[EH] Make rethrow's target a try label (#3568)

I was previously mistaken about `rethrow`'s argument rule and thought
it only counted `catch`'s depth. But it turns out it follows the same
rule `delegate`'s label: the immediate argument follows the same rule as
when computing branch labels, but it only can target `try` labels
(semantically it targets that `try`'s corresponding `catch`); otherwise
it will be a validation failure. Unlike `delegate`, `rethrow`'s label
denotes not where to rethrow, but which exception to rethrow. For
example,
```wasm
try $l0
catch ($l0)
  try $l1
  catch ($l1)
   rethrow $l0 ;; rethrow the exception caught by 'catch ($l0)'
  end
end
```

Refer to this comment for the more detailed informal semantics:
https://github.com/WebAssembly/exception-handling/issues/146#issuecomment-777714491

---

This also reverts some of `delegateTarget` -> `exceptionTarget` changes
done in #3562 in the validator. Label validation rules apply differently
for `delegate` and `rethrow` for try-catch. For example, this is valid:
```wasm
try $l0
  try
  delegate $l0
catch ($l0)
end
```
But this is NOT valid:
```wasm
try $l0
catch ($l0)
  try
  delegate $l0
end
```
So `try`'s label should be used within try-catch range (not catch-end
range) for `delegate`s.

But for the `rethrow` the rule is different. For example, this is valid:
```wasm
try $l0
catch ($l0)
  rethrow $l0
end
```
But this is NOT valid:
```wasm
try $l0
  rethrow $l0
catch ($l0)
end
```
So the `try`'s label should be used within catch-end range instead.

4 files changed, 15 insertions, 15 deletions

diff --git a/test/binaryen.js/exception-handling.js b/test/binaryen.js/exception-handling.js
index 40dc9d489..a8d3dd3a4 100644
--- a/test/binaryen.js/exception-handling.js
+++ b/test/binaryen.js/exception-handling.js
@@ -3,7 +3,7 @@ function cleanInfo(info) {
   for (var x in info) {
     // Filter out address pointers and only print meaningful info
     if (x == 'id' || x == 'type' || x == 'name' || x == 'event' ||
-        x == 'depth' || x == 'hasCatchAll' || x == 'delegateTarget' ||
+        x == 'target' || x == 'hasCatchAll' || x == 'delegateTarget' ||
         x == 'isDelegate') {
       ret[x] = info[x];
     }
@@ -21,19 +21,19 @@ module.setFeatures(binaryen.Features.ReferenceTypes |
 
 var event_ = module.addEvent("e", 0, binaryen.i32, binaryen.none);
 
-// (try
+// (try $l0
 //   (do
 //     (throw $e (i32.const 0))
 //   )
 //   (catch
 //     (drop (pop i32))
-//     (rethrow 0)
+//     (rethrow $l0)
 //   )
 // )
 var throw_ = module.throw("e", [module.i32.const(0)]);
-var rethrow = module.rethrow(0);
+var rethrow = module.rethrow("l0");
 var try_catch = module.try(
-  '',
+  "l0",
   throw_,
   ["e"],
   [
diff --git a/test/binaryen.js/exception-handling.js.txt b/test/binaryen.js/exception-handling.js.txt
index 350d543e5..434546126 100644
--- a/test/binaryen.js/exception-handling.js.txt
+++ b/test/binaryen.js/exception-handling.js.txt
@@ -3,7 +3,7 @@
  (type $i32_=>_none (func (param i32)))
  (event $e (attr 0) (param i32))
  (func $test
-  (try
+  (try $l0
    (do
     (throw $e
      (i32.const 0)
@@ -13,7 +13,7 @@
     (drop
      (pop i32)
     )
-    (rethrow 0)
+    (rethrow $l0)
    )
   )
   (try $try_outer
@@ -35,6 +35,6 @@
 )
 
 getExpressionInfo(throw) = {"id":48,"type":1,"event":"e"}
-getExpressionInfo(rethrow) = {"id":49,"type":1,"depth":0}
-getExpressionInfo(try_catch) = {"id":47,"type":1,"name":"","hasCatchAll":0,"delegateTarget":"","isDelegate":0}
+getExpressionInfo(rethrow) = {"id":49,"type":1,"target":"l0"}
+getExpressionInfo(try_catch) = {"id":47,"type":1,"name":"l0","hasCatchAll":0,"delegateTarget":"","isDelegate":0}
 getExpressionInfo(try_delegate) = {"id":47,"type":0,"name":"try_outer","hasCatchAll":1,"delegateTarget":"","isDelegate":0}
diff --git a/test/binaryen.js/expressions.js b/test/binaryen.js/expressions.js
index 688321675..1ffd45362 100644
--- a/test/binaryen.js/expressions.js
+++ b/test/binaryen.js/expressions.js
@@ -1585,14 +1585,14 @@ console.log("# Rethrow");
 (function testRethrow() {
   const module = new binaryen.Module();
 
-  const theRethrow = binaryen.Rethrow(module.rethrow(0));
+  const theRethrow = binaryen.Rethrow(module.rethrow("l0"));
   assert(theRethrow instanceof binaryen.Rethrow);
   assert(theRethrow instanceof binaryen.Expression);
-  assert(theRethrow.depth === 0);
+  assert(theRethrow.target === "l0");
   assert(theRethrow.type === binaryen.unreachable);
 
-  theRethrow.depth = 1
-  assert(theRethrow.depth === 1);
+  theRethrow.target = "l1";
+  assert(theRethrow.target === "l1");
   theRethrow.type = binaryen.f64;
   theRethrow.finalize();
   assert(theRethrow.type === binaryen.unreachable);
@@ -1601,7 +1601,7 @@ console.log("# Rethrow");
   assert(
     theRethrow.toText()
     ==
-    "(rethrow 1)\n"
+    "(rethrow $l1)\n"
   );
 
   module.dispose();
diff --git a/test/binaryen.js/expressions.js.txt b/test/binaryen.js/expressions.js.txt
index 8467c55c0..1efbd9e13 100644
--- a/test/binaryen.js/expressions.js.txt
+++ b/test/binaryen.js/expressions.js.txt
@@ -297,7 +297,7 @@
 )
 
 # Rethrow
-(rethrow 1)
+(rethrow $l1)
 
 # TupleMake
 (tuple.make