From 8f641157c617a5372c473b1cc6d879a3546a22a4 Mon Sep 17 00:00:00 2001
From: Stefan VanBuren <stefan@vanburen.xyz>
Date: Mon, 31 Aug 2026 09:17:36 -0400
Subject: [PATCH] fix(injected): clamp injected regions that run to the end of
 the file

Treesitter reports a region that extends to EOF as ending at
(line count, col 0), which is one row past the last line. That end row
is used directly as a buffer index, so merge_ranges_with_prefix hits
nvim_buf_get_lines' strict indexing and the whole format call fails with
"Index out of bounds", leaving the buffer unformatted.

Fenced languages hide this: a closing ``` ends the region before EOF.
Delimiter-less blocks do not, so a YAML block scalar as the last thing
in the file reproduces it every time, as does an unclosed markdown fence.

Clamp the end of such a region to the end of the final line.

Fixes #452
---
 lua/conform/formatters/injected.lua | 7 +++++++
 tests/injected/eof.md               | 5 +++++
 tests/injected/eof.md.formatted     | 5 +++++
 3 files changed, 17 insertions(+)
 create mode 100644 tests/injected/eof.md
 create mode 100644 tests/injected/eof.md.formatted

diff --git a/lua/conform/formatters/injected.lua b/lua/conform/formatters/injected.lua
index 6f0cecb4..7a1b3999 100644
--- a/lua/conform/formatters/injected.lua
+++ b/lua/conform/formatters/injected.lua
@@ -256,6 +256,13 @@ return {
               -- The types are wrong. included_regions should be Range[][] not integer[][]
               ---@diagnostic disable-next-line: param-type-mismatch
               local start_row, start_col, _, end_row, end_col, _ = unpack(region)
+              -- Treesitter reports a region that runs to the end of the file as ending at
+              -- (line count, col 0), one row past the last line. Left alone that overruns
+              -- every buffer index below. Clamp it to the end of the final line.
+              if end_row >= #lines then
+                end_row = #lines - 1
+                end_col = #(lines[#lines] or "")
+              end
               accum_range(regions, { lang, start_row + 1, start_col, end_row + 1, end_col })
             end
           end
diff --git a/tests/injected/eof.md b/tests/injected/eof.md
new file mode 100644
index 00000000..1e2f8693
--- /dev/null
+++ b/tests/injected/eof.md
@@ -0,0 +1,5 @@
+text
+
+```lua
+local foo = "bar"
+local bar = 2
diff --git a/tests/injected/eof.md.formatted b/tests/injected/eof.md.formatted
new file mode 100644
index 00000000..ad75c1bf
--- /dev/null
+++ b/tests/injected/eof.md.formatted
@@ -0,0 +1,5 @@
+text
+
+```lua
+>local foo = "bar"
+local bar = 2<