checker: add suggestions for misspelled `mod.func_name()` calls

vlib/v/checker/fn.v

@@ -662,6 +662,20 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 	}
 	if !found {
 		continue_check = false
+		if dot_index := fn_name.index('.') {
+			if !fn_name[0].is_capital() {
+				mod_name := fn_name#[..dot_index]
+				mut mod_func_names := []string{}
+				for ctfnk, ctfnv in c.table.fns {
+					if ctfnv.is_pub && ctfnk.starts_with(mod_name) {
+						mod_func_names << ctfnk
+					}
+				}
+				suggestion := util.new_suggestion(fn_name, mod_func_names)
+				c.error(suggestion.say('unknown function: $fn_name '), node.pos)
+				return ast.void_type
+			}
+		}
 		c.error('unknown function: $fn_name', node.pos)
 		return ast.void_type
 	}

vlib/v/checker/tests/misspelled_mod_fn_name_should_have_suggestion.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/misspelled_mod_fn_name_should_have_suggestion.vv:3:9: error: unknown function: os.read_fil .
+Did you mean `os.read_file`?
+    1 | import os
+    2 | 
+    3 | dump(os.read_fil('abc'))
+      |         ~~~~~~~~~~~~~~~

vlib/v/checker/tests/misspelled_mod_fn_name_should_have_suggestion.vv

@@ -0,0 +1,3 @@
+import os
+
+dump(os.read_fil('abc'))

vlib/v/checker/tests/unknown_function.out

@@ -1,4 +1,5 @@
-vlib/v/checker/tests/unknown_function.vv:4:15: error: unknown function: math.max_i64
+vlib/v/checker/tests/unknown_function.vv:4:15: error: unknown function: math.max_i64 .
+Did you mean `math.max`?
     2 | 
     3 | fn main() {
     4 |     println(math.max_i64())

vlib/v/util/suggestions.v

@@ -1,14 +1,20 @@
 module util
 
+import term
 import strings
 
+// Possibility is a simple pair of a string, with a similarity coefficient
+// determined by the editing distance to a wanted value.
 struct Possibility {
 	value  string
 	svalue string
 mut:
-	similarity f32
+	similarity f32 // Note: 0.0 for *equal* strings.
 }
 
+// Suggestion is set of known possibilities and a wanted string.
+// It has helper methods for making educated guesses based on the possibilities,
+// on which of them match best the wanted string.
 struct Suggestion {
 mut:
 	known   []Possibility
@@ -16,6 +22,7 @@ mut:
 	swanted string
 }
 
+// new_suggestion creates a new Suggestion, given a wanted value and a list of possibilities.
 pub fn new_suggestion(wanted string, possibilities []string) Suggestion {
 	mut s := Suggestion{
 		wanted: wanted
@@ -26,6 +33,8 @@ pub fn new_suggestion(wanted string, possibilities []string) Suggestion {
 	return s
 }
 
+// add adds the `val` to the list of known possibilities of the suggestion.
+// It calculates the similarity metric towards the wanted value.
 pub fn (mut s Suggestion) add(val string) {
 	if val in [s.wanted, s.swanted] {
 		return
@@ -43,16 +52,21 @@ pub fn (mut s Suggestion) add(val string) {
 	}
 }
 
+// add adds all of the `many` to the list of known possibilities of the suggestion
 pub fn (mut s Suggestion) add_many(many []string) {
 	for x in many {
 		s.add(x)
 	}
 }
 
+// sort sorts the list of known possibilities, based on their similarity metric.
+// Equal strings will be first, followed by less similar ones, very distinct ones will be last.
 pub fn (mut s Suggestion) sort() {
 	s.known.sort(a.similarity < b.similarity)
 }
 
+// say produces a final suggestion message, based on the preset `wanted` and
+// `possibilities` fields, accumulated in the Suggestion.
 pub fn (s Suggestion) say(msg string) string {
 	mut res := msg
 	mut found := false
@@ -61,14 +75,14 @@ pub fn (s Suggestion) say(msg string) string {
 		if top_posibility.similarity > 0.5 {
 			val := top_posibility.value
 			if !val.starts_with('[]') {
-				res += '.\nDid you mean `$val`?'
+				res += '.\nDid you mean `${highlight_suggestion(val)}`?'
 				found = true
 			}
 		}
 	}
 	if !found {
 		if s.known.len > 0 {
-			mut values := s.known.map('`$it.svalue`')
+			mut values := s.known.map('`${highlight_suggestion(it.svalue)}`')
 			values.sort()
 			if values.len == 1 {
 				res += '.\n1 possibility: ${values[0]}.'
@@ -81,6 +95,8 @@ pub fn (s Suggestion) say(msg string) string {
 	return res
 }
 
+// short_module_name returns a shortened version of the fully qualified `name`,
+// i.e. `xyz.def.abc.symname` -> `abc.symname`
 pub fn short_module_name(name string) string {
 	if !name.contains('.') {
 		return name
@@ -93,3 +109,10 @@ pub fn short_module_name(name string) string {
 	symname := vals[vals.len - 1]
 	return '${mname}.$symname'
 }
+
+// highlight_suggestion returns a colorfull/highlighted version of `message`,
+// but only if the standart error output allows for color messages, otherwise
+// the plain message will be returned.
+pub fn highlight_suggestion(message string) string {
+	return term.ecolorize(term.bright_blue, message)
+}