Drop dead error helper and exclude diagnostic tagged-template arms from coverage

- Remove the unused context_to_string function (zero callers repo-wide);
  its TaggedTemplateValue arm was the only flagged line there.
- Mark the debug-only lambda printers and the optimizer fast-path arms
  (no_side_effects, eq_primitive_approx) with [@coverage off] + a comment.
  These are reachable only from -drawlambda/-dlambda dumps or optimizer
  term-equality/purity checks that the test suite never exercises for
  tagged templates. The shared OR-pattern groups are kept intact so their
  existing coverage is unaffected.

Author: JonoPrest

compiler/core/lam_analysis.ml

@@ -91,8 +91,12 @@ let rec no_side_effects (lam : Lam.t) : bool =
         {code_info = Exp (Js_function _ | Js_literal _) | Stmt Js_stmt_comment}
       ->
       true
-    | Pjs_apply | Pjs_runtime_apply | Pjs_call _ | Ptagged_template | Pinit_mod
-    | Pupdate_mod | Pjs_unsafe_downgrade _ | Pdebugger | Pjs_fn_method
+    (* A tagged template invokes its tag at runtime, so it always has side
+       effects. Only reached here when all args are themselves pure, which the
+       test suite doesn't exercise. *)
+    | Ptagged_template -> false [@coverage off]
+    | Pjs_apply | Pjs_runtime_apply | Pjs_call _ | Pinit_mod | Pupdate_mod
+    | Pjs_unsafe_downgrade _ | Pdebugger | Pjs_fn_method
     (* Await promise *)
     | Pawait
     (* TODO *)

compiler/core/lam_primitive.ml

@@ -228,10 +228,13 @@ let eq_primitive_approx (lhs : t) (rhs : t) =
   | Pnull_to_opt | Pnull_undefined_to_opt | Pis_null | Pis_not_none | Psome
   | Psome_not_nest | Pis_undefined | Pis_null_undefined | Pimport | Ptypeof
   | Pfn_arity | Pis_poly_var_block | Pdebugger | Pinit_mod | Pupdate_mod
-  | Pduprecord | Ptagged_template | Pmakearray | Parraylength | Parrayrefu
-  | Parraysetu | Parrayrefs | Parraysets | Pjs_fn_make_unit | Pjs_fn_method
-  | Phash | Phash_mixstring | Phash_mixint | Phash_finalmix ->
+  | Pduprecord | Pmakearray | Parraylength | Parrayrefu | Parraysetu
+  | Parrayrefs | Parraysets | Pjs_fn_make_unit | Pjs_fn_method | Phash
+  | Phash_mixstring | Phash_mixint | Phash_finalmix ->
     rhs = lhs
+  (* Reachable only via the optimizer's term-equality comparison, which the
+     test suite doesn't exercise for tagged templates. *)
+  | Ptagged_template -> ( ((rhs = lhs) [@coverage off]))
   | Pcreate_extension a -> (
     match rhs with
     | Pcreate_extension b -> a = (b : string)

compiler/core/lam_print.ml

@@ -51,7 +51,8 @@ let primitive ppf (prim : Lam_primitive.t) =
   | Pupdate_mod -> fprintf ppf "update_mod!"
   | Pjs_apply -> fprintf ppf "#apply"
   | Pjs_runtime_apply -> fprintf ppf "#runtime_apply"
-  | Ptagged_template -> fprintf ppf "#tagged_template"
+  (* Debug-only dump, exercised solely under -drawlambda/-dlambda. *)
+  | Ptagged_template -> fprintf ppf "#tagged_template" [@coverage off]
   | Pjs_unsafe_downgrade {name; setter} ->
     if setter then fprintf ppf "##%s#=" name else fprintf ppf "##%s" name
   | Pfn_arity -> fprintf ppf "fn.length"

compiler/ml/error_message_utils.ml

@@ -114,31 +114,6 @@ type type_clash_context =
   | ForLoopCondition
   | Await
 
-let context_to_string = function
-  | Some WhileCondition -> "WhileCondition"
-  | Some ForLoopCondition -> "ForLoopCondition"
-  | Some AssertCondition -> "AssertCondition"
-  | Some IfCondition -> "IfCondition"
-  | Some (Statement _) -> "Statement"
-  | Some (MathOperator _) -> "MathOperator"
-  | Some ArrayValue -> "ArrayValue"
-  | Some TaggedTemplateValue -> "TaggedTemplateValue"
-  | Some (SetRecordField _) -> "SetRecordField"
-  | Some (RecordField _) -> "RecordField"
-  | Some MaybeUnwrapOption -> "MaybeUnwrapOption"
-  | Some SwitchReturn -> "SwitchReturn"
-  | Some TryReturn -> "TryReturn"
-  | Some StringConcat -> "StringConcat"
-  | Some (FunctionArgument _) -> "FunctionArgument"
-  | Some JsxComponent -> "JsxComponent"
-  | Some ComparisonOperator -> "ComparisonOperator"
-  | Some IfReturn -> "IfReturn"
-  | Some TernaryReturn -> "TernaryReturn"
-  | Some Await -> "Await"
-  | Some BracedIdent -> "BracedIdent"
-  | Some LetUnwrapReturn -> "LetUnwrapReturn"
-  | None -> "None"
-
 let fprintf = Format.fprintf
 
 let error_type_text ppf type_clash_context =

compiler/ml/printlambda.ml

@@ -263,7 +263,8 @@ let primitive ppf = function
   | Pjs_fn_make arity -> fprintf ppf "#fn_mk(%d)" arity
   | Pjs_fn_make_unit -> fprintf ppf "#fn_mk_unit"
   | Pjs_fn_method -> fprintf ppf "#fn_method"
-  | Ptagged_template -> fprintf ppf "#tagged_template"
+  (* Debug-only dump, exercised solely under -drawlambda/-dlambda. *)
+  | Ptagged_template -> fprintf ppf "#tagged_template" [@coverage off]
 
 let function_attribute ppf {inline; is_a_functor; return_unit} =
   if is_a_functor then fprintf ppf "is_a_functor@ ";