gnu: rust-bootstrap: Add support for riscv64-linux.

* gnu/packages/rust.scm (%mrustc-source): Add patch.
(rust-bootstrap)[native-inputs]: On riscv64-linux add gcc@9:lib.
[supported-systems]: Add riscv64-linux. Adjust comment.
* gnu/packages/patches/mrustc-riscv64-support.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.

gnu/local.mk

@@ -1509,6 +1509,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/mono-mdoc-timestamping.patch		\
   %D%/packages/patches/mosaicatcher-unbundle-htslib.patch	\
   %D%/packages/patches/mrrescue-support-love-11.patch		\
+  %D%/packages/patches/mrustc-riscv64-support.patch		\
   %D%/packages/patches/mtools-mformat-uninitialized.patch	\
   %D%/packages/patches/mumps-build-parallelism.patch		\
   %D%/packages/patches/mumps-shared-libseq.patch		\

gnu/packages/patches/mrustc-riscv64-support.patch

@@ -0,0 +1,48 @@
+Patch sent upstream for review:
+https://github.com/thepowersgang/mrustc/pull/276
+
+diff --git a/src/trans/target.cpp b/src/trans/target.cpp
+index 420a2870..4d5eefb3 100644
+--- a/src/trans/target.cpp
++++ b/src/trans/target.cpp
+@@ -65,6 +65,13 @@ const TargetArch ARCH_POWERPC64LE = {
+     { /*atomic(u8)=*/true, true, true, true,  true },
+     TargetArch::Alignments(2, 4, 8, 16, 4, 8, 8)
+ };
++// This is a guess
++const TargetArch ARCH_RISCV64 = {
++    "riscv64",
++    64, false,
++    { /*atomic(u8)=*/true, true, true, true,  true },
++    TargetArch::Alignments(2, 4, 8, 16, 4, 8, 8)
++};
+ TargetSpec  g_target;
+ 
+ 
+@@ -455,6 +462,13 @@ namespace
+                 ARCH_POWERPC64LE
+                 };
+         }
++        else if(target_name == "riscv64-unknown-linux-gnu")
++        {
++            return TargetSpec {
++                "unix", "linux", "gnu", {CodegenMode::Gnu11, false, "riscv64-unknown-linux-gnu", BACKEND_C_OPTS_GNU},
++                ARCH_RISCV64
++                };
++        }
+         else if(target_name == "i586-pc-windows-gnu")
+         {
+             return TargetSpec {
+diff --git a/tools/common/target_detect.h b/tools/common/target_detect.h
+index a052da6b..42fea91a 100644
+--- a/tools/common/target_detect.h
++++ b/tools/common/target_detect.h
+@@ -34,6 +34,8 @@
+ #  define DEFAULT_TARGET_NAME "powerpc64-unknown-linux-gnu"
+ # elif defined(__powerpc64__) && defined(__LITTLE_ENDIAN__)
+ #  define DEFAULT_TARGET_NAME "powerpc64le-unknown-linux-gnu"
++# elif defined(__riscv) && __riscv_xlen == 64
++#  define DEFAULT_TARGET_NAME "riscv64-unknown-linux-gnu"
+ # else
+ #  warning "Unable to detect a suitable default target (linux-gnu)"
+ # endif

gnu/packages/rust.scm

@@ -137,7 +137,8 @@
       (file-name (git-file-name name (git-version version revision commit)))
       (sha256
        (base32
-        "0f7kh4n2663sn0z3xib8gzw0s97qpvwag40g2vs3bfjlrbpgi9z0")))))
+        "0f7kh4n2663sn0z3xib8gzw0s97qpvwag40g2vs3bfjlrbpgi9z0"))
+      (patches (search-patches "mrustc-riscv64-support.patch")))))
 
 ;;; Rust 1.54 is special in that it is built with mrustc, which shortens the
 ;;; bootstrap path.
@@ -172,6 +173,10 @@
        ;; https://github.com/thepowersgang/mrustc/issues/266
        ;; https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105860
        ("gcc" ,gcc-9)
+       ;; TODO: STARTFILE_PREFIX_SPEC is fixed on gcc<10 on core-updates.
+       ,@(if (target-riscv64?)
+           `(("gcc:lib" ,gcc-9 "lib"))
+           '())
        ("flex" ,flex)
        ("pkg-config" ,pkg-config)
        ;; Required for the libstd sources.
@@ -338,9 +343,11 @@
 safety and thread safety guarantees.")
     (home-page "https://github.com/thepowersgang/mrustc")
 
-    ;; So far mrustc is (x86_64|aarch64)-only.  It may support i686 soon:
+    ;; The intermediate generated code is known to be inefficient and
+    ;; therefore the build process needs 8GB of RAM while building.
+    ;; It may support i686 soon:
     ;; <https://github.com/thepowersgang/mrustc/issues/78>.
-    (supported-systems '("x86_64-linux" "aarch64-linux"))
+    (supported-systems '("x86_64-linux" "aarch64-linux" "riscv64-linux"))
 
     ;; Dual licensed.
     (license (list license:asl2.0 license:expat))))