Implement some handling of preprocessor macros

corpus/preprocessor.spicy

@@ -0,0 +1,47 @@
+# Preprocessor macros with varying contents.
+
+@if 1
+# foo
+@else
+
+@endif
+
+@if 1
+1;
+
+2;
+@else
+
+2;
+@endif
+
+@if ! 1
+1;
+@endif
+
+@if 1 ==1
+
+1;
+@endif
+
+# FIXME(bbannier): Empty lines in bodies are currently not removed.
+@if
+
+@else
+
+@endif
+
+
+# We allow constructs like `import` in preprocessor macros.
+@if 1
+import foo ;
+
+@endif
+
+# We allow field declarations in preprocessor macros.
+type X = unit {
+# FIXME(bbannier): preprocessor macros inherit indention, we might not want to do that.
+@if 1
+: bytes   &size=1;
+@endif
+};

corpus/preprocessor.spicy.expected

@@ -0,0 +1,41 @@
+# Preprocessor macros with varying contents.
+
+@if 1
+# foo
+@else
+@endif
+
+@if 1
+1;
+2;
+@else
+2;
+@endif
+
+@if !1
+1;
+@endif
+
+@if 1 == 1
+1;
+@endif
+
+# FIXME(bbannier): Empty lines in bodies are currently not removed.
+@if
+
+@else
+
+@endif
+
+# We allow constructs like `import` in preprocessor macros.
+@if 1
+import foo;
+@endif
+
+# We allow field declarations in preprocessor macros.
+type X = unit {
+    # FIXME(bbannier): preprocessor macros inherit indention, we might not want to do that.
+    @if 1
+    : bytes &size=1;
+    @endif
+};

src/query.scm

@@ -317,3 +317,28 @@
  (_)
  "{" @prepend_space
 )
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+; Preprocessor macros
+
+; Empty lines before preprocessor blocks are fine.
+(preproc) @allow_blank_line_before
+
+; `@if` always has a following expression so it needs a space.
+(preproc "@if" @append_space)
+
+; Newline handling. This is slightly nasty since preprocessor macros are
+; terminated by explicit newlines instead of `;`. Force a newline, but delete
+; original newlines so we do not insert stray empty lines or spaces.
+(preproc
+  "@if" (expression) @append_hardline
+  "\n" @delete
+)
+(preproc
+  "@else" @append_hardline
+  "\n" @delete
+)
+(preproc
+  "@endif" @prepend_hardline
+)
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;