Fix wasm non-web usage of env::now (#123)

A wasm embedder that hasn't enabled the `web` feature will still get the
`Instant::now()` function generated, even if they end up using a custom
`now` function. After all, the compiler/link don't have runtime
knowledge about how the program runs, so it could be that
`ThreadProfiler::initialize` is never called, so the default value of
`now_ns` that makes use of `Instant::now` could be called at any point.
 
This changes the `now_ns` function so it's defined only using `Instant`
for native target and wasm when web is enabled. In the other case, we
assume this is never called and it's a programmer error because we don't
have any time reporter.

Fixes #118.

puffin/src/lib.rs

@@ -535,17 +535,11 @@ impl GlobalProfiler {
 
 /// Returns a high-precision, monotonically increasing nanosecond count since unix epoch.
 #[inline]
+#[cfg(any(not(target_arch = "wasm32"), feature = "web"))]
 pub fn now_ns() -> NanoSecond {
     #[cfg(target_arch = "wasm32")]
     fn nanos_since_epoch() -> NanoSecond {
-        #[cfg(feature = "web")]
-        {
-            (js_sys::Date::new_0().get_time() * 1e6) as _
-        }
-        #[cfg(not(feature = "web"))]
-        {
-            0 // We won't get correct date-times, but that's fine.
-        }
+        (js_sys::Date::new_0().get_time() * 1e6) as _
     }
 
     #[cfg(not(target_arch = "wasm32"))]
@@ -566,6 +560,13 @@ pub fn now_ns() -> NanoSecond {
     START_TIME.0 + START_TIME.1.elapsed().as_nanos() as NanoSecond
 }
 
+#[inline]
+#[cfg(all(target_arch = "wasm32", not(feature = "web")))]
+pub fn now_ns() -> NanoSecond {
+    // This should be unused.
+    panic!("Wasm without the `web` feature requires passing a custom source of time via `ThreadProfiler::initialize`");
+}
+
 // ----------------------------------------------------------------------------
 
 // We currently store an Option<ProfilerScope> on the stack (None when profiling is off).