Keep attributes in front of a labeled parameter alias when formatting (#9111)

Why:
    The parser puts attributes written before `~a as x` on the alias
    pattern, and the formatter printed them after `as`, where they parse
    onto the inner pattern, so formatting changed the AST (#9101). Doc
    comments in that position were printed between `as` and the pattern.

Format modified (labeled parameter alias):
Example:
    let g = (@attr ~a as x: int) => x
    let h = (/** a */ ~a as x: int=3) => x
Before (rescript):
    let g = (~a as @attr x: int) => x
    let h = (
      ~a as /** a */
      x: int=3,
    ) => x
After (rescript):
    let g = (@attr ~a as x: int) => x
    let h = (
      /** a */
      ~a as x: int=3,
    ) => x


Claude-Session: https://claude.ai/code/session_01Kbf3gGkPma9LAZimuQjnrP

Signed-off-by: Christoph Knittel <ck@cca.io>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: cknitt

changes/labeled-alias-parameter-attributes.fix.md

@@ -0,0 +1 @@
+- Fix the formatter moving an attribute or doc comment in front of a labeled parameter alias like `~a as x: int` onto the alias.

compiler/syntax/src/res_printer.ml

@@ -5656,13 +5656,14 @@ and print_exp_fun_parameter ~state parameter cmt_tbl =
              ])
           cmt_tbl pattern.ppat_loc
       | (Labelled {txt = lbl} | Optional {txt = lbl}), pattern ->
-        (* ~b as c *)
+        (* ~b as c: the parser puts attributes before [~b] on [c] *)
         Doc.concat
           [
+            print_attributes ~state pattern.ppat_attributes cmt_tbl;
             Doc.text "~";
             print_ident_like lbl;
             Doc.text " as ";
-            print_pattern ~state pattern cmt_tbl;
+            print_pattern ~state {pattern with ppat_attributes = []} cmt_tbl;
           ]
     in
     let optional_label_suffix =

tests/syntax_tests/data/printer/comments/docCommentParameters.res

@@ -2,6 +2,7 @@ let f = (@res.doc("The runner") run: string) => run
 let f = (/** The runner */ run: string) => run
 let f = (/** Positional */ x, /** Labelled */ ~y, /** Typed */ ~z: int) => x + y + z
 let f = (/** Optional */ ~a=?, /** Default */ ~b=1, /** Aliased */ ~c as d) => d
+let f = (/** Typed alias */ ~a as x: int=3) => x
 let f = (/** Locally abstract */ type a, x: a) => x
 
 // A doc comment on a single parameter that would be hugged

tests/syntax_tests/data/printer/comments/expected/docCommentParameters.res.txt

@@ -19,9 +19,13 @@ let f = (
   ~a=?,
   /** Default */
   ~b=1,
-  ~c as /** Aliased */
-  d,
+  /** Aliased */
+  ~c as d,
 ) => d
+let f = (
+  /** Typed alias */
+  ~a as x: int=3,
+) => x
 let f = (
   /** Locally abstract */
   type a,

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

@@ -104,7 +104,7 @@ let _ = await {
 
 let f1 = async (~x, ~y) => x + y
 let f2 = async (@foo ~x, @bar ~y) => x + y
-let f3 = @foo async (~x as @bar @zz z, ~y) => x + y
+let f3 = @foo async (@bar @zz ~x as z, ~y) => x + y
 let f4 = async x => x
 let f5 = async x => async y => 3
 let f6 = async (~x1, ~x2) => async y => 3

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

@@ -75,6 +75,13 @@ let f = @attr (a, b) => @attr2 (c, d) => ()
 let f = (@attr ~a, @attr ~b) => ()
 let f = (@attr ~a, @attr ~b) => ()
 
+// Attributes before a labeled alias stay in front of the label
+let f = (@attr ~a as x: int) => x
+let f = (@attr ~a as x: int=?, @attr ~b as y: int=1) => x + y
+let f = (@attr ~a as (x, y), @attr ~b as {z}, @attr ~c as _) => x + y + z
+let f = (@attr @inner ~a as x: int) => x
+let f = (~a as @attr x: int) => x
+
 let f = (
   thisIsAVeryLongNaaaaaaaaaaaaaaaaaaameeeeeeeeeeeee,
   thisIsAVeryLongNaaaaaaaaaaaaaaaaaaameeeeeeeeeeeee,

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

@@ -75,6 +75,13 @@ let f = @attr (a, b) => @attr2 (c, d) => ()
 let f = (@attr ~a, @attr ~b) => ()
 let f = (@attr ~a, @attr ~b) => ()
 
+// Attributes before a labeled alias stay in front of the label
+let f = (@attr ~a as x: int) => x
+let f = (@attr ~a as x: int=?, @attr ~b as y: int=1) => x + y
+let f = (@attr ~a as (x, y), @attr ~b as {z}, @attr ~c as _) => x + y + z
+let f = (@attr ~a as @inner (x: int)) => x
+let f = (~a as @attr x: int) => x
+
 let f = (
   thisIsAVeryLongNaaaaaaaaaaaaaaaaaaameeeeeeeeeeeee,
   thisIsAVeryLongNaaaaaaaaaaaaaaaaaaameeeeeeeeeeeee,