Keep _-prefixed exotic identifiers escaped, reject \"_\"

CHANGELOG.md

@@ -14,6 +14,7 @@
 
 #### :boom: Breaking Change
 
+- `\"_"` is no longer allowed as a quoted identifier, as it couldn't be told apart from the `_` wildcard and placeholder. https://github.com/rescript-lang/rescript/pull/8795
 - Record syntax no longer creates or matches dicts, e.g. `let d: dict<int> = {foo: 1}`; it's an error pointing to `dict{...}`. https://github.com/rescript-lang/rescript/pull/8751
 - Remove the call-site `@inlined` attribute, which was parsed but never affected code generation. It is now reported as a misplaced attribute (warning 53). https://github.com/rescript-lang/rescript/pull/8734
 - Remove `@deriving(abstract)` and `@deriving(jsConverter)`. Use record types (with optional fields, mutable fields and `@as` renaming) and polymorphic variants directly instead. https://github.com/rescript-lang/rescript/pull/8729
@@ -30,6 +31,7 @@
 
 - Fix parser recovery and editor completion when a record or dict value is missing before a comma, e.g. `{a: , b: 1}`. https://github.com/rescript-lang/rescript/pull/8779
 - Derive module names from file names consistently, fixing files like `Button.test.res` that could not reference `Button`, and `.res`/`.resi` pairs with dotted or non-ASCII names. https://github.com/rescript-lang/rescript/pull/8797
+- Fix the formatter printing escaped identifiers starting with `_`, like `\"_'"`, in a form that doesn't parse. https://github.com/rescript-lang/rescript/pull/8795
 - Fix references to a `@val` global reading a binding with the same name, e.g. a toplevel `let` declared before them. https://github.com/rescript-lang/rescript/pull/8785
 - Evaluate the unused arguments of an inlined function call in parameter order. https://github.com/rescript-lang/rescript/pull/8786
 - Fix shadowed variables in generated JS colliding with identifiers or external names containing `$`. https://github.com/rescript-lang/rescript/pull/8778

analysis/src/utils.ml

@@ -250,7 +250,13 @@ let print_maybe_exotic_ident ?(allow_uident = false) txt =
     else if i == 0 then
       match String.unsafe_get txt i with
       | 'A' .. 'Z' when allow_uident -> loop (i + 1)
-      | 'a' .. 'z' | '_' | '$' -> loop (i + 1)
+      | 'a' .. 'z' | '$' -> loop (i + 1)
+      (* Same as Res_printer.classify_ident_content: a lone [_] or one
+         followed by another character is not scanned as an identifier. *)
+      | '_' when len > 1 -> (
+        match String.unsafe_get txt 1 with
+        | 'A' .. 'Z' | 'a' .. 'z' | '0' .. '9' | '_' | '$' -> loop (i + 1)
+        | _ -> "\"" ^ txt ^ "\"")
       | _ -> "\"" ^ txt ^ "\""
     else
       match String.unsafe_get txt i with

compiler/syntax/src/res_printer.ml

@@ -496,7 +496,13 @@ let classify_ident_content ?(allow_uident = false) ?(allow_hyphen = false) txt =
         match String.unsafe_get txt i with
         | '\\' -> UppercaseExoticIdent
         | 'A' .. 'Z' when allow_uident -> loop (i + 1)
-        | 'a' .. 'z' | '_' | '$' -> loop (i + 1)
+        | 'a' .. 'z' | '$' -> loop (i + 1)
+        (* The scanner reads [_] as an identifier only when one of these
+           follows; otherwise it is a wildcard. *)
+        | '_' when len > 1 -> (
+          match String.unsafe_get txt 1 with
+          | 'A' .. 'Z' | 'a' .. 'z' | '0' .. '9' | '_' | '$' -> loop (i + 1)
+          | _ -> ExoticIdent)
         | _ -> ExoticIdent
       else
         match String.unsafe_get txt i with
@@ -3338,6 +3344,10 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
       let constr = print_longident_location longident_loc cmt_tbl in
       let args = print_expression_args ~state args cmt_tbl in
       Doc.group (Doc.concat [constr; args])
+    | Pexp_ident ({txt = Longident.Lident "_"} as path) ->
+      (* The `_` placeholder of underscore apply sugar, see
+         [Parsetree_viewer.rewrite_underscore_apply]. *)
+      print_comments (Doc.text "_") cmt_tbl path.loc
     | Pexp_ident path -> print_lident_path path cmt_tbl
     | Pexp_tuple exprs ->
       Doc.group

compiler/syntax/src/res_scanner.ml

@@ -331,6 +331,13 @@ let scan_exotic_identifier scanner =
     scanner.err ~start_pos ~end_pos
       (Diagnostics.message "A quoted identifier can't be empty string.");
     Token.Lident ident)
+  else if name = "_" then (
+    (* `_` is the wildcard and the placeholder of underscore apply sugar;
+       a quoted `_` couldn't be told apart from them. *)
+    let end_pos = position scanner in
+    scanner.err ~start_pos ~end_pos
+      (Diagnostics.message "`_` can't be used as a quoted identifier.");
+    Token.Lident name)
   else if Ext_ident.is_uident name then Token.Lident ident
     (* Exotic ident with uppercase letter should be encoded to avoid confusing in OCaml parsetree *)
   else Token.Lident name

tests/analysis_tests/tests/src/CompletionExpressions.res

@@ -263,7 +263,7 @@ let arr = ["hello"]
 // arr->Belt.Array.map()
 //                     ^com
 
-type exoticPolyvariant = [#"some exotic" | #dollar$]
+type exoticPolyvariant = [#"some exotic" | #dollar$ | #"_"]
 
 let takesExotic = (e: exoticPolyvariant) => {
   ignore(e)

tests/analysis_tests/tests/src/expected/CompletionExpressions.res.txt

@@ -1330,19 +1330,31 @@ Path takesExotic
     "detail": "#\"some exotic\"",
     "documentation": {
       "kind": "markdown",
-      "value": "```rescript\n#\"some exotic\"\n```\n\n```rescript\n[#dollar$ | #\"some exotic\"]\n```"
+      "value": "```rescript\n#\"some exotic\"\n```\n\n```rescript\n[#\"_\" | #dollar$ | #\"some exotic\"]\n```"
     },
     "insertText": "#\"some exotic\"",
     "insertTextFormat": 2,
     "kind": 4,
     "label": "#\"some exotic\"",
     "tags": []
   },
+  {
+    "detail": "#\"_\"",
+    "documentation": {
+      "kind": "markdown",
+      "value": "```rescript\n#\"_\"\n```\n\n```rescript\n[#\"_\" | #dollar$ | #\"some exotic\"]\n```"
+    },
+    "insertText": "#\"_\"",
+    "insertTextFormat": 2,
+    "kind": 4,
+    "label": "#\"_\"",
+    "tags": []
+  },
   {
     "detail": "#dollar$",
     "documentation": {
       "kind": "markdown",
-      "value": "```rescript\n#dollar$\n```\n\n```rescript\n[#dollar$ | #\"some exotic\"]\n```"
+      "value": "```rescript\n#dollar$\n```\n\n```rescript\n[#\"_\" | #dollar$ | #\"some exotic\"]\n```"
     },
     "insertText": "#dollar$",
     "insertTextFormat": 2,

tests/syntax_tests/data/parsing/errors/scanner/expected/quotedUnderscore.res.txt

@@ -0,0 +1,22 @@
+
+  Syntax error!
+  syntax_tests/data/parsing/errors/scanner/quotedUnderscore.res:1:5-8
+
+  1 │ let \"_" = 1
+  2 │ let x = f(\"_")
+  3 │ 
+
+  `_` can't be used as a quoted identifier.
+
+
+  Syntax error!
+  syntax_tests/data/parsing/errors/scanner/quotedUnderscore.res:2:11-14
+
+  1 │ let \"_" = 1
+  2 │ let x = f(\"_")
+  3 │ 
+
+  `_` can't be used as a quoted identifier.
+
+let _ = 1
+let x [arity:1]__x = f __x

tests/syntax_tests/data/parsing/errors/scanner/quotedUnderscore.res

@@ -0,0 +1,2 @@
+let \"_" = 1
+let x = f(\"_")

tests/syntax_tests/data/printer/expr/expected/underscoreExoticIdent.res.txt

@@ -0,0 +1,11 @@
+let \"_'" = 2
+let \"_''" = 3
+let __' = 4
+let _a' = 5
+let f = (~\"_'") => \"_'"
+let a = #"_'"
+let b = #"_"
+type r = {\"_'": int}
+let r = {\"_'": 1}
+let g = r.\"_'"
+let x = f(a, _, c)

tests/syntax_tests/data/printer/expr/underscoreExoticIdent.res

@@ -0,0 +1,11 @@
+let \"_'" = 2
+let \"_''" = 3
+let __' = 4
+let _a' = 5
+let f = (~\"_'") => \"_'"
+let a = #"_'"
+let b = #"_"
+type r = {\"_'": int}
+let r = {\"_'": 1}
+let g = r.\"_'"
+let x = f(a, _, c)