Tracking Issue `--extern` crate modifiers

[petrochenkov]

This is a tracking issue for command line option syntax currently used by several modifiers affecting behavior of extern crates.
Specific tracking issues for those modifiers:

• Tracking issue for --extern priv #98399
• Tracking issue for --extern noprelude #98398
• Tracking issue for --extern nounused #98400
• Tracking Issue for --extern force #111302

The currently used syntax is

--extern [MODIFIERS:]NAME[=PATH]

Before stabilization it should likely be harmonized with command line syntax already used for native library modifiers, see item (3) in #93901 (comment).

• Modifiers can be converted to +/- form
  • priv - +priv or -pub
  • noprelude - -prelude
  • nounused - +used
• Position of the MODIFIERS component in the option can also be changed.

[jsgf]

Before stabilization it should likely be harmonized with command line syntax already used for native library modifiers, see item (3) in #93901 (comment).
Modifiers can be converted to +/- form

I don't really see the relationship between #[link] and --extern flag:. The latter controls how a Rust dependency is handled/consumed by this particular rustc invocation, whereas the former controls how an external non-Rust dependency is finally linked.

(I also think the whole #[link] mechanism is a bad idea and should be deprecated, not extended. Controlling how things are ultimately linked is the responsibility of the build system, and so the functionality should be moved to Cargo.)

[ehuss]

I don't think that is referring to #[link], just that it should use the same +/- syntax of the -l flag, like --extern +prev,-prelude:foo=libfoo.rlib or possibly rearranged to --extern foo:+prev,-prelude=libfoo.rlib