From 4494d81f8cf6faecf5ff43c0d1e69cd8777c5f77 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=A9r=C3=B4me=20Vouillon?= <jerome.vouillon@gmail.com>
Date: Thu, 17 Oct 2024 17:46:54 +0200
Subject: [PATCH] Source map: repeat mappings over new lines

Firefox assumes that a mapping stops at the end of a line, which is
inconvenient. When this happens, we repeat the mapping on the next line.
---
 compiler/lib/js_output.ml               | 53 ++++++++++++++++++-------
 compiler/tests-compiler/sourcemap.ml    |  2 +
 compiler/tests-sourcemap/dump.reference |  1 +
 3 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/compiler/lib/js_output.ml b/compiler/lib/js_output.ml
index 8cd987b758..cea059b47e 100644
--- a/compiler/lib/js_output.ml
+++ b/compiler/lib/js_output.ml
@@ -2154,22 +2154,45 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
               String.length filename >= 9 && String.sub filename 0 9 = "/builtin/")
         in
         let sm_mappings = Source_map.Mappings.decode sm.mappings in
-        let mappings =
-          List.rev_append_map !temp_mappings sm_mappings ~f:(fun (pos, m) ->
-              let gen_line = pos.PP.p_line + 1 in
-              let gen_col = pos.PP.p_col in
-              match m with
-              | Source_map.Gen { gen_col = _; gen_line = _ } ->
-                  Source_map.Gen { gen_col; gen_line }
-              | Source_map.Gen_Ori
-                  { gen_line = _; gen_col = _; ori_source; ori_line; ori_col } ->
-                  Source_map.Gen_Ori { gen_line; gen_col; ori_source; ori_line; ori_col }
-              | Source_map.Gen_Ori_Name
-                  { gen_line = _; gen_col = _; ori_source; ori_line; ori_col; ori_name }
-                ->
-                  Source_map.Gen_Ori_Name
-                    { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name })
+        let relocate pos m =
+          let gen_line = pos.PP.p_line + 1 in
+          let gen_col = pos.PP.p_col in
+          match m with
+          | Source_map.Gen { gen_col = _; gen_line = _ } ->
+              Source_map.Gen { gen_col; gen_line }
+          | Source_map.Gen_Ori m -> Source_map.Gen_Ori { m with gen_line; gen_col }
+          | Source_map.Gen_Ori_Name m ->
+              Source_map.Gen_Ori_Name { m with gen_line; gen_col }
+        in
+        let prepend mapping prev_mappings =
+          let rec prepend_rec pos mapping prev_mappings =
+            match mapping with
+            | [] -> prev_mappings
+            | (pos', m) :: rem ->
+                (*
+                Format.eprintf
+                  "%d,%d %d,%d@."
+                  pos'.PP.p_line
+                  pos'.PP.p_col
+                  pos.PP.p_line
+                  pos.PP.p_col;
+*)
+                if pos'.PP.p_line = pos.PP.p_line
+                   || (pos'.p_line = pos.p_line - 1 && pos.p_col = 0)
+                then prepend_rec pos' rem (relocate pos' m :: prev_mappings)
+                else if pos.p_col > 0
+                then
+                  let pos = { pos with p_col = 0 } in
+                  prepend_rec pos mapping (relocate pos m :: prev_mappings)
+                else
+                  let pos = { pos with p_line = pos.p_line - 1 } in
+                  prepend_rec pos mapping (relocate pos m :: prev_mappings)
+          in
+          match mapping with
+          | [] -> prev_mappings
+          | (pos, m) :: rem -> prepend_rec pos rem (relocate pos m :: prev_mappings)
         in
+        let mappings = prepend !temp_mappings sm_mappings in
         let mappings = Source_map.Mappings.encode mappings in
         Some
           { sm with
diff --git a/compiler/tests-compiler/sourcemap.ml b/compiler/tests-compiler/sourcemap.ml
index 4cbcbb09d2..2204eb5ca5 100644
--- a/compiler/tests-compiler/sourcemap.ml
+++ b/compiler/tests-compiler/sourcemap.ml
@@ -84,10 +84,12 @@ let%expect_test _ =
        13: //# sourceMappingURL=test.map
       /builtin/blackbox.ml:1:0 -> 5:7
       /builtin/blackbox.ml:1:0 -> 5:17
+      /builtin/blackbox.ml:1:0 -> 6:0
       /dune-root/test.ml:1:4 -> 6:12
       /dune-root/test.ml:1:7 -> 6:15
       /dune-root/test.ml:1:11 -> 6:18
       /dune-root/test.ml:1:12 -> 6:28
+      /dune-root/test.ml:1:12 -> 7:0
       /dune-root/test.ml:1:12 -> 7:7
       /builtin/blackbox.ml:1:0 -> 7:14
     |}]
diff --git a/compiler/tests-sourcemap/dump.reference b/compiler/tests-sourcemap/dump.reference
index 5feb760f7e..ca2f059261 100644
--- a/compiler/tests-sourcemap/dump.reference
+++ b/compiler/tests-sourcemap/dump.reference
@@ -3,4 +3,5 @@ sourcemap for test.bc.js
 /my/sourceRoot#b.ml:1:6 -> 14:   function f(<>x){return x - 1 | 0; }
 /my/sourceRoot#b.ml:1:10 -> 17:   function f(x){<>return x - 1 | 0; }
 /my/sourceRoot#b.ml:1:15 -> 35:   function f(x){return x - 1 | 0; <>}
+/my/sourceRoot#b.ml:1:15 -> 0:<>   var Testlib_B = [0, f];
 /my/sourceRoot#b.ml:1:15 -> 7:   var <>Testlib_B = [0, f];