Add warning when using $in at the beginning of a block

crates/nu-protocol/src/engine/state_working_set.rs

@@ -1132,6 +1132,21 @@ impl<'a> GetSpan for &'a StateWorkingSet<'a> {
     }
 }
 
+impl GetSpan for StateWorkingSet<'_> {
+    fn get_span(&self, span_id: SpanId) -> Span {
+        let num_permanent_spans = self.permanent_state.num_spans();
+        if span_id.get() < num_permanent_spans {
+            self.permanent_state.get_span(span_id)
+        } else {
+            *self
+                .delta
+                .spans
+                .get(span_id.get() - num_permanent_spans)
+                .expect("internal error: missing span")
+        }
+    }
+}
+
 impl miette::SourceCode for &StateWorkingSet<'_> {
     fn read_span<'b>(
         &'b self,

crates/nu-protocol/src/errors/parse_warning.rs

@@ -14,12 +14,20 @@ pub enum ParseWarning {
         span: Span,
         url: String,
     },
+
+    #[error("Found $in at the start of a command.")]
+    #[diagnostic(help("Using $in at the start of a command collects the pipeline input.\nIf you did mean to collect the pipeline input, replace this with the `collect` command."))]
+    UnnecessaryInVariable {
+        #[label("try removing this")]
+        span: Span,
+    },
 }
 
 impl ParseWarning {
     pub fn span(&self) -> Span {
         match self {
             ParseWarning::DeprecatedWarning { span, .. } => *span,
+            ParseWarning::UnnecessaryInVariable { span, .. } => *span,
         }
     }
 }