add -check-return support too (experimental ... too many notices)

vlib/v/checker/fn.v

@@ -3965,9 +3965,18 @@ fn (mut c Checker) resolve_return_type(node ast.CallExpr) ast.Type {
 }
 
 fn (mut c Checker) check_must_use_call_result(node &ast.CallExpr, f &ast.Fn, label string) {
-	if node.is_return_used || !f.is_must_use {
+	if node.is_return_used {
 		return
 	}
-	c.warn('return value must be used, ${label} `${f.name}` was tagged with `@[must_use]`',
-		node.pos)
+	if f.return_type == ast.void_type {
+		return
+	}
+	if f.is_must_use {
+		c.warn('return value must be used, ${label} `${f.name}` was tagged with `@[must_use]`',
+			node.pos)
+		return
+	}
+	if c.pref.is_check_return {
+		c.note('return value must be used', node.pos)
+	}
 }

vlib/v/help/build/build.txt

@@ -54,8 +54,9 @@ NB: the build flags are shared with the run command too:
     Define the provided flag.
     If `value` is not provided, it is assumed to be set to `true`.
     `value` can be any pure literal like `32` (i64),`34.98` (f64), `false` (bool),
-    `v` (char) or `"V rocks"` (string). In V code you can use `value := $d('<flag>', <default value>)`
-    to retrieve the value assigned to `<flag>`.
+    `v` (char) or `"V rocks"` (string).
+    In V code you can use `value := $d('<flag>', <default value>)` to retrieve the
+    value assigned to `<flag>`.
     If no flag identifier (or value) is assigned, `$d()` will return the passed `<default value>`.
 
   -g
@@ -245,10 +246,16 @@ NB: the build flags are shared with the run command too:
     NB: in the future, this will be turned ON by default,
     and will become an error, after vlib modules are cleaned up.
 
+  -check-return
+    Note about all calls, that ignore the return value of
+    the corresponding fn/method.
+    NB: this is still experimental, the rules for it will
+    change, it may be dropped completely, or it may become the default.
+
 For C-specific build flags, use `v help build-c`.
 For JS-specific build flags, use `v help build-js`.
 For Native-specific build flags, use `v help build-native`.
 For WebAssembly-specific build flags, use `v help build-wasm`.
 
 See also:
-   `v help run` for documentation regarding `v run`.
+   `v help run` for documentation regarding `v run`.

vlib/v/pref/pref.v

@@ -126,6 +126,7 @@ pub mut:
 	is_callstack       bool     // turn on callstack registers on each call when v.debug is imported
 	is_trace           bool     // turn on possibility to trace fn call where v.debug is imported
 	is_coverage        bool     // turn on code coverage stats
+	is_check_return    bool     // -check-return, will make V produce notices about *all* call expressions with unused results. NOTE: experimental!
 	eval_argument      string   // `println(2+2)` on `v -e "println(2+2)"`. Note that this source code, will be evaluated in vsh mode, so 'v -e 'println(ls(".")!)' is valid.
 	test_runner        string   // can be 'simple' (fastest, but much less detailed), 'tap', 'normal'
 	profile_file       string   // the profile results will be stored inside profile_file
@@ -956,6 +957,9 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 			'-check-unused-fn-args' {
 				res.show_unused_params = true
 			}
+			'-check-return' {
+				res.is_check_return = true
+			}
 			'-use-coroutines' {
 				res.use_coroutines = true
 				$if macos || linux {