"cargo install" should be robust against `deny(warnings)`

[RalfJung]

More than once have I encountered issues where I wanted to cargo install a crate that unfortunately had #![deny(warnings)] among its attributes. This is generally considered a bad thing for crates to do, but sadly that does not stop people from doing it. Now I have to wait until upstream fixes their code or fiddle with RUSTFLAGS=-Asomething which will also rebuild all dependencies, it's all not very nice.

As of right now, one example of a command that fails due to this issue is

cargo +stable install --git https://github.com/rust-lang/measureme --branch stable summarize

When building dependencies, cargo uses --cap-lints to make this a non-issue. However, when installing a crate from crates.io or from a git repo, then as far as I am concerned even the binary crate I am installing is a "dependency" -- it is not code I wrote or can do much about. It would be nice if cargo could treat it as such and apply --cap-lints. (I don't care whether it caps the lints to "allow" or "warn" as long as installation isn't blocked by a misplaced #![deny(warnings)].)

Related issues:

• cargo install should use --cap-lints #3453 seems to be the exact same issue and was closed as having been implemented. But I do see cargo install failing due to #![deny(warnings)] today so either that did not work or it did get reverted.

[epage]

#3453 links to #3221 but not seeing what it was that fixed it in that PR.

In #17107, I added cap-lint tests for cargo install but skipped git because of this bug as well as identified the root cause: #17107 (comment)

Note: there is an existing bug in cargo install --git where we download the git source and then load it as a path source so it is considered local and rustc and cargo lints are shown.

See

cargo/src/cargo/ops/cargo_install.rs

Lines 925 to 926 in d3b55f8

	let mut ws = if source_id.is_git() || source_id.is_path() {
	Workspace::new(pkg.manifest_path(), gctx)?

As weihang pointed out, #13259 might be another symptom of this root cause.