wasmprinter folding: change some panics to errors or successful round…

…trips (bytecodealliance#1840)

* wasmprinter folding: convert some panics to errors

* wasmprinter: roundtrip some unfoldable-but-valid `if` instructions

crates/wasmprinter/src/operator.rs

@@ -1699,13 +1699,16 @@ impl<'printer, 'state, 'a, 'b> PrintOperatorFolded<'printer, 'state, 'a, 'b> {
     }
 
     fn push_if(&mut self, ty: BlockType, plain: String) -> Result<()> {
-        let mut predicate = vec![self
+        let mut predicate = Vec::new();
+        if let Some(phrase) = self
             .control
             .last_mut()
-            .expect("no enclosing block")
+            .ok_or_else(|| anyhow!("no enclosing block"))?
             .folded
             .pop()
-            .expect("no predicate")];
+        {
+            predicate.push(phrase)
+        }
         if let Some(hint) = self.branch_hint.take() {
             predicate.push(hint);
         }
@@ -1826,7 +1829,7 @@ impl<'printer, 'state, 'a, 'b> PrintOperatorFolded<'printer, 'state, 'a, 'b> {
 
         self.control
             .last_mut()
-            .expect("end without outer block")
+            .ok_or_else(|| anyhow!("end without outer block"))?
             .folded
             .push(inst);
         Ok(())

tests/local/folding/fold-unfoldable-if.wat

@@ -0,0 +1,9 @@
+(module
+  (func
+    i32.const 0
+    try_table (param i32)
+      if
+      end
+    end
+  )
+)

tests/snapshots/local/folding/fold-unfoldable-if.wat.print

@@ -0,0 +1,11 @@
+(module
+  (type (;0;) (func))
+  (type (;1;) (func (param i32)))
+  (func (;0;) (type 0)
+    i32.const 0
+    try_table (type 1) (param i32) ;; label = @1
+      if ;; label = @2
+      end
+    end
+  )
+)

tests/snapshots/local/folding/fold-unfoldable-if.wat.print-folded

@@ -0,0 +1,10 @@
+(module
+  (type (;0;) (func))
+  (type (;1;) (func (param i32)))
+  (func (;0;) (type 0)
+    (i32.const 0)
+    (try_table (type 1) (param i32) ;; label = @1
+      (if ;; label = @2
+        (then)))
+  )
+)