Start work on improving subtype error messages (#7404)

* structure for improving subtype errors

* variant configuration errors

* different constructor counts

* format

* update test output

* different type kinds

* missing fn

* record fields mismatch

* more hints about what types we are looking at

* Revert "more hints about what types we are looking at"

This reverts commit 55476b9.

* refactor

* format

* cleanup

* expand comments tracking work left to do

* add comments

* changelog

* change fn name

* pr comments

Author: zth

CHANGELOG.md

@@ -33,6 +33,7 @@
 - Show in error messages when coercion can be used to fix a type mismatch. https://github.com/rescript-lang/rescript/pull/7505
 - Remove deprecated pipe last (`|>`) syntax. https://github.com/rescript-lang/rescript/pull/7512
 - Improve error message for pipe (`->`) syntax. https://github.com/rescript-lang/rescript/pull/7520
+- Improve a few error messages around various subtyping issues. https://github.com/rescript-lang/rescript/pull/7404
 
 # 12.0.0-alpha.13
 

compiler/ml/ast_untagged_variants.ml

@@ -71,6 +71,17 @@ type block_type =
   | ObjectType
   | UnknownType
 
+let block_type_to_user_visible_string = function
+  | IntType -> "int"
+  | StringType -> "string"
+  | FloatType -> "float"
+  | BigintType -> "bigint"
+  | BooleanType -> "bool"
+  | InstanceType i -> Instance.to_string i
+  | FunctionType -> "function"
+  | ObjectType -> "object"
+  | UnknownType -> "unknown"
+
 (*
   Type of the runtime representation of a tag.
   Can be a literal (case with no payload), or a block (case with payload).
@@ -89,6 +100,16 @@ type tag = {name: string; tag_type: tag_type option}
 type block = {tag: tag; tag_name: string option; block_type: block_type option}
 type switch_names = {consts: tag array; blocks: block array}
 
+let tag_type_to_user_visible_string = function
+  | String _ -> "string"
+  | Int _ -> "int"
+  | Float _ -> "float"
+  | BigInt _ -> "bigint"
+  | Bool _ -> "bool"
+  | Null -> "null"
+  | Undefined -> "undefined"
+  | Untagged block_type -> block_type_to_user_visible_string block_type
+
 let untagged = "unboxed"
 
 let block_type_can_be_undefined = function