fix downcast_raw error when using with_span_field_formatter (#15)

with_span_field_formatter changes the generic types of the returned `Self`, so it's not valid to pass through `self.get_context`; we instead need to refetch `get_context` based on the new type. added a unit tests that validates this works.

src/lib.rs

@@ -396,7 +396,11 @@ where
 }
 
 // pub methods
-impl<S, F> IndicatifLayer<S, F> {
+impl<S, F> IndicatifLayer<S, F>
+where
+    S: Subscriber + for<'a> LookupSpan<'a>,
+    F: for<'writer> FormatFields<'writer> + 'static,
+{
     #[deprecated(since = "0.2.3", note = "use get_stderr_writer() instead")]
     pub fn get_fmt_writer(&self) -> IndicatifWriter<writer::Stderr> {
         self.get_stderr_writer()
@@ -443,10 +447,16 @@ impl<S, F> IndicatifLayer<S, F> {
             progress_style: self.progress_style,
             span_child_prefix_indent: self.span_child_prefix_indent,
             span_child_prefix_symbol: self.span_child_prefix_symbol,
-            get_context: self.get_context,
-            get_stderr_writer_context: self.get_stderr_writer_context,
-            get_stdout_writer_context: self.get_stdout_writer_context,
-            get_multi_progress_context: self.get_multi_progress_context,
+            get_context: WithContext(IndicatifLayer::<S, F2>::get_context),
+            get_stderr_writer_context: WithStderrWriter(
+                IndicatifLayer::<S, F2>::get_stderr_writer_context,
+            ),
+            get_stdout_writer_context: WithStdoutWriter(
+                IndicatifLayer::<S, F2>::get_stdout_writer_context,
+            ),
+            get_multi_progress_context: WithMultiProgress(
+                IndicatifLayer::<S, F2>::get_multi_progress_context,
+            ),
             inner: self.inner,
         }
     }

src/tests.rs

@@ -14,6 +14,7 @@ use tracing_subscriber::fmt::format::DefaultFields;
 use tracing_subscriber::fmt::MakeWriter;
 use tracing_subscriber::layer::SubscriberExt;
 
+use crate::filter::hide_indicatif_span_fields;
 use crate::span_ext::IndicatifSpanExt;
 use crate::suspend_tracing_indicatif;
 use crate::IndicatifLayer;
@@ -858,6 +859,23 @@ hello world
     );
 }
 
+#[test]
+fn test_with_span_field_formatter() {
+    let indicatif_layer = IndicatifLayer::new()
+        .with_span_field_formatter(hide_indicatif_span_fields(DefaultFields::new()));
+
+    let subscriber = tracing_subscriber::registry()
+        .with(tracing_subscriber::fmt::layer().with_writer(indicatif_layer.get_stderr_writer()))
+        .with(indicatif_layer);
+
+    tracing::subscriber::with_default(subscriber, || {
+        let _span = info_span!("foo");
+        _span.pb_start();
+
+        suspend_tracing_indicatif(|| {});
+    });
+}
+
 // These don't actually run anything, but exist to type check macros.
 #[allow(dead_code)]
 fn type_check_indicatif_println() {