Make wasmtime_internal_error's error formatting more-compatible with anyhow's (#12306)

For the "caused by" error chain, anyhow doesn't use tabs and only prints a
number when there are multiple causes.

Author: fitzgen

crates/error/src/context.rs

@@ -73,7 +73,7 @@ mod sealed {
 /// failed to convert `999` into a `u8` (max = `255`)
 ///
 /// Caused by:
-/// 	0: out of range integral type conversion attempted
+///     out of range integral type conversion attempted
 ///     "#.trim(),
 /// );
 /// ```

crates/error/src/error.rs

@@ -446,8 +446,14 @@ impl fmt::Debug for Error {
 
         if let Some(source) = inner.source() {
             f.write_str("\n\nCaused by:\n")?;
+            let multiple_causes = source.source().is_some();
             for (i, e) in Chain::new(source).enumerate() {
-                writeln!(f, "\t{i}: {e}")?;
+                if multiple_causes {
+                    write!(f, "{i: >5}: ")?;
+                } else {
+                    write!(f, "    ")?;
+                }
+                writeln!(f, "{e}")?;
             }
         }
 
@@ -717,8 +723,8 @@ impl Error {
     /// cannot frob the blobbins
     ///
     /// Caused by:
-    /// 	0: failed to bonkinate
-    /// 	1: root cause
+    ///     0: failed to bonkinate
+    ///     1: root cause
     ///         "#.trim(),
     ///     ),
     /// );

crates/error/tests/tests.rs

@@ -640,11 +640,39 @@ fn fmt_debug() {
     let error = Error::msg("whoops").context("uh oh").context("yikes");
     let actual = format!("{error:?}");
 
-    let expected = "yikes\n\
-                        \n\
-                        Caused by:\n\
-                        \t0: uh oh\n\
-                        \t1: whoops\n";
+    let expected = "\
+yikes
+
+Caused by:
+    0: uh oh
+    1: whoops
+";
+
+    #[cfg(feature = "backtrace")]
+    {
+        assert!(actual.starts_with(expected));
+        if let BacktraceStatus::Captured = error.backtrace().status() {
+            assert!(actual.contains("Stack backtrace:"));
+        }
+    }
+
+    #[cfg(not(feature = "backtrace"))]
+    {
+        assert_eq!(actual, expected);
+    }
+}
+#[test]
+fn fmt_debug_with_single_cause() {
+    let error = Error::msg("whoops").context("uh oh");
+    let actual = format!("{error:?}");
+
+    // NB: the causes are only numbered when there are multiple of them.
+    let expected = "\
+uh oh
+
+Caused by:
+    whoops
+";
 
     #[cfg(feature = "backtrace")]
     {