Expansion error in pest-derive with tags

[veeenu]

Describe the bug
On Rust 1.70 and 1.71, when using tags, the derive macro errors out with the following:

error[E0433]: failed to resolve: use of undeclared crate or module `alloc`
 --> src/lib.rs:3:10
  |
3 | #[derive(Parser)]
  |          ^^^^^^ use of undeclared crate or module `alloc`
  |
  = help: add `extern crate alloc` to use the `alloc` crate
  = help: consider importing this enum:
          std::borrow::Cow
  = note: this error originates in the derive macro `Parser` (in Nightly builds, run with -Z macro-backtrace for more info)

To Reproduce
Here is a minimum reproducible example:

# Cargo.toml
[package]
name = "pest-issue"
version = "0.1.0"
edition = "2021"

[dependencies]
pest = "2.7.1"
pest_derive = { version = "2.7.1", features = ["std", "grammar-extras"] }

program = { SOI ~ rule* ~ EOI }
rule = !{ "{" ~ (item ~ ";")* ~ (#tail_item = item)? ~ "}" }
item = _{ "foo" | "bar" }

// lib.rs
use pest_derive::Parser;

#[derive(Parser)]
#[grammar = "grammar.pest"]
struct ExampleParser;

Expected behavior
The generated code should not emit the error.

Additional context

• Adding extern crate alloc at the top of lib.rs makes the error go away -- it is a feasible workaround, but it should probably not be asked of the users to do that.
• This is the only snippet that appears to reference alloc in the expanded code:

  state
      .optional(|state| {
          self::item(state)
              .and_then(|state| {
                  state.tag_node(alloc::borrow::Cow::Borrowed("tail_item"))
              })
      })

• I found this. I haven't had time to try it myself yet, but could prepending :: so that we have ::alloc be a potential solution?

Thank you for all your work on this library!

[tomtau]

haven't had time to try it myself yet, but could prepending :: so that we have ::alloc be a potential solution?

Possibly, that will resolve it from the crate root, so that should normally work with default features?

[veeenu]

Prepending the :: did not work. However, changing the Cow path conditional on the std feature flag being active works. HTH!