Second large refactor for WebIDL bindings

This commit is the second, and hopefully last massive, refactor for
using WebIDL bindings internally in `wasm-bindgen`. This commit actually
fully executes on the task at hand, moving `wasm-bindgen` to internally
using WebIDL bindings throughout its code generation, anyref passes,
etc. This actually fixes a number of issues that have existed in the
anyref pass for some time now!

The main changes here are to basically remove the usage of `Descriptor`
from generating JS bindings. Instead two new types are introduced:
`NonstandardIncoming` and `NonstandardOutgoing` which are bindings lists
used for incoming/outgoing bindings. These mirror the standard
terminology and literally have variants which are the standard values.
All `Descriptor` types are now mapped into lists of incoming/outgoing
bindings and used for process in wasm-bindgen. All JS generation has
been refactored and updated to now process these lists of bindings
instead of the previous `Descriptor`.

In other words this commit takes `js2rust.rs` and `rust2js.rs` and first
splits them in two. Interpretation of `Descriptor` and what to do for
conversions is in the binding selection modules. The actual generation
of JS from the binding selection is now performed by `incoming.rs` and
`outgoing.rs`. To boot this also deduplicates all the code between the
argument handling of `js2rust.rs` and return value handling of
`rust2js.rs`. This means that to implement a new binding you only need
to implement it one place and it's implemented for free in the other!

This commit is not the end of the story though. I would like to add a
mdoe to `wasm-bindgen` that literally emits a WebIDL bindings section.
That's left for a third (and hopefully final) refactoring which is also
intended to optimize generated JS for bindings.

This commit currently loses the optimization where an imported is hooked
up by value directly whenever a shim isn't needed. It's planned that
the next refactoring to emit a webidl binding section that can be added
back in. It shouldn't be too too hard hopefully since all the
scaffolding is in place now.

cc #1524

crates/cli-support/src/anyref.rs

@@ -1,43 +1,45 @@
-use crate::descriptor::{Closure, Descriptor, Function};
-use crate::webidl::{AuxImport, ImportBinding, WasmBindgenAux, WebidlCustomSection};
+use crate::webidl::{NonstandardIncoming, NonstandardOutgoing};
+use crate::webidl::{NonstandardWebidlSection, WasmBindgenAux};
 use failure::Error;
 use std::collections::HashSet;
 use walrus::Module;
 use wasm_bindgen_anyref_xform::Context;
+use wasm_webidl_bindings::ast;
 
 pub fn process(module: &mut Module) -> Result<(), Error> {
     let mut cfg = Context::default();
     cfg.prepare(module)?;
     let bindings = module
         .customs
-        .get_typed_mut::<WebidlCustomSection>()
+        .get_typed_mut::<NonstandardWebidlSection>()
         .expect("webidl custom section should exist");
 
+    // Transform all exported functions in the module, using the bindings listed
+    // for each exported function.
     for (export, binding) in bindings.exports.iter_mut() {
-        let (args, ret) = extract_anyrefs(binding, 0);
+        let ty = module.types.get(binding.wasm_ty);
+        let args = Arguments::Incoming(&mut binding.incoming);
+        let (args, ret) = extract_anyrefs(ty, args);
         cfg.export_xform(*export, &args, ret);
-        process_closure_arguments(&mut cfg, binding);
     }
 
-    for (import, kind) in bindings.imports.iter_mut() {
-        let binding = match kind {
-            ImportBinding::Function(f) => f,
-            ImportBinding::Constructor(f) => f,
-            ImportBinding::Method(f) => f,
-        };
-        let (args, ret) = extract_anyrefs(binding, 0);
+    // Transform all imported functions in the module, using the bindings listed
+    // for each imported function.
+    for (import, binding) in bindings.imports.iter_mut() {
+        let ty = module.types.get(binding.wasm_ty);
+        let args = Arguments::Outgoing(&mut binding.outgoing);
+        let (args, ret) = extract_anyrefs(ty, args);
         cfg.import_xform(*import, &args, ret);
-        process_closure_arguments(&mut cfg, binding);
     }
 
-    let aux = module
-        .customs
-        .get_typed_mut::<WasmBindgenAux>()
-        .expect("webidl custom section should exist");
-    for import in aux.import_map.values_mut() {
-        match import {
-            AuxImport::Closure(f) => process_closure(&mut cfg, f),
-            _ => {}
+    // And finally transform all table elements that are used as function
+    // pointers for closures and such.
+    for (idx, binding) in bindings.elems.iter_mut() {
+        let ty = module.types.get(binding.wasm_ty);
+        let args = Arguments::Incoming(&mut binding.incoming);
+        let (args, ret) = extract_anyrefs(ty, args);
+        if let Some(new) = cfg.table_element_xform(*idx, &args, ret) {
+            *idx = new;
         }
     }
 
@@ -55,7 +57,7 @@ pub fn process(module: &mut Module) -> Result<(), Error> {
         .collect::<HashSet<_>>();
     module
         .customs
-        .get_typed_mut::<WebidlCustomSection>()
+        .get_typed_mut::<NonstandardWebidlSection>()
         .expect("webidl custom section should exist")
         .imports
         .retain(|id, _| remaining_imports.contains(id));
@@ -68,44 +70,9 @@ pub fn process(module: &mut Module) -> Result<(), Error> {
     Ok(())
 }
 
-/// Process the `function` provided to ensure that all references to `Closure`
-/// descriptors are processed below.
-fn process_closure_arguments(cfg: &mut Context, function: &mut Function) {
-    for arg in function.arguments.iter_mut() {
-        process_descriptor(cfg, arg);
-    }
-    process_descriptor(cfg, &mut function.ret);
-
-    fn process_descriptor(cfg: &mut Context, descriptor: &mut Descriptor) {
-        match descriptor {
-            Descriptor::Ref(d)
-            | Descriptor::RefMut(d)
-            | Descriptor::Option(d)
-            | Descriptor::Slice(d)
-            | Descriptor::Vector(d) => process_descriptor(cfg, d),
-            Descriptor::Closure(c) => process_closure(cfg, c),
-            Descriptor::Function(c) => process_function(cfg, c),
-            _ => {}
-        }
-    }
-
-    fn process_function(cfg: &mut Context, function: &mut Function) {
-        let (args, ret) = extract_anyrefs(&function, 2);
-        if let Some(new) = cfg.table_element_xform(function.shim_idx, &args, ret) {
-            function.shim_idx = new;
-        }
-        process_closure_arguments(cfg, function);
-    }
-}
-
-/// Ensure that the `Closure` is processed in case any of its arguments
-/// recursively contain `anyref` and such.
-fn process_closure(cfg: &mut Context, closure: &mut Closure) {
-    let (args, ret) = extract_anyrefs(&closure.function, 2);
-    if let Some(new) = cfg.table_element_xform(closure.shim_idx, &args, ret) {
-        closure.shim_idx = new;
-    }
-    process_closure_arguments(cfg, &mut closure.function);
+enum Arguments<'a> {
+    Incoming(&'a mut [NonstandardIncoming]),
+    Outgoing(&'a mut [NonstandardOutgoing]),
 }
 
 /// Extract a description of the anyref arguments from the function signature
@@ -120,19 +87,53 @@ fn process_closure(cfg: &mut Context, closure: &mut Closure) {
 /// the wasm abi arguments described by `f` start at. For closures this is 2
 /// because two synthetic arguments are injected into the wasm signature which
 /// aren't present in the `Function` signature.
-fn extract_anyrefs(f: &Function, offset: usize) -> (Vec<(usize, bool)>, bool) {
-    let mut args = Vec::new();
-    let mut cur = offset;
-    if f.ret.abi_returned_through_pointer() {
-        cur += 1;
-    }
-    for arg in f.arguments.iter() {
-        if arg.is_anyref() {
-            args.push((cur, true));
-        } else if arg.is_ref_anyref() {
-            args.push((cur, false));
+fn extract_anyrefs(ty: &walrus::Type, args: Arguments<'_>) -> (Vec<(usize, bool)>, bool) {
+    let mut ret = Vec::new();
+
+    // First find all the `anyref` arguments in the input type, and we'll
+    // assume that they're owned anyref arguments for now (the `true`)
+    for (i, arg) in ty.params().iter().enumerate() {
+        if *arg == walrus::ValType::Anyref {
+            ret.push((i, true));
         }
-        cur += arg.abi_arg_count();
     }
-    (args, f.ret.is_anyref())
+
+    // Afterwards look through the argument list (specified with various
+    // bindings) to find any borrowed anyref values and update our
+    // transformation metadata accordingly. if we find one then the binding no
+    // longer needs to remember its borrowed but rather it's just a simple cast
+    // from wasm anyref to JS any.
+    match args {
+        Arguments::Incoming(incoming) => {
+            for binding in incoming {
+                let expr = match binding {
+                    NonstandardIncoming::BorrowedAnyref {
+                        val: ast::IncomingBindingExpression::Get(expr),
+                    } => expr.clone(),
+                    _ => continue,
+                };
+                ret.iter_mut().find(|p| p.0 == expr.idx as usize).unwrap().1 = false;
+                let new_binding = ast::IncomingBindingExpressionAs {
+                    ty: walrus::ValType::Anyref,
+                    expr: Box::new(expr.into()),
+                };
+                *binding = NonstandardIncoming::Standard(new_binding.into());
+            }
+        }
+        Arguments::Outgoing(outgoing) => {
+            for binding in outgoing {
+                let idx = match binding {
+                    NonstandardOutgoing::BorrowedAnyref { idx } => *idx,
+                    _ => continue,
+                };
+                ret.iter_mut().find(|p| p.0 == idx as usize).unwrap().1 = false;
+                let new_binding = ast::OutgoingBindingExpressionAs {
+                    idx,
+                    ty: ast::WebidlScalarType::Any.into(),
+                };
+                *binding = NonstandardOutgoing::Standard(new_binding.into());
+            }
+        }
+    }
+    (ret, ty.results() == &[walrus::ValType::Anyref])
 }