add resource support to C# generator (#939)

This adds resource support to the C# generator and also fixes a few
miscellaneous issues that were preventing tests from passing.

I believe the code generated for imported resources is reasonably ergonomic,
although I'm pretty new to C#, so I'm open to suggestions to make it more
idiomatic.

As with other languages, the code for exported resources is a bit less
ergonomic, I'm using an `interface` to represent the API, but I also want
implementations to inherit code which manages the resource handle, for which I'm
currently using a `class`.  The upshot is that implementing an exported resource
involves both extending a `class` and implementing an `interface`.  Again, I'm
open to suggestions about improving this.  Note that exporting resources tends
to be a lot less common in end-user code than importing them, so we probably
don't need to obsess over ergonomics in that case.



disable C# runtime tests on non-Windows platforms

As of this writing, they only work on Windows, although Linux support is on the
horizon.



Update lib.rs

Fixes lots of warnings and makes the C# code more idiomatic

rename rep_table.cs to RepTable.cs; address PR feedback



per-resource RepTables; use zero as handle sentinel



add comments explaining handle mgmt for exported resources

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

crates/csharp/Cargo.toml

@@ -23,6 +23,7 @@ wasm-metadata = { workspace = true }
 heck = { workspace = true }
 clap = { workspace = true, optional = true }
 anyhow = { workspace = true }
+indexmap = { workspace = true }
 
 [dev-dependencies]
 test-helpers = { path = '../test-helpers' }

crates/csharp/src/RepTable.cs

@@ -0,0 +1,46 @@
+/**
+ * This class is used to assign a unique integer identifier to instances of
+ * exported resources, which the host will use as its "core representation" per
+ * https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#definition-types.
+ * The identifier may be used to retrieve the corresponding instance e.g. when
+ * lifting a handle as part of the canonical ABI implementation.
+ */
+internal class RepTable<T> {
+    private List<object> list = new List<object>();
+    private int? firstVacant = null;
+
+    private class Vacant {
+        internal int? next;
+
+        internal Vacant(int? next) {
+            this.next = next;
+        }
+    }
+
+    internal int Add(T v) {
+        int rep;
+        if (firstVacant.HasValue) {
+            rep = firstVacant.Value;
+            firstVacant = ((Vacant) list[rep]).next;
+            list[rep] = v;
+        } else {
+            rep = list.Count;
+            list.Add(v);
+        }
+        return rep;
+    }
+
+    internal T Get(int rep) {
+        if (list[rep] is Vacant) {
+            throw new ArgumentException("invalid rep");
+        }
+        return (T) list[rep];
+    }
+
+    internal T Remove(int rep) {
+        var val = Get(rep);
+        list[rep] = new Vacant(firstVacant);
+        firstVacant = rep;
+        return (T) val;
+    }
+}