Improve C/C++ indentation flow for single statement blocks (#34549)

Before this, indentation did not automatically increase after
if/for/while/do/else statements in C++, and only increased after if/for
in C. This led to Zed using last line logic when inserting lines *after*
the indented statement, as well as not indenting the statement itself,
resulting in irregular indentation during typing.

Just adding indentation (similar to C) creates a new problem: now if a
scope is started with a brace on a new line, that brace is indented.
Thus we need to deindent it.

Using else_clause in the indent guide results in the else statement
being indented forward as well, so we need to deindent that too.

Note: the most significant issue for me is the one where indentation
jumps forward when inserting lines after indented lines. Unfortunately,
it appears that fixing that issue requires all of these other changes. I
would have preferred a simpler fix, but I'm not sure if disabling last
line behavior for C/C++ is appropriate as it probably breaks something
else, like cases where the file is incomplete and the statements can't
be parsed properly.

Editing flow before this change:

[Screencast From 2025-07-16
08-31-36.webm](https://github.com/user-attachments/assets/3dea86c5-47bd-47c2-aee8-b0aa613948e6)

Editing flow after this change:

[Screencast From 2025-07-16
08-35-36.webm](https://github.com/user-attachments/assets/7ef23e60-1ee3-49fd-90f9-d53f909ca674)

(note: the "else" snippet is completely breaking the flow here, but I
think that comes from clangd by default? Unfortunately I haven't found a
way to disable it cleanly but that is a separate problem that happens
right now too.)

Release Notes:

- Improve indentation during typing for C/C++ around if/for/while/do
blocks

crates/languages/src/c/config.toml

@@ -2,6 +2,10 @@ name = "C"
 grammar = "c"
 path_suffixes = ["c"]
 line_comments = ["// "]
+decrease_indent_patterns = [
+  { pattern = "^\\s*\\{.*\\}?\\s*$", valid_after = ["if", "for", "while", "do", "switch", "else"] },
+  { pattern = "^\\s*else\\s*$", valid_after = ["if"] }
+]
 autoclose_before = ";:.,=}])>"
 brackets = [
     { start = "{", end = "}", close = true, newline = true },

crates/languages/src/c/indents.scm

@@ -3,7 +3,17 @@
   (assignment_expression)
   (if_statement)
   (for_statement)
+  (while_statement)
+  (do_statement)
+  (else_clause)
 ] @indent
 
 (_ "{" "}" @end) @indent
 (_ "(" ")" @end) @indent
+
+(if_statement) @start.if
+(for_statement) @start.for
+(while_statement) @start.while
+(do_statement) @start.do
+(switch_statement) @start.switch
+(else_clause) @start.else

crates/languages/src/cpp/config.toml

@@ -2,6 +2,10 @@ name = "C++"
 grammar = "cpp"
 path_suffixes = ["cc", "hh", "cpp", "h", "hpp", "cxx", "hxx", "c++", "ipp", "inl", "ixx", "cu", "cuh", "C", "H"]
 line_comments = ["// ", "/// ", "//! "]
+decrease_indent_patterns = [
+  { pattern = "^\\s*\\{.*\\}?\\s*$", valid_after = ["if", "for", "while", "do", "switch", "else"] },
+  { pattern = "^\\s*else\\s*$", valid_after = ["if"] }
+]
 autoclose_before = ";:.,=}])>"
 brackets = [
     { start = "{", end = "}", close = true, newline = true },

crates/languages/src/cpp/indents.scm

@@ -1,7 +1,19 @@
 [
     (field_expression)
     (assignment_expression)
+    (if_statement)
+    (for_statement)
+    (while_statement)
+    (do_statement)
+    (else_clause)
 ] @indent
 
 (_ "{" "}" @end) @indent
 (_ "(" ")" @end) @indent
+
+(if_statement) @start.if
+(for_statement) @start.for
+(while_statement) @start.while
+(do_statement) @start.do
+(switch_statement) @start.switch
+(else_clause) @start.else