checker: improve errors with compile-time field access (#8373)

vlib/v/checker/checker.v

@@ -3377,13 +3377,19 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 					node.field_expr.position())
 			}
 			if node.field_expr is ast.SelectorExpr {
+				left_pos := node.field_expr.expr.position()
+				if c.comptime_fields_type.len == 0 {
+					c.error('compile time field access can only be used when iterating over `T.fields`',
+						left_pos)
+				}
 				expr_name := node.field_expr.expr.str()
 				if expr_name in c.comptime_fields_type {
 					return c.comptime_fields_type[expr_name]
 				}
+				c.error('unknown `\$for` variable `$expr_name`', left_pos)
+			} else {
+				c.error('expected selector expression e.g. `$(field.name)`', node.field_expr.position())
 			}
-			c.error('compile time field access can only be used when iterating over `T.fields`',
-				node.field_expr.position())
 			return table.void_type
 		}
 		ast.ConcatExpr {
@@ -4458,12 +4464,14 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 		}
 		if node.is_comptime { // Skip checking if needed
 			// smartcast field type on comptime if
+			mut comptime_field_name := ''
 			if branch.cond is ast.InfixExpr {
 				if branch.cond.op == .key_is {
 					left := branch.cond.left
 					got_type := (branch.cond.right as ast.Type).typ
 					if left is ast.SelectorExpr {
-						c.comptime_fields_type[left.expr.str()] = got_type
+						comptime_field_name = left.expr.str()
+						c.comptime_fields_type[comptime_field_name] = got_type
 						is_comptime_type_is_expr = true
 					} else if left is ast.Type {
 						is_comptime_type_is_expr = true
@@ -4488,6 +4496,9 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 			} else if !is_comptime_type_is_expr {
 				node.branches[i].stmts = []
 			}
+			if comptime_field_name.len > 0 {
+				c.comptime_fields_type.delete(comptime_field_name)
+			}
 			c.skip_flags = cur_skip_flags
 		} else {
 			c.stmts(branch.stmts)

vlib/v/checker/tests/comptime_field_selector_not_in_for_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/comptime_field_selector_not_in_for_err.vv:9:5: error: compile time field access can only be used when iterating over `T.fields`
+vlib/v/checker/tests/comptime_field_selector_not_in_for_err.vv:9:5: error: expected selector expression e.g. `$(field.name)`
     7 |     mut t := T{}
     8 |     name := 'test'
     9 |     t.$name = '3'

vlib/v/checker/tests/comptime_field_selector_not_name_err.out

@@ -3,5 +3,19 @@ vlib/v/checker/tests/comptime_field_selector_not_name_err.vv:10:7: error: expect
     9 |         $if f.typ is string {
    10 |             t.$f = '3'
       |                ^
-   11 |         }
-   12 |     }
+   11 |             fv := Foo{}
+   12 |             _ = t.$fv.name
+vlib/v/checker/tests/comptime_field_selector_not_name_err.vv:12:11: error: unknown `$for` variable `fv`
+   10 |             t.$f = '3'
+   11 |             fv := Foo{}
+   12 |             _ = t.$fv.name
+      |                    ~~
+   13 |         }
+   14 |     }
+vlib/v/checker/tests/comptime_field_selector_not_name_err.vv:15:9: error: compile time field access can only be used when iterating over `T.fields`
+   13 |         }
+   14 |     }
+   15 |     _ = t.$f.name
+      |            ^
+   16 | }
+   17 |

vlib/v/checker/tests/comptime_field_selector_not_name_err.vv

@@ -8,8 +8,11 @@ fn test<T>() {
 	$for f in T.fields {
 		$if f.typ is string {
 			t.$f = '3'
+			fv := Foo{}
+			_ = t.$fv.name
 		}
 	}
+	_ = t.$f.name
 }
 
 fn main() {