So I've been working through the Rust book, and have been doing fine, until I got to the "Advanced Features" chapter. Is the book wrong, or have I missed something subtle? Example code below is taken from the "Advanced Lifetimes" section of chapter 19 of the Rust book.
Here's the code from an early example, using a single lifetime for Parser.
This does fail to compile as predicted, but the error I get is different from the book's.
/*
Expected: error[E0491]: in type `&'c Context<'s>`, reference has a longer
lifetime than the data it references
--> src/lib.rs:4:5
*/
/*
I get:
error[E0515]: cannot return value referencing function parameter `context`
--> src/main.rs:37:9
|
37 | Parser { context: &context }.parse()
| ^^^^^^^^^^^^^^^^^^--------^^^^^^^^^^
| | |
| | `context` is borrowed here
| returns a value referencing data owned by the current function
*/
fn main3() {
struct Context<'a>(&'a str);
struct Parser<'a> {
context: &'a Context<'a>,
}
impl<'a> Parser<'a> {
fn parse(&self) -> Result<(), &'a str> {
Err(&self.context.0[1..])
}
}
fn parse_context(context: Context) -> Result<(), &str> {
Parser { context: &context }.parse()
}
}
This is code from the example with two lifetimes, but no subtyping. This should create a compilation error according to the book, but in fact does not.
fn main2() {
struct Context<'s>(&'s str);
struct Parser<'c, 's> {
context: &'c Context<'s>,
}
impl<'c, 's> Parser<'c, 's> {
fn parse(&self) -> Result<(), &'s str> {
Err(&self.context.0[1..])
}
}
fn parse_context(context: Context) -> Result<(), &str> {
Parser { context: &context }.parse()
}
}
This is the final code example, with lifetime subtyping. It compiles fine, as expected.
#![allow(unused_variables, dead_code)]
fn main() {
struct Context<'s>(&'s str);
struct Parser<'c, 's: 'c> {
context: &'c Context<'s>,
}
impl<'c, 's> Parser<'c, 's> {
fn parse(&self) -> Result<(), &'s str> {
Err(&self.context.0[1..])
}
}
fn parse_context(context: Context) -> Result<(), &str> {
Parser { context: &context }.parse()
}
}
.