compiler: print asserted source line on failure

vlib/compiler/preludes/tests_assertions.v

@@ -0,0 +1,35 @@
+module main
+
+import os
+import term
+
+////////////////////////////////////////////////////////////////////
+/// This file will get compiled as part of the main program,
+/// for a _test.v file.
+/// The methods defined here are called back by the test program's
+/// assert statements, on each success/fail. The goal is to make
+/// customizing the look & feel of the assertions results easier,
+/// since it is done in normal V code, instead of in embedded C ...
+////////////////////////////////////////////////////////////////////
+
+fn cb_assertion_failed(filename string, line int, sourceline string, funcname string){
+	color_on := term.can_show_color_on_stderr()
+	use_relative_paths := match os.getenv('VERROR_PATHS') {
+		'absolute' { false }
+		else { true }
+	}
+	final_filename := if use_relative_paths { filename } else { os.realpath( filename ) }
+	final_funcname := funcname.replace('main__','')
+
+	mut fail_message := 'FAILED assertion'
+	if color_on { fail_message = term.bold(term.red(fail_message)) }
+	
+	eprintln('$final_filename:$line: $fail_message')
+	eprintln('Function: $final_funcname')
+	eprintln('Source  : $sourceline')
+}
+
+fn cb_assertion_ok(filename string, line int, sourceline string, funcname string){
+	//do nothing for now on an OK assertion
+	//println('OK ${line:5d}|$sourceline ')
+}