v: add an -assert aborts/backtraces option to ease debugging

cmd/v/help/build-c.txt

@@ -205,3 +205,13 @@ see also `v help build`.
       Write all C flags into `file.txt`, one flag per line.
       If `file.txt` is `-`, then write the flags to stdout, one flag per line.
 
+   -assert aborts
+      Call abort() after an assertion failure. Debuggers usually
+      install signal handlers for SIGABRT, so your program will stop and you
+      will get a backtrace. If you are running your program outside of a
+      debugger, you will most likely get a core dump file.
+      
+   -assert backtraces
+      Call print_backtrace() after an assertion failure. Note that
+      backtraces are not implemented yet on all combinations of
+      platform/compiler.

vlib/v/gen/c/assert.v

@@ -33,9 +33,7 @@ fn (mut g Gen) gen_assert_stmt(original_assert_statement ast.AssertStmt) {
 		g.writeln('\tg_test_fails++;')
 		metaname_fail := g.gen_assert_metainfo(node)
 		g.writeln('\tmain__cb_assertion_failed(&$metaname_fail);')
-		if 'abort_on_assert' in g.pref.compile_defines_all {
+		g.gen_assert_postfailure_mode(node)
-			g.writeln('\tabort();')
-		}
 		g.writeln('\tlongjmp(g_jump_buffer, 1);')
 		g.writeln('\t// TODO')
 		g.writeln('\t// Maybe print all vars in a test function if it fails?')
@@ -48,14 +46,24 @@ fn (mut g Gen) gen_assert_stmt(original_assert_statement ast.AssertStmt) {
 		g.writeln(' {')
 		metaname_panic := g.gen_assert_metainfo(node)
 		g.writeln('\t__print_assert_failure(&$metaname_panic);')
-		if 'abort_on_assert' in g.pref.compile_defines_all {
+		g.gen_assert_postfailure_mode(node)
-			g.writeln('\tabort();')
-		}
 		g.writeln('\tv_panic(_SLIT("Assertion failed..."));')
 		g.writeln('}')
 	}
 }
 
+fn (mut g Gen) gen_assert_postfailure_mode(node ast.AssertStmt) {
+	match g.pref.assert_failure_mode {
+		.default {}
+		.aborts {
+			g.writeln('\tabort();')
+		}
+		.backtraces {
+			g.writeln('\tprint_backtrace();')
+		}
+	}
+}
+
 fn (mut g Gen) gen_assert_metainfo(node ast.AssertStmt) string {
 	mod_path := cestring(g.file.path)
 	fn_name := g.fn_decl.name

vlib/v/pref/pref.v

@@ -17,6 +17,12 @@ pub enum BuildMode {
 	build_module
 }
 
+pub enum AssertFailureMode {
+	default
+	aborts
+	backtraces
+}
+
 pub enum GarbageCollectionMode {
 	no_gc
 	boehm_full // full garbage collection mode
@@ -167,6 +173,7 @@ pub mut:
 	is_help             bool // -h, -help or --help was passed
 	gc_mode             GarbageCollectionMode = .no_gc // .no_gc, .boehm, .boehm_leak, ...
 	is_cstrict          bool                  // turn on more C warnings; slightly slower
+	assert_failure_mode AssertFailureMode // whether to call abort() or print_backtrace() after an assertion failure
 	// checker settings:
 	checker_match_exhaustive_cutoff_limit int = 10
 }
@@ -197,6 +204,23 @@ pub fn parse_args(known_external_commands []string, args []string) (&Preferences
 				res.arch = target_arch_kind
 				res.build_options << '$arg $target_arch'
 			}
+			'-assert' {
+				assert_mode := cmdline.option(current_args, '-assert', '')
+				match assert_mode {
+					'aborts' {
+						res.assert_failure_mode = .aborts
+					}
+					'backtraces' {
+						res.assert_failure_mode = .backtraces
+					}
+					else {
+						eprintln('unknown assert mode `-gc $assert_mode`, supported modes are:`')
+						eprintln('  `-assert aborts`     .... calls abort() after assertion failure')
+						eprintln('  `-assert backtraces` .... calls print_backtrace() after assertion failure')
+						exit(1)
+					}
+				}
+			}
 			'-show-timings' {
 				res.show_timings = true
 			}