In 2018 edition, warn about cargo installing the current crate

[matklad]

Running cargo install installs the binaries for the project in current working directory. This might be a surprising behavior for people coming from other ecosystems, especially the node one. The confusion here is that, in Cargo, you don't need to "install" a library to actually use it. At the same time, installing local crates seems like a niche use-case, one typically installs crates from crates.io.

So let's fix confusion by printing a warning, in the next edition, when a user invokes cargo install. The warning should say:

• If you care coming from other ecosystems, you probably have meant cargo build
• If you really want to install current package, run cargo install --path .

The code for install command is here:

cargo/src/bin/commands/install.rs

Line 81 in d8b2031

To check the current edition, one can use this method:

cargo/src/cargo/core/manifest.rs

Line 409 in d8b2031

pub fn edition(&self) -> Edition {

The tests for install are here:

https://github.com/rust-lang/cargo/blob/d8b20310217e5e2bc515c13111ab9a7789709e42/tests/testsuite/install.rs

[dwijnand]

I'll take a crack at this.

[dwijnand]

@matklad how should the different behaviour when varying rust editions be tested? I can't find any prior art in https://github.com/rust-lang/cargo/issues?utf8=%E2%9C%93&q=sort%3Aupdated-desc+2018

[matklad]

@dwijnand here's a example of a test with an edition: bbb2b92#diff-851ff422b9a34b41d197ada1851b7fbbR1469