musl: add musl-static and musl-dynamic aliases

[Gankra]

As noted elsewhere, the Rust team is planning to change the meaning of *-unknown-linux-musl from "+crt-static" to "-crt-static". This fixes a longstanding issue with the musl target, which should ideally default to the proper semantics for musl-libc distros like Alpine.

However this is a breaking change, and lots of people do want the static/portable binary solution that the current behaviour provides. We should introduce two new "magic" targets that cargo-dist desugars:

• *-musl-static desugars to *-musl with the -Ctarget-feature=+crt-static -Clink-self-contained=yes
• *-musl-dynamic desugars to *-musl with whatever the opposite of the above is

(plain *-musl would then just be "whatever rustc says today")

Some initial sketching/prep of this design has already been made for both the shell installer and npm installer (some random excerpts):

cargo-dist/cargo-dist/src/tasks.rs

Lines 1985 to 1990 in 1fe2cb9

	// Likewise, the default for musl will change in the future, so
	// we can future-proof this by adding the flag now
	// See: https://github.com/axodotdev/cargo-dist/issues/486
	if target.ends_with("linux-musl") {
	rustflags.push_str(" -Ctarget-feature=+crt-static -Clink-self-contained=yes");
	}

cargo-dist/cargo-dist/src/tasks.rs

Lines 1210 to 1240 in 1fe2cb9

	const X64_MACOS: &str = "x86_64-apple-darwin";
	const ARM64_MACOS: &str = "aarch64-apple-darwin";
	const X64_GNU: &str = "x86_64-unknown-linux-gnu";
	const X64_MUSL: &str = "x86_64-unknown-linux-musl";
	const X64_MUSL_STATIC: &str = "x86_64-unknown-linux-musl-static";
	const X64_MUSL_DYNAMIC: &str = "x86_64-unknown-linux-musl-dynamic";
	let mut has_x64_apple = false;
	let mut has_arm_apple = false;
	let mut has_gnu_linux = false;
	let mut has_static_musl_linux = false;
	// Currently always false, someday this build will exist
	let has_dynamic_musl_linux = false;
	for &variant_idx in &release.variants {
	let variant = self.variant(variant_idx);
	let target = &variant.target;
	if target == X64_MACOS {
	has_x64_apple = true;
	}
	if target == ARM64_MACOS {
	has_arm_apple = true;
	}
	if target == X64_GNU {
	has_gnu_linux = true;
	}
	if target == X64_MUSL {
	has_static_musl_linux = true;
	}
	}
	let do_rosetta_fallback = has_x64_apple && !has_arm_apple;
	let do_gnu_to_musl_fallback = !has_gnu_linux && has_static_musl_linux;
	let do_musl_to_musl_fallback = has_static_musl_linux && !has_dynamic_musl_linux;

cargo-dist/cargo-dist/templates/installer/installer.sh.j2

Lines 460 to 473 in e522919

	if ldd --version 2>&1 | grep -q 'musl'; then
	_clibtype="musl-dynamic"
	# glibc, but is it a compatible glibc?
	else
	# Parsing version out from line 1 like:
	# ldd (Ubuntu GLIBC 2.35-0ubuntu3.1) 2.35
	_local_glibc="$(ldd --version | head -1 | awk -F' ' '{ print $NF }')"
	if [ "$(echo "${_local_glibc}" | awk -F. '{ print $1 }')" = $BUILDER_GLIBC_MAJOR ] && [ "$(echo "${_local_glibc}" | awk -F. '{ print $2 }')" -ge $BUILDER_GLIBC_SERIES ]; then
	_clibtype="gnu"
	else
	_clibtype="musl-static"
	fi
	fi

In particular:

• both of those installers are aware of the concept of musl-static and musl-dynamic, and generate fallbacks for best-effort coverage of linux-gnu and linux-musl-dynamic (e.g. if there's no gnu build, say the musl-static build is a gnu build -- ditto for lack of musl-dynamic).
• both of those installers now internally do their analyses to select one of
  • linux-gnu (lld reports glibc with sufficient version)
  • linux-musl-dynamic (lld reports musl, no version checks yet)
  • linux-musl-static (either glibc is too old, or an unknown libc is reported)

The piece that's missing is:

• Us accepting linux-musl-static and linux-musl-dynamic as targets in [workspace.manifest.dist]
• Us desugarring those internally so that we pass the right flags/triple to cargo
• BUT keeping the sugar around in places like artifact names and the fallback/coverage logic in tasks.rs
• Also emit some metadata in dist-manifest.json indicating the crt-static-ness (may also want this for msvc targets?)

For this to work properly, work should also be done in:

• Oranda's artifact code to understand these sugars, and how to detect/select them in installer widgets
• axoproject's target triple listings / pretty-printing
• cargo-binstall's artifact name guessing, not sure where that is

Note also that we don't currently have a story for building musl-dynamic on github CI. We might need custom infra or docker images to get a host Alpine system or something?