crates/wit-parser/src/ast/resolve.rs

@@ -1,4 +1,4 @@
-use super::{Error, ParamList, ResultList, ValueKind};
+use super::{Error, ParamList, ResultList, ValueKind, VariableId};
 use crate::ast::toposort::toposort;
 use crate::*;
 use anyhow::{anyhow, bail, Result};
@@ -186,6 +186,7 @@ impl<'a> Resolver<'a> {
                                     docs: Docs::default(),
                                     document: doc,
                                     functions: IndexMap::new(),
+                                    wildcard: None,
                                 });
                                 DocumentItem::Interface(id)
                             });
@@ -205,6 +206,7 @@ impl<'a> Resolver<'a> {
                                 docs: Docs::default(),
                                 document: doc,
                                 functions: IndexMap::new(),
+                                wildcard: None,
                             })
                         }),
                 };
@@ -531,6 +533,7 @@ impl<'a> Resolver<'a> {
             name: name.map(|s| s.to_string()),
             functions: IndexMap::new(),
             types: IndexMap::new(),
+            wildcard: None,
         });
         if let Some(name) = name {
             self.document_interfaces[document.index()]
@@ -563,11 +566,21 @@ impl<'a> Resolver<'a> {
             match field {
                 ast::InterfaceItem::Value(value) => match &value.kind {
                     ValueKind::Func(func) => {
-                        self.define_interface_name(&value.name, TypeOrItem::Item("function"))?;
-                        let func = self.resolve_function(&value.docs, value.name.name, func)?;
-                        let prev = self.interfaces[interface_id]
-                            .functions
-                            .insert(value.name.name.to_string(), func);
+                        if !matches!(value.name, VariableId::Wildcard(_)) {
+                            self.define_interface_name(
+                                value.name(),
+                                value.span(),
+                                TypeOrItem::Item("function"),
+                            )?;
+                        }
+                        let func = self.resolve_function(&value.docs, value.name(), func)?;
+                        let prev = if matches!(value.name, VariableId::Wildcard(_)) {
+                            self.interfaces[interface_id].wildcard.replace(func)
+                        } else {
+                            self.interfaces[interface_id]
+                                .functions
+                                .insert(value.name().to_string(), func)
+                        };
                         assert!(prev.is_none());
                     }
                 },
@@ -646,7 +659,9 @@ impl<'a> Resolver<'a> {
                 name: Some(def.name.name.to_string()),
                 owner,
             });
-            self.define_interface_name(&def.name, TypeOrItem::Type(id))?;
+            let name = def.name.name;
+            let span = def.name.span;
+            self.define_interface_name(name, span, TypeOrItem::Type(id))?;
         }
         Ok(())
     }
@@ -675,7 +690,7 @@ impl<'a> Resolver<'a> {
                 name: Some(name.name.to_string()),
                 owner,
             });
-            self.define_interface_name(name, TypeOrItem::Type(id))?;
+            self.define_interface_name(name.name, name.span, TypeOrItem::Type(id))?;
         }
         Ok(())
     }
@@ -758,12 +773,12 @@ impl<'a> Resolver<'a> {
         }
     }
 
-    fn define_interface_name(&mut self, name: &ast::Id<'a>, item: TypeOrItem) -> Result<()> {
-        let prev = self.type_lookup.insert(name.name, (item, name.span));
+    fn define_interface_name(&mut self, name: &'a str, span: Span, item: TypeOrItem) -> Result<()> {
+        let prev = self.type_lookup.insert(name, (item, span));
         if prev.is_some() {
             Err(Error {
-                span: name.span,
-                msg: format!("name `{}` is defined more than once", name.name),
+                span,
+                msg: format!("name `{}` is defined more than once", name),
             }
             .into())
         } else {

crates/wit-parser/src/lib.rs

@@ -8,7 +8,7 @@ use std::path::Path;
 pub mod abi;
 mod ast;
 use ast::lex::Span;
-pub use ast::SourceMap;
+pub use ast::{expand, SourceMap};
 mod sizealign;
 pub use sizealign::*;
 mod resolve;
@@ -289,6 +289,9 @@ pub struct Interface {
 
     /// The document that this interface belongs to.
     pub document: DocumentId,
+
+    /// TODO: A templated function.
+    pub wildcard: Option<Function>,
 }
 
 #[derive(Debug, Clone, PartialEq)]

crates/wit-parser/src/resolve.rs

@@ -742,6 +742,9 @@ impl Remap {
         for (_, func) in iface.functions.iter_mut() {
             self.update_function(func);
         }
+        if let Some(func) = &mut iface.wildcard {
+            self.update_function(func);
+        }
     }
 
     fn update_function(&self, func: &mut Function) {