Rollup merge of rust-lang#104748 - lqd:download_lld, r=jyn514

Ensure `lld` is supported with `download-ci-llvm`

This PR:
- ensures LLD's step in bootstrap's dist, but it's not strictly necessary since dist will already package it when it's present.
- makes bootstrap's `native::LLD` step support using the packaged `ci-llvm/bin/lld`, instead of building it from source (which would most likely not be available today, nor in the future where `download-ci-llvm = if-available` is the default).

If I understand correctly, `--enable-full-tools` will also enable `rust.lld`, and this is why LLD is already packaged today in the `rust-dev` component on the main targets (and why `-Zgcc-ld=lld` does work there).

That means it's likely that this PR will not be able to land before I've reworked and landed rust-lang#101792: if LLD is available in `download-ci-llvm`, the `needs-rust-lld` tests should start being executed on the x64 macOS test builders, and CI would fail today.

I've tested locally that building with `download-ci-llvm = true` and `lld = true` with the LLVM submodule unregistered was successful, and that `rust-lld` and the various `lld-wrapper`s are present and `-Zgcc-ld=lld` works as well, on a few different platforms:
- `x86_64-unknown-linux-gnu`
- `aarch64-apple-darwin`
- `x86_64-pc-windows-msvc` (with `-Clinker=rust-lld` rather than `-Zgcc-ld=lld`)
- `x86_64-apple-darwin`, with the `MACOSX_DEPLOYMENT_TARGET` workaround for rust-lang#101653

I don't think we really need to bump the `download-ci-llvm-stamp` in this case, since `./build/$triple/ci-llvm/bin/lld` is present on all the above targets already, but have added it mechanically, and it should probably be removed to avoid unnecessary downloads/churn.

Fixes rust-lang#98340
Supersedes rust-lang#100010

src/bootstrap/dist.rs

@@ -2067,6 +2067,9 @@ impl Step for RustDev {
 
         builder.ensure(crate::native::Llvm { target });
 
+        // We want to package `lld` to use it with `download-ci-llvm`.
+        builder.ensure(crate::native::Lld { target });
+
         let src_bindir = builder.llvm_out(target).join("bin");
         // If updating this list, you likely want to change
         // src/bootstrap/download-ci-llvm-stamp as well, otherwise local users

src/bootstrap/download-ci-llvm-stamp

@@ -1,4 +1,4 @@
 Change this file to make users of the `download-ci-llvm` configuration download
 a new version of LLVM from CI, even if the LLVM submodule hasn’t changed.
 
-Last change is for: https://github.com/rust-lang/rust/pull/102790
+Last change is for: https://github.com/rust-lang/rust/pull/104748

src/bootstrap/native.rs

@@ -63,13 +63,13 @@ impl LdFlags {
     }
 }
 
-// This returns whether we've already previously built LLVM.
-//
-// It's used to avoid busting caches during x.py check -- if we've already built
-// LLVM, it's fine for us to not try to avoid doing so.
-//
-// This will return the llvm-config if it can get it (but it will not build it
-// if not).
+/// This returns whether we've already previously built LLVM.
+///
+/// It's used to avoid busting caches during x.py check -- if we've already built
+/// LLVM, it's fine for us to not try to avoid doing so.
+///
+/// This will return the llvm-config if it can get it (but it will not build it
+/// if not).
 pub fn prebuilt_llvm_config(
     builder: &Builder<'_>,
     target: TargetSelection,
@@ -823,8 +823,21 @@ impl Step for Lld {
         }
         let target = self.target;
 
-        let LlvmResult { llvm_config, llvm_cmake_dir } =
-            builder.ensure(Llvm { target: self.target });
+        let LlvmResult { llvm_config, llvm_cmake_dir } = builder.ensure(Llvm { target });
+
+        // The `dist` step packages LLD next to LLVM's binaries for download-ci-llvm. The root path
+        // we usually expect here is `./build/$triple/ci-llvm/`, with the binaries in its `bin`
+        // subfolder. We check if that's the case, and if LLD's binary already exists there next to
+        // `llvm-config`: if so, we can use it instead of building LLVM/LLD from source.
+        let ci_llvm_bin = llvm_config.parent().unwrap();
+        if ci_llvm_bin.is_dir() && ci_llvm_bin.file_name().unwrap() == "bin" {
+            let lld_path = ci_llvm_bin.join(exe("lld", target));
+            if lld_path.exists() {
+                // The following steps copying `lld` as `rust-lld` to the sysroot, expect it in the
+                // `bin` subfolder of this step's out dir.
+                return ci_llvm_bin.parent().unwrap().to_path_buf();
+            }
+        }
 
         let out_dir = builder.lld_out(target);
         let done_stamp = out_dir.join("lld-finished-building");