cargo init with existing lib source file with hyphen in name makes bad Cargo.toml

[BartMassey]

Problem

(That is an ugly title. Improvements welcome.)

Background

cargo init has an obscure but convenient feature for small programs. Consider the following:

$ mkdir small
$ cd small
$ touch small.rs
$ cargo init --lib

This will produce a Cargo.toml that looks like

[package]
name = "small"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]

[lib]
name = "small"
path = "small.rs"

It will not produce src/lib.rs in this case: everything is self-contained in a single directory. I find this quite convenient.

(Note that for this to work, the directory name and the source file name have to be spelled the same.)

Hyphens

OK, let's try this with a different name.

$ mkdir dashed-name
$ cd dashed-name
$ touch dashed-name.rs
$ cargo init --lib

We get a warning message this time:

warning: compiling this new package may not work due to invalid workspace configuration

failed to parse manifest at `/tmp/dashed-name/./Cargo.toml`

Caused by:
  library target names cannot contain hyphens: dashed-name
     Created library package

Sure enough, the generated Cargo.toml now looks like this.

[package]
name = "dashed-name"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]

[lib]
name = "dashed-name"
path = "dashed-name.rs"

And sure enough, trying to build using this will fail.

error: failed to parse manifest at `/tmp/dashed-name/Cargo.toml`

Caused by:
  library target names cannot contain hyphens: dashed-name

Steps

See above.

Possible Solution(s)

In order of preference:

1. Omit lib.name from the generated declaration. It is not needed for libs anyhow. This will cause the hyphens to be defaulted to underscores when deriving lib.name from package.name.

2. Replace hyphens with underscores when generating lib.name.

3. Just get rid of this whole defaulted-source feature of cargo init. I really like it, and would prefer that it stay, but it may be more trouble than it's worth.

Notes

Vaguely relevant issues: #2775, #6827, #9333 (fixed, closed).

This issue does not affect cargo init --bin, since hyphens are allowed in bin.name. One could also not generate bin.name in the case that there is only one bin in the generated Cargo.toml, but that's a separate issue.

Version

cargo 1.64.0 (387270bc7 2022-09-16)
release: 1.64.0
commit-hash: 387270bc7f446d17869c7f208207c73231d6a252
commit-date: 2022-09-16
host: x86_64-unknown-linux-gnu
libgit2: 1.4.2 (sys:0.14.2 vendored)
libcurl: 7.83.1-DEV (sys:0.4.55+curl-7.83.1 vendored ssl:OpenSSL/1.1.1q)
os: Debian n/a [64-bit]