vfmt: fix formatting of submodules with common prefixes (fix #15582)

vlib/v/ast/str.v

@@ -164,12 +164,42 @@ fn stringify_fn_after_name(node &FnDecl, mut f strings.Builder, t &Table, cur_mo
 	}
 	f.write_string(')')
 	if node.return_type != void_type {
-		mut rs := util.no_cur_mod(t.type_to_str(node.return_type), cur_mod)
+		sreturn_type := util.no_cur_mod(t.type_to_str(node.return_type), cur_mod)
+		short_sreturn_type := shorten_full_name_based_on_aliases(sreturn_type, m2a)
+		f.write_string(' ' + short_sreturn_type)
+	}
+}
+
+struct StringifyModReplacement {
+	mod    string
+	alias  string
+	weight int
+}
+
+fn shorten_full_name_based_on_aliases(input string, m2a map[string]string) string {
+	// Shorten the full names to their aliases, but replace the longer mods first, so that:
+	//   `import user.project`
+	//   `import user.project.routes`
+	// will lead to replacing `user.project.routes` first to `routes`, NOT `user.project.routes` to `project.routes`.
+	// Also take into account the nesting level, so `a.e.c.d` will be shortened before `a.xyz.b`, even though they are the same length.
+	mut replacements := []StringifyModReplacement{cap: m2a.len}
 	for mod, alias in m2a {
-			rs = rs.replace(mod, alias)
+		if input.contains(mod) {
+			replacements << StringifyModReplacement{
+				mod: mod
+				alias: alias
+				weight: mod.count('.') * 100 + mod.len
 			}
-		f.write_string(' ' + rs)
 		}
+	}
+	mut res := input.clone()
+	if replacements.len > 0 {
+		replacements.sort(a.weight > b.weight)
+		for r in replacements {
+			res = res.replace(r.mod, r.alias)
+		}
+	}
+	return res
 }
 
 // Expressions in string interpolations may have to be put in braces if they

vlib/v/fmt/tests/shorten_longer_module_paths_first_keep.vv

@@ -0,0 +1,16 @@
+module app
+
+import user.project
+import user.project.routes
+
+pub fn new_app() ?project.Application {
+	return project.Application{
+		data: get_router()!.route_name
+	}
+}
+
+fn get_router() ?routes.Router {
+	return routes.Router{
+		route_name: 'Root'
+	}
+}