Enable link-arg link kind inside of #[link] attribute

[zetanumbers]

#99427 (comment)

...
This would help to make link-arg usable in #[link] attributes and e.g. wrap libc and libgcc into a group (*) in the libc crate like

#[link(kind = "link-arg", name = "--start-group")]
#[link(kind = "static", name = "c")]
#[link(kind = "static", name = "gcc")]
#[link(kind = "link-arg", name = "--end-group")]

(*) to address cyclic dependencies between them

This is an analogue of CMake's LINKER: prefix (https://cmake.org/cmake/help/git-stage/command/target_link_options.html#handling-compiler-driver-differences), and was discussed as a possible future extension in the link modifier RFC (https://github.com/rust-lang/rfcs/blob/master/text/2951-native-link-modifiers.md#support-linkarg--string-in-addition-to-the-modifiers).

[rustbot]

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[petrochenkov]

r? @petrochenkov

[zetanumbers]

Add unstable book entry

[bjorn3]

I thought we had this ability in the past but removed it because it depends on the exact linker that is used.

[petrochenkov]

@bjorn3
The previous version (#[link_args]) was removed in #83820.
It was different than this one and more similar to the -C link-arg flag - its relative order with -l flags and #[link] attribute wasn't respected, an when used in a rlib it wasn't recorded in rlib metadata and wasn't later used during the final link.

This version is an attribute version of -l link-arg, so the order is respected, and it's preserved in metadata, and it otherwise behaves like a native library.
This new setup was described as a future possibility and potential replacement for #[link_args] in https://github.com/rust-lang/rfcs/blob/master/text/2951-native-link-modifiers.md#support-linkarg--string-in-addition-to-the-modifiers.

depends on the exact linker that is used

The portability here is relatively good because -Wl, is added automatically if the current linker flavor requires it (-verbatim is the default).
There are also some relevant docs at enum LinkerFlavor - basically in practice for each given target there is a basic linker flavor with its own interface and two "modifiers" cc and lld that can be added to it. lld doesn't change the interface, and cc requires adding -Wl, which can be done automatically, so use cases like --(start,end)-group in #99427 (comment) should work.

[petrochenkov]

r=me after squashing commits into one.
@rustbot author

[zetanumbers]

@bors r=petrochenkov

[bors]

@zetanumbers: 🔑 Insufficient privileges: Not in reviewers

[petrochenkov]

@bors r+

[bors]

📌 Commit 08bfe49 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 08bfe49 with merge 25432b7...

[petrochenkov]

@bors r+

[bors]

📌 Commit f7617c1 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit f7617c1 with merge 0079625...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 0079625 to master...

[rust-timer]

Finished benchmarking commit (0079625): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.3%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

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)	-1.6%	[-1.6%, -1.6%]	1
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: 672.048s -> 673.226s (0.18%)
Artifact size: 313.42 MiB -> 313.37 MiB (-0.01%)