transformer: disable generic str_intp opt (fix #25896) (#25897)

Author: kbkpbot

vlib/v/tests/generics/generics_str_intp_test.v

@@ -0,0 +1,27 @@
+module main
+
+import strings
+
+pub struct MyStruct[T] {
+pub mut:
+	result T
+}
+
+pub fn (it MyStruct[T]) indent_str[T]() string {
+	mut res := strings.new_builder(32)
+	res.write_string('${it.result}')
+	return res.str()
+}
+
+fn test_generics_str_intp() {
+	x := MyStruct[int]{
+		result: 100
+	}
+
+	y := MyStruct[string]{
+		result: 'hello'
+	}
+
+	assert x.indent_str() == '100'
+	assert y.indent_str() == 'hello'
+}

vlib/v/transformer/transformer.v

@@ -1268,6 +1268,11 @@ pub fn (mut t Transformer) simplify_nested_interpolation_in_sb(mut onode ast.Stm
 		return false
 	}
 	original := nexpr.args[0].expr as ast.StringInterLiteral
+	if original.exprs.len != original.expr_types.len {
+		// This should be a generic type, e.g., `${it}` where `it` is type of T
+		// first time, `T` maybe `int`, but second time, `T` maybe `string`
+		return false
+	}
 	// only very simple string interpolations, without any formatting, like the following examples
 	// can be optimised to a list of simpler string builder calls, instead of using str_intp:
 	// >> sb.write_string('abc ${num}')