E4215

Compiler diagnostic name: expect_template_string.

This diagnostic described a non-template right-hand side of the <+ operator and is not emitted by the current compiler. The parser now rejects this syntax with E3002.

The <+ template-writing operator writes a string template to the value on its left. The right-hand side must be a string literal, an interpolated string, or a multiline string template. Other expressions cannot be expanded as template content.

Erroneous example

pub fn render(value : String) -> String {
  let builder = StringBuilder()
  builder <+ value
  builder.to_string()
}

The right-hand side is a variable reference, not a template string.

Suggestion

Use a string template on the right-hand side.

pub fn render(value : String) -> String {
  let builder = StringBuilder()
  builder <+ "\{value}"
  builder.to_string()
}