Explicit support for whole-archive linker option

[leeopop]

1. The problem

Some libraries using __attribute__((constructor)) suffers when their libraries are not linked via whole-archive linker option.
However, current cargo build script does not support this functionality.

2. Alternatives

We may use RUSTFLAGS option to give custom linker option, however I expect that this is not a desired use case of cargo.

whole-archive is not needed when dynamically loading a shared library.
(the loader explicitly invoke the constructor function)
(in static linking case, linker finds no explicit linkage so that the library is omitted)

3. Solution

Add another link type to cargo's link-lib list.
Currently, we have static, dylib, and framework.
As whole-archive works on static linking case, we may name it as static-whole.

e.g. cargo:rustc-link-lib=static-whole=foo at the build script.

I'm aware of the risk when the linker flag is fully open.
I expect that this is the reason why linker flags are not open to build scripts.
Thus, I suggest to explicit support a single type of linking: static-whole along with other specialized link types.

4. Implementation strategy

Cargo internally manages the static-whole database.
At the final linking state, cargo opens a whole-archive, listing all static-whole libraries from the depending crates, and closes whole-archive.

[leeopop]

ping @alexcrichton @ehuss

My previous writing is not that clear.
To be more specific, I would like to propose a concrete abstraction for the --whole-archive compiler option.

[leeopop]

Here is an example of possible workaround of this topic.
https://github.com/ANLAB-KAIST/rust-dpdk/blob/9773c2b61676bdf16b94c9d4c00660b998481779/Dockerfile#L44

However, I think it is better to have this feature as standard cargo options.

[dcsommer]

@ehuss I think this issue can be closed now that 53ec56c is in?

[ehuss]

Seems like it, thanks!

[dcsommer]

Just a note: there are still some issues with whole archive linking. However, the issues are on the rustc side, not cargo. See:

• Formal support for linking rlibs using a non-Rust linker rust#73632
• Linking modifier whole-archive and link time cfg are not supported on bundled static libraries rust#99429
• rustc --test ignores static:-whole-archive link modifiers rust#100066