Benches: improved user experience for adding new benchmarking files

[Walther]

Currently, creating benchmarks for a crate looks something like this:

# Project structure
.
├── Cargo.toml
├── README.md
├── benches
│   ├── various.rs
│   ├── file.rs
│   ├── names.rs
│   └── here.rs
├── src
│   ├── various.rs
│   ├── file.rs
│   ├── names.rs
│   └── here.rs
# Mirroring the src/ file names in benches/ makes sense, for easy discoverability
...

# Cargo.toml

[dev-dependencies]
divan = "0.1.11"

[[bench]]
name = "various"
harness = false

[[bench]]
name = "file"
harness = false

[[bench]]
name = "names"
harness = false

[[bench]]
name = "here"
harness = false

# However, here it feels like a chore

This gets repetitive very quickly, and doesn't feel like the ideal developer experience.

Would it be possible to consider:

• automatically detecting the files under benches/ so that the [[bench]] sections aren't required
• setting harness = false by default and letting people override it to harness = true when needed

in order to simplify the process of adding new benchmarks into just adding a new file to the benches/ directory?

---

A related issue and comment for historical context: rust-lang/rust#29553 (comment)

[nvzqz]

Perhaps if a main entrypoint is detected, then harness = false becomes set? Detecting main could also remove the need to list every benchmark in Cargo.toml.

[epage]

The [[bench]]s are needed only because of harness = false. if you used the unstable harness, auto-discovery would work.

I can't remember if cargo already has an issue for "defaults for target kinds" but that is how I've been talking about solving this.

Either way, this is a cargo issue, so moving it (once I'm off mobile).

[epage]

Closing as a duplicate of #6945. If there is a reason we should keep these separate, let us know!