errors: Even more downcast using buck2_error

Summary: I'm not sure if we should keep the `EvalMessage::from_anyhow` function around. Tbh libraries should never accept `anyhow::Error`s like this, and they certainly shouldn't downcast them. But whatever, I don't care too much right now

Reviewed By: wendy728

Differential Revision: D50213712

fbshipit-source-id: 1f1f8d871d2b690a5ced0e8b9817074675e978fb

starlark/src/analysis/types.rs

@@ -146,7 +146,9 @@ impl Display for EvalMessage {
 }
 
 impl EvalMessage {
-    /// Convert from an `anyhow::Error`, including some type checking, to an `EvalMessage`
+    /// Convert from an `anyhow::Error` to an `EvalMessage`.
+    ///
+    /// This will attempt to downcast the error to a `Diagnostic`.
     pub fn from_anyhow(file: &Path, x: &anyhow::Error) -> Self {
         match x.downcast_ref::<Diagnostic>() {
             Some(d) => Self::from_diagnostic(file, d),