[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.

1 files changed, 108 insertions, 11 deletions

diff --git a/test/exception-handling.wast b/test/exception-handling.wast
index 399fef14c..45cb21910 100644
--- a/test/exception-handling.wast
+++ b/test/exception-handling.wast
@@ -130,17 +130,6 @@
         )
       )
     )
-
-    ;; rethrow
-    (try
-      (do
-        (throw $e-i32 (i32.const 0))
-      )
-      (catch $e-i32
-        (drop (pop i32))
-        (rethrow 0)
-      )
-    )
   )
 
   (func $delegate-test
@@ -200,4 +189,112 @@
       (delegate 0)
     )
   )
+
+  (func $rethrow-test
+    ;; Simple try-catch-rethrow
+    (try $l0
+      (do
+        (call $foo)
+      )
+      (catch $e-i32
+        (drop (pop i32))
+        (rethrow $l0) ;; by label
+      )
+      (catch_all
+        (rethrow 0) ;; by depth
+      )
+    )
+
+    ;; When there are both a branch and a rethrow that target the same try
+    ;; label. Because binaryen only allows blocks and loops to be targetted by
+    ;; branches, we wrap the try with a block and make branches that block
+    ;; instead, resulting in the br and rethrow target different labels in the
+    ;; output.
+    (try $l0
+      (do
+        (call $foo)
+      )
+      (catch $e-i32
+        (drop (pop i32))
+        (rethrow $l0)
+      )
+      (catch_all
+        (br $l0)
+      )
+    )
+
+    ;; One more level deep
+    (try $l0
+      (do
+        (call $foo)
+      )
+      (catch_all
+        (try
+          (do
+            (call $foo)
+          )
+          (catch $e-i32
+            (drop (pop i32))
+            (rethrow $l0) ;; by label
+          )
+          (catch_all
+            (rethrow 1) ;; by depth
+          )
+        )
+      )
+    )
+
+    ;; Interleaving block
+    (try $l0
+      (do
+        (call $foo)
+      )
+      (catch_all
+        (try
+          (do
+            (call $foo)
+          )
+          (catch $e-i32
+            (drop (pop i32))
+            (block $b0
+              (rethrow $l0) ;; by label
+            )
+          )
+          (catch_all
+            (block $b1
+              (rethrow 2) ;; by depth
+            )
+          )
+        )
+      )
+    )
+
+    ;; Within nested try, but rather in 'try' part and not 'catch'
+    (try $l0
+      (do
+        (call $foo)
+      )
+      (catch_all
+        (try
+          (do
+            (rethrow $l0) ;; by label
+          )
+          (catch_all)
+        )
+      )
+    )
+    (try $l0
+      (do
+        (call $foo)
+      )
+      (catch_all
+        (try
+          (do
+            (rethrow 1) ;; by depth
+          )
+          (catch_all)
+        )
+      )
+    )
+  )
 )