Support linking to rust dylib with --crate-type staticlib

[bjorn3]

This allows for example dynamically linking libstd, while statically linking the user crate into an executable or C dynamic library. For this two unstable flags (-Z staticlib-allow-rdylib-deps and -Z staticlib-prefer-dynamic) are introduced. Without the former you get an error. The latter is the equivalent to -C prefer-dynamic for the staticlib crate type to indicate that dynamically linking is preferred when both options are available, like for libstd. Care must be taken to ensure that no crate ends up being merged into two distinct staticlibs that are linked together. Doing so will cause a linker error at best and undefined behavior at worst. In addition two distinct staticlibs compiled by different rustc may not be combined under any circumstances due to some rustc private symbols not being mangled.

To successfully link a staticlib, --print native-static-libs can be used while compiling to ask rustc for the linker flags necessary when linking the staticlib. This is an existing flag which previously only listed native libraries. It has been extended to list rust dylibs too. Trying to locate libstd yourself to link against it is not supported and may break if for example the libstd of multiple rustc versions are put in the same directory.

For an example on how to use this see the src/test/run-make-fulldeps/staticlib-dylib-linkage/ test.

[rustbot]

r? @nagisa

(rustbot has picked a reviewer for you, use r? to override)

[nagisa]

r? rust-lang/compiler

(I would've assigned petrochenkov, but IIRC they're busy too)

[Noratrieb]

r? compiler

[eholk]

r? compiler

[eholk]

r? compiler

(trying again since @Nilstrieb already rerolled)

[jackh726]

Okay clearly we're just playing spin the bottle for review here. So let me nominate to make sure we can find the right person in a meeting.

[pnkfelix]

self-assigning for both review and for bring up at T-lang meeting to find out if they want to weigh in on this feature.

r? @pnkfelix

@rustbot label: +I-lang-nominated -I-compiler-nominated

[joshtriplett]

I feel like this isn't a lang issue at all.

[joshtriplett]

We discussed this in today's @rust-lang/lang meeting.

We agreed that we're comfortable with T-compiler handling this issue.

We also felt like, in general, we'd love to have some kind of Rust team whose purview directly encompasses linking and similar issues like this.

[bjorn3]

Looks like the test doesn't work when cross-compiling. Disabled it for cross compiling just like a similar test for cdylib.

[bjorn3]

@bors r=pnkfelix

The only change since the last review is a new ignore-cross-compile directive for a test that needs it.

[bors]

📌 Commit 47be060 has been approved by pnkfelix

It is now in the queue for this repository.

[bors]

⌛ Testing commit 47be060 with merge d55a160f23727197173156b4dcd1b17712340a6c...

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

error: failed to push some refs to 'https://github.com/rust-lang-nursery/rust-toolstate'
Sleeping for 3 seconds before retrying push
From https://github.com/rust-lang-nursery/rust-toolstate
 * branch            master     -> FETCH_HEAD
thread 'main' panicked at 'Failed to update toolstate repository with new data', toolstate.rs:436:9
HEAD is now at 839b1a6 (windows CI update)
Build completed unsuccessfully in 0:00:53

[bors]

💔 Test failed - checks-actions

[Dylan-DPC]

@bors retry
@bors treeclosed=1000

[bors]

⌛ Testing commit 47be060 with merge 63fc57b...

[bors]

☀️ Test successful - checks-actions
Approved by: pnkfelix
Pushing 63fc57b to master...

[rust-timer]

Finished benchmarking commit (63fc57b): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.5%, -1.5%]	2
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 657.481s -> 657.842s (0.05%)