Linking modifiers for native libraries

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+- Feature Name: `native_link_modifiers`
+- Start Date: 2020-06-12
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Provide an extensible mechanism for tweaking linking behavior of native libraries
+both in `#[link]` attributes (`#[link(modifiers = "+foo,-bar")]`)
+and on command line (`-l static:+foo,-bar=mylib`).
+
+# Motivation
+[motivation]: #motivation
+
+Occasionally some tweaks to linking behavior of native libraries are necessary,
+and currently there's no way to apply them.
+
+For example, some static libraries may need to be linked as a whole archive
+without throwing any object files away because some symbols in them appear to be unused
+while actually having some effect. \
+This RFC introduces modifier `whole-archive` to address this.
+
+In other cases we need to link to a library located at some specific path
+or not matching the default naming conventions. \
+This RFC introduces modifier `verbatim` to pass such libraries to the linker.
+
+This RFC also reformulates the `static-nobundle` linking kind as a modifier `bundle`
+thus providing an opportunity to change the static linking default to non-bundling
+on some future edition boundary, and hopefully unblocking its stabilization.
+
+The generic syntax provides a way to add more such modifiers in the future
+without introducing new linking kinds.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This is an advanced feature not expected to be used commonly,
+see the reference-level explanation if you know that you need some of these modifiers.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+## Existing syntax of linking attributes and options
+
+- Attributes: `#[link(name = "string", kind = "string", cfg(predicate))]`
+(some components are optional.)
+- Command line options: `-l kind=name:rename` (some components are optional).
+
+## Proposed extensions to the syntax
+
+- Attributes: `#[link(/* same */, modifiers = "+foo,-bar")]`.
+- Command line options: `-l kind:+foo,-bar=name:rename`.
+
+The modifiers are boolean and applied only to the single library specified with `name`. \
+`+` means enable, `-` means disable, multiple options can be separated by commas,
+the last boolean value specified for the given modifier wins. \
+The notation is borrowed from
+[target features](https://doc.rust-lang.org/rustc/codegen-options/index.html#target-feature)
+in general and should have the same semantics.
+
+If the `:rename` component is specified on the command line, then in addition to the name
+and linking kind the modifiers will be updated as well (using concatenation).
+
+## Specific modifiers
+
+### `bundle`
+
+Only compatible with the `static` linking kind.
+
+`+bundle` means objects from the static library are bundled into the produced crate
+(a rlib, for example) and are used from this crate later during linking of the final binary.
+
+`-bundle` means the static library is included into the produced rlib "by name"
+and object files from it are included only during linking of the final binary,
+the file search by that name is also performed during final linking.
+
+This modifier is supposed to supersede the `static-nobundle` linking kind defined by
+[RFC 1717](https://github.com/rust-lang/rfcs/pull/1717).
+
+The default for this modifier is currently `+bundle`,
+but it could be changed later on some future edition boundary.
+
+### `verbatim`
+
+`+verbatim` means that `rustc` won't add any target-specified library prefixes or suffixes
+(like `lib` or `.a`) to the library name.
+
+For `ld`-like linkers `rustc` will use the `-l:filename` syntax (note the colon)
+when passing the library, so the linker won't add any prefixes or suffixes as well. \
+See [`-l namespec`](https://sourceware.org/binutils/docs/ld/Options.html) in `ld` documentation
+for more details.
+
+The default for this modifier is `-verbatim`.
+
+This RFC changes the behavior of `raw-dylib` linking kind specified by
+[RFC 2627](https://github.com/rust-lang/rfcs/pull/2627).
+The `.dll` suffix (or other target-specified suffixes for other targets)
+is now added automatically. \
+If your DLL doesn't have the `.dll` suffix, it can be specified with `+verbatim`.
+
+### `whole-archive`
+
+Only compatible with the `static` linking kind.
+
+`+whole-archive` means that the static library is linked as a whole archive
+without throwing any object files away.
+
+This modifier translates to `--whole-archive` for `ld`-like linkers,
+and to `/WHOLEARCHIVE` for `link.exe`.
+
+The default for this modifier is `-whole-archive`.
+
+### `as-needed`
+
+Only compatible with the `dynamic` linking kind.
+
+`+as-needed` means that the library will be actually linked only if it satisfies some
+undefined symbols at the point at which it is specified on the command line,
+making it similar to static libraries in this regard.
+
+This modifier translates to `--as-needed` for `ld`-like linkers.
+
+The default for this modifier is unclear, some targets currently specify it as `+as-needed`,
+some do not. We may want to try making `+as-needed` a default for all targets.
+
+## Stability story
+
+The modifier syntax can be stabilized independently from any specific modifiers.
+
+All the specific modifiers start unstable and can be stabilized independently from each other
+given enough demand.
+
+## Relative order of `-l` and `-Clink-arg(s)` options
+
+This RFC also proposes to guarantee that the relative order of `-l` and `-Clink-arg(s)`
+command line options of `rustc` is preserved when passing them to linker. \
+(Currently they are passed independently and the order is not guaranteed.)
+
+This provides ability to tweak linking of individual libraries on the command line
+by using raw linker options. \
+An equivalent of order-preserving `-Clink-arg`, but in an attribute form,
+is not provided at this time.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Some extra complexity in parsing the modifiers
+and converting them into a form suitable for the linker.
+
+Not all modifiers are applicable to all targets and linkers,
+but that's true for many existing `-C` options as well.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+
+## Alternative: rely on raw linker options
+
+The primary alternative for the (relatively cross-platform) `whole-archive` and `as-needed`
+modifiers is to rely on more target-specific raw linker options more.
+
+(Note, that raw linker options don't cover the `bundle` and `verbatim` modifiers
+that are `rustc`-specific.)
+
+The modifier support is removed from the command line options,
+the desired effect is achieved by something like this.
+```sh
+-Clink-arg=-Wl,--whole-archive -lfoo -Clink-arg=-Wl,--no-whole-archive
+```
+
+Note the `-Wl,` that is needed when using `gcc` as a linker,
+but not when using an `ld`-like linker directly.
+So this solution is not only more target-specific, but also more linker specific as well.
+
+The `-Wl,` part can potentially be added automatically though, there's some prior art from CMake
+regarding this, see the `LINKER:` modifier for
+[`target_link_options`](https://cmake.org/cmake/help/git-stage/command/target_link_options.html).
+
+Relying on raw linker options while linking with attributes will requires introducing
+a new attribute, see the paragraph about `#[link(arg = "string")]` in "Future possibilities".
+
+# Prior art
+[prior-art]: #prior-art
+
+`gcc` provides the `-Wl,foo` command line syntax (and some other similar options) for passing
+arbitrary options directly to the linker.
+
+The relative order of `-Wl` options and `-l` options linking the libraries is preserved.
+
+`cl.exe` provides `/link link-opts` for passing options directly to the linker,
+but the options supported by `link.exe` are generally order-independent,
+so it is not as relevant to modifying behavior of specific libraries as with `ld`-like linkers.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+What about the `framework` linking kind? What does it mean? What modifiers is it compatible with?
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+## New modifiers
+
+### `dedup`
+
+`rustc` doesn't currently deduplicate linked libraries
+[in general](https://github.com/rust-lang/rust/issues/73319).
+
+The reason is that *sometimes* the linked libraries need to be duplicated on the command line.
+
+However, such cases are rare and we may want to deduplicate the libraries by default,
+but provide the `-dedup` modifier as an opt-out for these rare cases.
+
+Introducing the `dedup` modifier with the current `-dedup` default doesn't make much sense.
+
+## Support `#[link(arg = "string")]` in addition to the modifiers
+
+`ld` supports some other niche per-library options, for example `--copy-dt-needed-entries`.
+
+`ld` also supports order-dependent options like `--start-group`/`--end-group`
+applying to groups of libraries.
+
+We may want to avoid new modifiers for all possible cases like this and provide an order-preserving
+analogue of `-C link-arg`, but in the attribute form. \
+It may also resolve issues with the existing unstable attribute
+[`#[link_args]`](https://github.com/rust-lang/rust/issues/29596)
+and serve as its replacement.
+
+Some analogue of
+[CMake's `LINKER:`](https://cmake.org/cmake/help/git-stage/command/target_link_options.html)
+mentioned above can improve portability here.