fmt: add single-line ternary expr support for constants (#19681)

vlib/v/fmt/fmt.v

@@ -2225,7 +2225,8 @@ pub fn (mut f Fmt) if_expr(node ast.IfExpr) {
 	f.inside_comptime_if = node.is_comptime
 	mut is_ternary := node.branches.len == 2 && node.has_else
 		&& branch_is_single_line(node.branches[0]) && branch_is_single_line(node.branches[1])
-		&& (node.is_expr || f.is_assign || f.is_struct_init || f.single_line_fields)
+		&& (node.is_expr || f.is_assign || f.inside_const || f.is_struct_init
+		|| f.single_line_fields)
 	f.single_line_if = is_ternary
 	start_pos := f.out.len
 	start_len := f.line_len

vlib/v/fmt/tests/comptime_if_expr_script_keep.vv

@@ -1,3 +1,5 @@
+const enable_debug = $if debug { true } $else { false }
+
 mut pre_built_str := ''
 $if prebuilt ? {
 	the_date := if $env('DATE') != '' { $env('DATE') } else { '##DATE##' }

vlib/v/fmt/tests/if_ternary_expected.vv

@@ -1,3 +1,19 @@
+const (
+	spacing_error = if true { true } else { false }
+	too_long_line = if b.no_cstep {
+		'TMP1/${b.nexpected_steps:1d}'
+	} else {
+		'${b.cstep:1d}/${b.nexpected_steps:1d}'
+	}
+	too_many_branches = if true {
+		true
+	} else if true {
+		true
+	} else {
+		false
+	}
+)
+
 fn main() {
 	// This line is too long
 	sprogress := if b.no_cstep {

vlib/v/fmt/tests/if_ternary_input.vv

@@ -1,3 +1,9 @@
+const (
+	spacing_error = if true {true}else{ false}
+	too_long_line = if b.no_cstep { 'TMP1/${b.nexpected_steps:1d}' } else { '${b.cstep:1d}/${b.nexpected_steps:1d}' }
+	too_many_branches = if true { true } else if true { true } else { false }
+)
+
 fn main() {
 	// This line is too long
 	sprogress := if b.no_cstep { 'TMP1/${b.nexpected_steps:1d}' } else { '${b.cstep:1d}/${b.nexpected_steps:1d}' }

vlib/v/fmt/tests/if_ternary_keep.vv

@@ -1,3 +1,7 @@
+import os
+
+const exe_extension = if os.user_os() == 'windows' { '.exe' } else { '' }
+
 struct Foo {
 	n int
 	s string