Add context to error message when unit is expected (rescript-lang#7045)

* add some context to error message when unit is expected * changelog

Author: zth

CHANGELOG.md

@@ -27,6 +27,7 @@
 - Improve output of `@variadic` bindings. https://github.com/rescript-lang/rescript-compiler/pull/7030
 - Improve error messages around JSX components. https://github.com/rescript-lang/rescript-compiler/pull/7038
 - Improve output of record copying. https://github.com/rescript-lang/rescript-compiler/pull/7043
+- Provide additional context in error message when `unit` is expected. https://github.com/rescript-lang/rescript-compiler/pull/7045
 
 #### :house: Internal
 

jscomp/build_tests/super_errors/expected/function_call_mismatch.res.expected

@@ -9,4 +9,7 @@
  8 │ }
 
  This function call returns: string
- But it's expected to return: unit
+ But it's expected to return: unit
+
+ - Did you mean to assign this to a variable?
+ - If you don't care about the result of this expression, you can assign it to _ via let _ = ... or pipe it to ignore via expression->ignore

jscomp/build_tests/super_errors/expected/warnings2.res.expected

@@ -8,4 +8,7 @@
  4 │ }
 
  This has type: int
- But it's expected to have type: unit
+ But it's expected to have type: unit
+
+ - Did you mean to assign this to a variable?
+ - If you don't care about the result of this expression, you can assign it to _ via let _ = ... or pipe it to ignore via expression->ignore

jscomp/ml/error_message_utils.ml

@@ -165,6 +165,15 @@ let print_extra_type_clash_help ppf trace type_clash_context =
  \  - Use a tuple, if your array is of fixed length. Tuples can mix types \
  freely, and compiles to a JavaScript array. Example of a tuple: `let \
  myTuple = (10, \"hello\", 15.5, true)"
+ | ( _,
+ [
+ ({Types.desc = Tconstr (_p1, _, _)}, _); ({desc = Tconstr (p2, _, _)}, _);
+ ] )
+ when Path.same Predef.path_unit p2 ->
+ fprintf ppf
+ "\n\n\
+ \  - Did you mean to assign this to a variable?\n\
+ \  - If you don't care about the result of this expression, you can assign it to @{<info>_@} via @{<info>let _ = ...@} or pipe it to @{<info>ignore@} via @{<info>expression->ignore@}\n\n"
  | _ -> ()
 
 let type_clash_context_from_function sexp sfunct =