[bug#49471] [PATCH] gnu: lld: Update to 12.0.0.

[Leo Prikler <leo.prikler@student.tugraz.at>]

* gnu/packages/llvm.scm (llvm-subprojects-origin, %llvm-project-12.0.0-hash):
New variables.
(lld-11, lld-12): New variables.
(lld): Point to lld-11.
---
This patch is an alternative to the one sent by terramorpha.

Supposed advantages of it are:
* lld still points to lld-11, not breaking any packages that rely on it
* llvm-subprojects-origin can be reused by other LLVM subprojects
  (think of libcxx and libcxxabi, which both require each other, or
  libunwind, which requires libcxx)

Disadvantages are:
* llvm-project is a beefy tarball and somewhat of a pain to unpack
* Both this version and the original use an unclean approach to building
  subprojects, that is propagated by LLVM.  Perhaps we ought to instead
  clean up their mess?

Regards,
Leo

 gnu/packages/llvm.scm | 43 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/gnu/packages/llvm.scm b/gnu/packages/llvm.scm
index 575c63c713..1ddbd39e59 100644
--- a/gnu/packages/llvm.scm
+++ b/gnu/packages/llvm.scm
@@ -92,6 +92,22 @@ as \"x86_64-linux\"."
   (string-append "https://github.com/llvm/llvm-project/releases/download"
                  "/llvmorg-" version "/" component "-" version ".src.tar.xz"))
 
+(define* (llvm-subprojects-origin components version hash
+                                  #:optional (patches '()))
+  (origin
+    (method url-fetch)
+    (uri (llvm-uri "llvm-project" version))
+    (sha256 (base32 hash))
+    (patches (map search-patch patches))
+    (modules '((guix build utils)
+               (ice-9 ftw)))
+    (snippet
+     `(for-each
+       (lambda (file)
+         (unless (member file '("." ".." ,@components))
+           (delete-file-recursively file)))
+       (scandir ".")))))
+
 (define* (clang-runtime-from-llvm llvm hash
                                   #:optional (patches '()))
   (package
@@ -882,7 +898,7 @@ of programming tools as well as libraries with equivalent functionality.")
 (define-public clang clang-9)
 (define-public clang-toolchain clang-toolchain-9)
 
-(define-public lld
+(define-public lld-11
   (package
     (name "lld")
     (version "11.0.0")
@@ -906,6 +922,31 @@ of programming tools as well as libraries with equivalent functionality.")
 components which highly leverage existing libraries in the larger LLVM Project.")
     (license license:asl2.0))) ; With LLVM exception
 
+(define %llvm-project-12.0.0-hash "0a1n88dmbwp1ww7h3ykf0s001cndk1bm3i2cj1y2sh548f4nilcy")
+
+(define-public lld-12
+  (package
+    (inherit lld-11)
+    (version "12.0.0")
+    (source (llvm-subprojects-origin '("lld" "libunwind") version
+                                     %llvm-project-12.0.0-hash))
+    (arguments
+     `(,@(package-arguments lld-11)
+       #:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'dissect
+           (lambda _
+             ;; Find libunwind in unpacked sources
+             (substitute* "lld/MachO/CMakeLists.txt"
+               (("\\$\\{LLVM_MAIN_SRC_DIR\\}/\\.\\./libunwind")
+                (string-append (getcwd) "/libunwind")))
+             ;; chdir to actual source folder
+             (chdir "lld"))))))
+    (inputs
+     `(("llvm" ,llvm-12)))))
+
+(define-public lld lld-11)
+
 (define-public lldb
   (package
     (name "lldb")
-- 
2.32.0