From 7e80119efb48184edf3a6a544297708bcb4de849 Mon Sep 17 00:00:00 2001
From: Ted Driggs <ted.driggs@outlook.com>
Date: Wed, 1 Feb 2023 13:38:06 -0800
Subject: [PATCH] Add Error::unsupported_shape_with_expected

Unsupported shape errors can now indicate what they expected to receive.
---
 core/src/error/kind.rs | 19 ++++++++++++++++---
 core/src/error/mod.rs  | 12 +++++++++++-
 core/src/util/shape.rs |  5 ++++-
 3 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/core/src/error/kind.rs b/core/src/error/kind.rs
index 0030e0d..2b4eda6 100644
--- a/core/src/error/kind.rs
+++ b/core/src/error/kind.rs
@@ -15,7 +15,10 @@ pub(in crate::error) enum ErrorKind {
     Custom(String),
     DuplicateField(FieldName),
     MissingField(FieldName),
-    UnsupportedShape(DeriveInputShape),
+    UnsupportedShape {
+        observed: DeriveInputShape,
+        expected: Option<String>,
+    },
     UnknownField(ErrorUnknownField),
     UnexpectedFormat(MetaFormat),
     UnexpectedType(String),
@@ -39,7 +42,7 @@ impl ErrorKind {
             DuplicateField(_) => "Duplicate field",
             MissingField(_) => "Missing field",
             UnknownField(_) => "Unexpected field",
-            UnsupportedShape(_) => "Unsupported shape",
+            UnsupportedShape { .. } => "Unsupported shape",
             UnexpectedFormat(_) => "Unexpected meta-item format",
             UnexpectedType(_) => "Unexpected literal type",
             UnknownValue(_) => "Unknown literal value",
@@ -69,7 +72,17 @@ impl fmt::Display for ErrorKind {
             DuplicateField(ref field) => write!(f, "Duplicate field `{}`", field),
             MissingField(ref field) => write!(f, "Missing field `{}`", field),
             UnknownField(ref field) => field.fmt(f),
-            UnsupportedShape(ref shape) => write!(f, "Unsupported shape `{}`", shape),
+            UnsupportedShape {
+                ref observed,
+                ref expected,
+            } => {
+                write!(f, "Unsupported shape `{}`", observed)?;
+                if let Some(expected) = &expected {
+                    write!(f, ". Expected {}.", expected)?;
+                }
+
+                Ok(())
+            }
             UnexpectedFormat(ref format) => write!(f, "Unexpected meta-item format `{}`", format),
             UnexpectedType(ref ty) => write!(f, "Unexpected literal type `{}`", ty),
             UnknownValue(ref val) => write!(f, "Unknown literal value `{}`", val),
diff --git a/core/src/error/mod.rs b/core/src/error/mod.rs
index 92b3bc5..f86d58e 100644
--- a/core/src/error/mod.rs
+++ b/core/src/error/mod.rs
@@ -120,7 +120,17 @@ impl Error {
 
     /// Creates a new error for a struct or variant that does not adhere to the supported shape.
     pub fn unsupported_shape(shape: &str) -> Self {
-        Error::new(ErrorKind::UnsupportedShape(shape.into()))
+        Error::new(ErrorKind::UnsupportedShape {
+            observed: shape.into(),
+            expected: None,
+        })
+    }
+
+    pub fn unsupported_shape_with_expected<T: fmt::Display>(shape: &str, expected: &T) -> Self {
+        Error::new(ErrorKind::UnsupportedShape {
+            observed: shape.into(),
+            expected: Some(expected.to_string()),
+        })
     }
 
     pub fn unsupported_format(format: &str) -> Self {
diff --git a/core/src/util/shape.rs b/core/src/util/shape.rs
index 9ab1048..5a90891 100644
--- a/core/src/util/shape.rs
+++ b/core/src/util/shape.rs
@@ -180,7 +180,10 @@ impl ShapeSet {
         if self.contains_shape(shape) {
             Ok(())
         } else {
-            Err(crate::Error::unsupported_shape(shape.description()))
+            Err(crate::Error::unsupported_shape_with_expected(
+                shape.description(),
+                self,
+            ))
         }
     }