fix(lsp): small bugs in snippet-parser #18998

This fixes the following bugs:
`${1:else_text}` -> format with if_text: "else_text"
`${1:-else_text}` -> format with if_text: "else_text"
`${1:}` in `format` (eg. empty else_text) -> error.
`${1:}` (eg. empty placeholder) -> error.

Thanks hrsh7th :)

1 files changed, 45 insertions, 12 deletions

diff --git a/runtime/lua/vim/lsp/_snippet.lua b/runtime/lua/vim/lsp/_snippet.lua
index 28064f36e9..910deba556 100644
--- a/runtime/lua/vim/lsp/_snippet.lua
+++ b/runtime/lua/vim/lsp/_snippet.lua
@@ -156,10 +156,10 @@ P.seq = function(...)
   return function(input, pos)
     local values = {}
     local new_pos = pos
-    for _, parser in ipairs(parsers) do
+    for i, parser in ipairs(parsers) do
       local result = parser(input, new_pos)
       if result.parsed then
-        table.insert(values, result.value)
+        values[i] = result.value
         new_pos = result.pos
       else
         return P.unmatch(pos)
@@ -272,22 +272,48 @@ S.format = P.any(
       S.open,
       S.int,
       S.colon,
-      P.any(
-        P.seq(S.question, P.take_until({ ':' }, { '\\' }), S.colon, P.take_until({ '}' }, { '\\' })),
-        P.seq(S.plus, P.take_until({ '}' }, { '\\' })),
-        P.seq(S.minus, P.take_until({ '}' }, { '\\' }))
-      ),
+      P.seq(S.question, P.opt(P.take_until({ ':' }, { '\\' })), S.colon, P.opt(P.take_until({ '}' }, { '\\' }))),
       S.close
     ),
     function(values)
       return setmetatable({
         type = Node.Type.FORMAT,
         capture_index = values[3],
-        if_text = values[5][2].esc,
-        else_text = (values[5][4] or {}).esc,
+        if_text = values[5][2] and values[5][2].esc or '',
+        else_text = values[5][4] and values[5][4].esc or '',
       }, Node)
     end
-  )
+  ),
+  P.map(
+    P.seq(S.dollar, S.open, S.int, S.colon, P.seq(S.plus, P.opt(P.take_until({ '}' }, { '\\' }))), S.close),
+    function(values)
+      return setmetatable({
+        type = Node.Type.FORMAT,
+        capture_index = values[3],
+        if_text = values[5][2] and values[5][2].esc or '',
+        else_text = '',
+      }, Node)
+    end
+  ),
+  P.map(
+    P.seq(S.dollar, S.open, S.int, S.colon, S.minus, P.opt(P.take_until({ '}' }, { '\\' })), S.close),
+    function(values)
+      return setmetatable({
+        type = Node.Type.FORMAT,
+        capture_index = values[3],
+        if_text = '',
+        else_text = values[6] and values[6].esc or '',
+      }, Node)
+    end
+  ),
+  P.map(P.seq(S.dollar, S.open, S.int, S.colon, P.opt(P.take_until({ '}' }, { '\\' })), S.close), function(values)
+    return setmetatable({
+      type = Node.Type.FORMAT,
+      capture_index = values[3],
+      if_text = '',
+      else_text = values[5] and values[5].esc or '',
+    }, Node)
+  end)
 )
 
 S.transform = P.map(
@@ -333,12 +359,19 @@ S.tabstop = P.any(
 
 S.placeholder = P.any(
   P.map(
-    P.seq(S.dollar, S.open, S.int, S.colon, P.many(P.any(S.toplevel, S.text({ '$', '}' }, { '\\' }))), S.close),
+    P.seq(S.dollar, S.open, S.int, S.colon, P.opt(P.many(P.any(S.toplevel, S.text({ '$', '}' }, { '\\' })))), S.close),
     function(values)
       return setmetatable({
         type = Node.Type.PLACEHOLDER,
         tabstop = values[3],
-        children = values[5],
+        -- insert empty text if opt did not match.
+        children = values[5] or {
+          setmetatable({
+            type = Node.Type.TEXT,
+            raw = '',
+            esc = '',
+          }, Node),
+        },
       }, Node)
     end
   )