checker: warn on unused imported functions used via import math { sin, cos }

Author: Delta456

vlib/v/checker/checker.v

@@ -2489,6 +2489,9 @@ fn (mut c Checker) import_stmt(node ast.Import) {
 			if !func.is_pub {
 				c.error('module `${node.mod}` function `${sym.name}()` is private', sym.pos)
 			}
+			if func.usages != 1 {
+				c.warn('module `${node.mod}` function `${sym.name}()` is unused', sym.pos)
+			}
 			continue
 		}
 		if _ := c.file.global_scope.find_const(name) {

vlib/v/checker/tests/import_sym_fn_unused_warning_err.out

@@ -0,0 +1,10 @@
+vlib/v/checker/tests/import_sym_fn_unused_warning_err.vv:1:15: warning: module `math` function `sin()` is unused
+    1 | import math { sin, cos }
+      |               ~~~
+    2 | 
+    3 | fn main() {}
+vlib/v/checker/tests/import_sym_fn_unused_warning_err.vv:1:20: warning: module `math` function `cos()` is unused
+    1 | import math { sin, cos }
+      |                    ~~~
+    2 | 
+    3 | fn main() {}

vlib/v/checker/tests/import_sym_fn_unused_warning_err.vv

@@ -0,0 +1,3 @@
+import math { sin, cos }
+
+fn main() {}