In debug mode log all imported uncaught exceptions

This commit updates the `--debug` output of `wasm-bindgen` from the CLI
to catch all JS exceptions from imported functions, log such, and then
rethrow. It's hoped that this can be used when necessary to learn more
information about thrown exceptions and where an uncaught exception
could be causing issues with Rust code.

Closes #1176

crates/cli-support/src/js/mod.rs

@@ -2410,6 +2410,9 @@ impl<'a, 'b> SubContext<'a, 'b> {
         // Build up our shim's state, and we'll use that to guide whether we
         // actually emit an import here or not.
         let mut shim = Rust2Js::new(self.cx);
+        if shim.cx.config.debug {
+            shim.catch_and_rethrow(true);
+        }
         shim.catch(import.catch)
             .variadic(import.variadic)
             .process(descriptor.unwrap_function())?;

crates/cli-support/src/js/rust2js.rs

@@ -36,6 +36,7 @@ pub struct Rust2Js<'a, 'b: 'a> {
 
     /// Whether or not we're catching JS exceptions
     catch: bool,
+    catch_and_rethrow: bool,
 
     /// Whether or not the last argument is a slice representing variadic arguments.
     variadic: bool,
@@ -53,6 +54,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
             arg_idx: 0,
             ret_expr: String::new(),
             catch: false,
+            catch_and_rethrow: false,
             variadic: false,
         }
     }
@@ -62,6 +64,11 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         self
     }
 
+    pub fn catch_and_rethrow(&mut self, catch_and_rethrow: bool) -> &mut Self {
+        self.catch_and_rethrow = catch_and_rethrow;
+        self
+    }
+
     pub fn variadic(&mut self, variadic: bool) -> &mut Self {
         self.variadic = variadic;
         self
@@ -505,6 +512,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         let Rust2Js {
             // fields which may affect whether we do nontrivial work
             catch,
+            catch_and_rethrow,
             finally,
             js_arguments,
             prelude,
@@ -520,6 +528,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         } = self;
 
         !catch &&
+            !catch_and_rethrow &&
             !variadic &&
             prelude.is_empty() &&
             finally.is_empty() &&
@@ -639,7 +648,21 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
                 ",
                 &invoc, catch
             );
-        };
+        } else if self.catch_and_rethrow {
+            invoc = format!(
+                "\
+                try {{\n\
+                    {}
+                }} catch (e) {{\n\
+                    console.error(\"wasm-bindgen: imported JS function that \
+                                    was not marked as `catch` threw an error:\", \
+                                    e);
+                    throw e;
+                }}\
+                ",
+                &invoc,
+            );
+        }
 
         if self.finally.len() > 0 {
             invoc = format!(