Passing rustc compilation flags as arguments to the cargo build command line.

[ptal]

A typical use-case is with the --explain XXX compilation flags. Currently we need to use cargo build -v, copy the compilation line and add this flag by hand.

This could be solved by adding an option, let's say, --rustc-flags to the cargo build sub-command.

[steveklabnik]

This has been suggested and rejected numerous times https://github.com/rust-lang/cargo/search?q=compiler+flags&type=Issues&utf8=%E2%9C%93

[alexcrichton]

Yes this is a dupe of #544 and #595

[ptal]

OK. Suppose that --explain E0001 gives contextual information about why a pattern is unreachable, what would be the preferred way to add it to cargo?