Expand trybuild-like for tests missing in cargo metadata

[Robbepop]

Original idea: dtolnay/trybuild#35
Suggested approach: dtolnay/trybuild#35 (comment)

Solution Summary

cargo expand should be able to expand the code of a test file somename.rs in the tests directory of a given crate via --test somename even though somename.rs exists but "somename" is not part of the test targets listed for the given crate by cargo-metadata.

Technical Details

The expansion should be done in a similar way as the trybuild crate handles building and testing of its tests:

• pull out the code under test (e.g. our somename.rs) into a temp. directory
• run rustc against that
  • just like trybuild would
  • using --pretty=expanded instead of building or testing anything

[tamuratak]

I have figured out how to deal the issue. Let's take proc-macro-workshop/builder as an example:

• https://github.com/tamuratak/proc-macro-workshop/tree/use_examples_dir/builder

1. Make an examples directory and move test codes to the directory.

Cargo.toml
examples/
 01-parse.rs
 02-create-builder.rs
 03-call-setters.rs
 04-call-build.rs 
 05-method-chaining.rs
 06-optional-field.rs
 07-repeated-field.rs
 08-unrecognized-attribute.rs
 08-unrecognized-attribute.stderr
 09-redefined-prelude-types.rs
src/
 lib.rs
tests/
 progress.rs

2. Edit tests/progress.rs as follows:

#[test]
fn tests() {
    let t = trybuild::TestCases::new();
    t.pass("examples/01-parse.rs");
    //t.pass("examples/02-create-builder.rs");
    //t.pass("examples/03-call-setters.rs");
    //t.pass("examples/04-call-build.rs");
    //t.pass("examples/05-method-chaining.rs");
    //t.pass("examples/06-optional-field.rs");
    //t.pass("examples/07-repeated-field.rs");
    //t.compile_fail("examples/08-unrecognized-attribute.rs");
    //t.pass("examples/09-redefined-prelude-types.rs");
}

3. Execute cargo expand --example 01-parse. We can expand examples/01-parse.rs. We can also test the lib with cargo test --tests.

$  cargo expand --example 01-parse
    Checking derive_builder v0.0.0 (/Users/tamura/src/github/proc-macro-workshop/builder)
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use derive_builder::Builder;
pub struct Command {
    executable: String,
    args: Vec<String>,
    env: Vec<String>,
    current_dir: String,
}
fn main() {}