parser: support const x := 123, to make extracting locals as constants less annoying while prototyping

vlib/v/fmt/tests/allow_const_with_decl_assign_expected.vv

@@ -0,0 +1,19 @@
+const a = 123
+
+const b = 'abc'
+
+const c = rune(123)
+
+const d = u64(2 * c)
+
+const f = func()
+
+fn func() int {
+	return 42
+}
+
+dump(a)
+dump(b)
+dump(c)
+dump(d)
+dump(f)

vlib/v/fmt/tests/allow_const_with_decl_assign_input.vv

@@ -0,0 +1,19 @@
+const a := 123
+
+const b := 'abc'
+
+const c := rune(123)
+
+const d := u64(2 * c)
+
+const f := func()
+
+fn func() int {
+	return 42
+}
+
+dump(a)
+dump(b)
+dump(c)
+dump(d)
+dump(f)

vlib/v/parser/parser.v

@@ -3713,10 +3713,13 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 		if p.tok.kind == .comma {
 			p.error_with_pos('const declaration do not support multiple assign yet', p.tok.pos())
 		}
+		// Allow for `const x := 123`, and for `const x = 123` too.
+		// Supporting `const x := 123` in addition to `const x = 123`, makes extracting local variables to constants much less annoying, while prototyping:
 		if p.tok.kind == .decl_assign {
-			p.error_with_pos('cannot use `:=` to declare a const, use `=` instead', p.tok.pos())
+			p.check(.decl_assign)
-		}
+		} else {
 			p.check(.assign)
+		}
 		end_comments << p.eat_comments()
 		if p.tok.kind == .key_fn {
 			p.error('const initializer fn literal is not a constant')

vlib/v/parser/tests/const_init_decl_assign_err.out

@@ -1,5 +0,0 @@
-vlib/v/parser/tests/const_init_decl_assign_err.vv:2:4: error: cannot use `:=` to declare a const, use `=` instead
-    1 | const (
-    2 |     a := 43
-      |       ~~
-    3 | )

vlib/v/parser/tests/const_init_decl_assign_err.vv

@@ -1,3 +0,0 @@
-const (
-	a := 43
-)