rust: Modify Rust to be usable as host-tool only

PATCH New sam.voss at gmail.com 3 participants 5 messages Jul 17, 2018 – Aug 22, 2018

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sam.voss at gmail.com

author

2018-07-17 04:29 UTC #0 lore raw

PATCH 1/1 rust: Modify Rust to be usable as host-tool only

From: Sam Voss <[email protected]>

Modify host-rust virtual package to default to host-rust-bin when no other selection has been made, as long as the host supports rust. This allows host only tools to still use rust when the target architecture does not support it.

Add target-specific variable which is used to differentiate host and target arch requirements (BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS).

A target package shall depend on this variable where a host package will use the previously defined BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS. The new "target" version is selectable for the same set of architectures as before, but now depends on the host variant.

Signed-off-by Sam Voss <[email protected]>

---

Note: this has been tested on x86_64 host compiling to:

- x86_64 qemu target (building package to be upstreamed shortly)
- mips64el (based on librsvg build failures seen at
http://autobuild.buildroot.net/results/f33/f335ed517b402c094ed3b10a3da4cdc23620dbd6/defconfig)
this target was only tested to compile, with no runtime tests.

 package/rust-bin/rust-bin.mk |  5 ++++-
 package/rust/rust.mk         |  1 +
 package/rustc/Config.in.host | 21 ++++++++++++++++++---
 package/rustc/rustc.mk       |  2 ++
 4 files changed, 25 insertions(+), 4 deletions(-)

package/rust-bin/rust-bin.mk +4 -1

index e10dbcebd0..c426da8d15 100644
@@ -15,7 +15,10 @@ HOST_RUST_BIN_SOURCE = rustc-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME).tar.xz
 
 HOST_RUST_BIN_EXTRA_DOWNLOADS = \
 	rust-std-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME).tar.xz \
-	rust-std-$(RUST_BIN_VERSION)-$(RUSTC_TARGET_NAME).tar.xz
+
+ifeq ($(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS),y)
+HOST_RUST_BIN_EXTRA_DOWNLOADS += rust-std-$(RUST_BIN_VERSION)-$(RUSTC_TARGET_NAME).tar.xz
+endif
 
 HOST_RUST_BIN_LIBSTD_HOST_PREFIX = rust-std-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME)/rust-std-$(RUSTC_HOST_NAME)

package/rust/rust.mk +1 -0

index af5c366495..c11dbb23b0 100644
@@ -19,6 +19,7 @@ HOST_RUST_DEPENDENCIES = \
 	host-python \
 	$(BR2_CMAKE_HOST_DEPENDENCY)
 
+
 ifeq ($(BR2_PACKAGE_JEMALLOC),y)
 HOST_RUST_DEPENDENCIES += jemalloc
 HOST_RUST_JEMALLOC_ENABLED = true

package/rustc/Config.in.host +18 -3

index 2ae8f89d3f..d7f0efcd3f 100644
@@ -14,6 +14,19 @@ config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
 	depends on BR2_TOOLCHAIN_USES_GLIBC
 	depends on BR2_HOSTARCH = "x86_64" || BR2_HOSTARCH = "x86"
 
+config BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
+	bool
+	default y if BR2_i386
+	default y if BR2_x86_64
+	default y if BR2_aarch64
+	default y if BR2_arm && !BR2_ARM_CPU_ARMV4 && !BR2_ARM_CPU_ARMV5 \
+	        && !(BR2_ARM_CPU_ARMV7A && BR2_ARM_EABI)
+	default y if BR2_powerpc || BR2_powerpc64 || BR2_powerpc64le
+	default y if (BR2_mips || BR2_mipsel) && !BR2_MIPS_CPU_MIPS32R6
+	default y if (BR2_mips64 || BR2_mips64el) && !BR2_MIPS_CPU_MIPS64R6 \
+		&& BR2_MIPS_NABI64
+	depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
+
 config BR2_PACKAGE_HOST_RUSTC_ARCH
 	string
 	default "armv7"  if BR2_ARM_CPU_ARMV7A

@@ -66,9 +79,11 @@ config BR2_PACKAGE_HOST_RUST_BIN
 
 endchoice
 
+endif
+
 config BR2_PACKAGE_PROVIDES_HOST_RUSTC
 	string
 	default "host-rust" if BR2_PACKAGE_HOST_RUST
-	default "host-rust-bin" if BR2_PACKAGE_HOST_RUST_BIN
-
-endif
+	# Default to host-rust-bin as long as host arch supports it
+	default "host-rust-bin" if !BR2_PACKAGE_HOST_RUST
+	depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS

package/rustc/rustc.mk +2 -0

index bed74f3c2c..6eea9b4fc5 100644
@@ -7,7 +7,9 @@
 RUSTC_ARCH = $(call qstrip,$(BR2_PACKAGE_HOST_RUSTC_ARCH))
 RUSTC_ABI = $(call qstrip,$(BR2_PACKAGE_HOST_RUSTC_ABI))
 
+ifeq ($(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS),y)
 RUSTC_TARGET_NAME = $(RUSTC_ARCH)-unknown-linux-gnu$(RUSTC_ABI)
+endif
 
 ifeq ($(HOSTARCH),x86)
 RUSTC_HOST_ARCH = i686

Signature · 1 line

2.18.0

Eric Le Bihan

2018-07-17 11:18 UTC #1 lore raw

PATCH 1/1 rust: Modify Rust to be usable as host-tool only

Hi!

Quoted 10 lines

From: Sam Voss <[email protected]>
Modify host-rust virtual package to default to host-rust-bin when no other selection has been made, as long as the host supports rust. This allows host only tools to still use rust when the target architecture does not support it.
Add target-specific variable which is used to differentiate host and target arch requirements (BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS).

Quoted 6 lines

A target package shall depend on this variable where a host package will use the previously defined BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS. The new "target" version is selectable for the same set of architectures as before, but now depends on the host variant.
Signed-off-by Sam Voss <[email protected]>

IIUC, the goal of this patch is to be able to build host packages which need rust, so host-rust-bin is set as default to avoid errors on empty variables and only the compiler and the host standard library are installed (not the target standard library). Only x86_64/x86 support is required, right?

I find it confusing that BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS and BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS are quite similar and that in the end BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS is only used to install the target standard library. Is that meant to cover the case where the host and the target are the same (x86_64) or the librsvg-related issue?

Would this be solved by introducing a virtual package for the target standard library as previously discussed [1]?

[1] http://patchwork.ozlabs.org/comment/1830134/

Regards,

--
ELB

Samuel Voss

author

2018-07-17 17:34 UTC #2 lore raw

PATCH 1/1 rust: Modify Rust to be usable as host-tool only

Eric,

Quoted 17 lines · Eric Le Bihan

Hi!
Quoted 15 lines
From: Sam Voss <[email protected]>
Modify host-rust virtual package to default to host-rust-bin when no other selection has been made, as long as the host supports rust. This allows host only tools to still use rust when the target architecture does not support it.
Add target-specific variable which is used to differentiate host and target arch requirements (BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS).
A target package shall depend on this variable where a host package will use the previously defined BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS. The new "target" version is selectable for the same set of architectures as before, but now depends on the host variant.

Quoted 6 lines

Quoted 1 line
Signed-off-by Sam Voss <[email protected]>
IIUC, the goal of this patch is to be able to build host packages which need rust, so host-rust-bin is set as default to avoid errors on empty variables and only the compiler and the host standard library are installed (not the target standard library). Only x86_64/x86 support is required, right?

That is correct. This boils down to being able to build a host tool on supported platforms when the target isn't.

Quoted 6 lines

I find it confusing that BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS and BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS are quite similar and that in the end BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS is only used to install the target standard library. Is that meant to cover the case where the host and the target are the same (x86_64) or the librsvg-related issue?

The target was the librsvg build issues, I'm not sure it does anything special for the target-same builds as it'll just fall into the same scenario as the previous itteration (nothing will be gated off).

Quoted 3 lines

Would this be solved by introducing a virtual package for the target standard library as previously discussed [1]?

I actually missed this conversation, and on paper I like the virtual package but I think it is a lot more work (which is OK, but it might be worth iterating to that point).

Thanks,

Sam

Thomas Petazzoni

2018-08-21 20:06 UTC #3 lore raw

PATCH 1/1 rust: Modify Rust to be usable as host-tool only

In reply to sam.voss at gmail.com

Hello Sam,

Thanks for taking the lead on this!

Quoted 10 lines · sam.voss at gmail.com

From: Sam Voss <[email protected]>
Modify host-rust virtual package to default to host-rust-bin when no other selection has been made, as long as the host supports rust. This allows host only tools to still use rust when the target architecture does not support it.
Add target-specific variable which is used to differentiate host and target arch requirements (BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS).

Quoted 6 lines

A target package shall depend on this variable where a host package will use the previously defined BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS. The new "target" version is selectable for the same set of architectures as before, but now depends on the host variant.
Signed-off-by Sam Voss <[email protected]>

I finally took a bit of time to look into this. Basically today, when we build host-rustc, we get:

- A Rust compiler capable of compiling code for the host machine, and cross-compiling code for the target.
- A Rust standard library for the host machine
- A Rust standard library for the target machine

However, this is restricted to Buildroot configurations where the target architecture is supported by Rust. But there are some host packages that we would like to build and run on the host (usually a x86 or x86-64 machine), and for these the restriction that host-rustc is only available when the target architecture is supported by Rust is too restrictive.

So basically, we would like:

- host-rustc to continue to install a Rust compiler that does native+cross, the host standard library and target standard library when the target architecture is supported by Rust.

- host-rustc to install a Rust compiler that does native build, and the host standard library, when the target architecture is NOT supported by Rust.

So the host-rustc package would have a different behavior depending on whether the target architecture is supported by Rust or not.

Quoted 18 lines

Note: this has been tested on x86_64 host compiling to:
- x86_64 qemu target (building package to be upstreamed shortly)
- mips64el (based on librsvg build failures seen at
http://autobuild.buildroot.net/results/f33/f335ed517b402c094ed3b10a3da4cdc23620dbd6/defconfig)
this target was only tested to compile, with no runtime tests.
 package/rust-bin/rust-bin.mk |  5 ++++-
 package/rust/rust.mk         |  1 +
 package/rustc/Config.in.host | 21 ++++++++++++++++++---
 package/rustc/rustc.mk       |  2 ++
 4 files changed, 25 insertions(+), 4 deletions(-)
package/rust-bin/rust-bin.mk +0 -0
index e10dbcebd0..c426da8d15 100644

package/rust-bin/rust-bin.mk HOST_RUST_BIN_SOURCE = rustc-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME).tar.xz · line 17

Quoted 3 lines

package/rust-bin/rust-bin.mk +0 -0

@@ -15,7 +15,10 @@ HOST_RUST_BIN_SOURCE = rustc-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME).tar.xz
 
 HOST_RUST_BIN_EXTRA_DOWNLOADS = \
 	rust-std-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME).tar.xz \

The ending backslash should be removed here, I guess it causes a check-package warning.

Quoted 13 lines

package/rust-bin/rust-bin.mk +4 -1

-	rust-std-$(RUST_BIN_VERSION)-$(RUSTC_TARGET_NAME).tar.xz
+
+ifeq ($(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS),y)
+HOST_RUST_BIN_EXTRA_DOWNLOADS += rust-std-$(RUST_BIN_VERSION)-$(RUSTC_TARGET_NAME).tar.xz
+endif
 
 HOST_RUST_BIN_LIBSTD_HOST_PREFIX = rust-std-$(RUST_BIN_VERSION)-$(RUSTC_HOST_NAME)/rust-std-$(RUSTC_HOST_NAME)

package/rust/rust.mk +0 -0

index af5c366495..c11dbb23b0 100644

package/rust/rust.mk HOST_RUST_DEPENDENCIES = \ · line 22

Quoted 4 lines

package/rust/rust.mk +1 -0

@@ -19,6 +19,7 @@ HOST_RUST_DEPENDENCIES = \
 	host-python \
 	$(BR2_CMAKE_HOST_DEPENDENCY)
 
+

Spurious change.

package/rustc/Config.in.host config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS · line 15

Quoted 10 lines

package/rust/rust.mk +0 -0

 ifeq ($(BR2_PACKAGE_JEMALLOC),y)
 HOST_RUST_DEPENDENCIES += jemalloc
 HOST_RUST_JEMALLOC_ENABLED = true

package/rustc/Config.in.host +0 -0

index 2ae8f89d3f..d7f0efcd3f 100644
@@ -14,6 +14,19 @@ config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
 	depends on BR2_TOOLCHAIN_USES_GLIBC
 	depends on BR2_HOSTARCH = "x86_64" || BR2_HOSTARCH = "x86"

I think you need to remove all the architecture dependencies here:
host-rust is always available, as long as the host architecture is x86-64 or x86. The glibc dependency should also be removed, as we don't have any way of expressing a "we depend on a host machine with glibc" dependency today.

package/rustc/Config.in.host config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS · line 27

Quoted 11 lines

package/rustc/Config.in.host +11 -0

+config BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
+	bool
+	default y if BR2_i386
+	default y if BR2_x86_64
+	default y if BR2_aarch64
+	default y if BR2_arm && !BR2_ARM_CPU_ARMV4 && !BR2_ARM_CPU_ARMV5 \
+	        && !(BR2_ARM_CPU_ARMV7A && BR2_ARM_EABI)
+	default y if BR2_powerpc || BR2_powerpc64 || BR2_powerpc64le
+	default y if (BR2_mips || BR2_mipsel) && !BR2_MIPS_CPU_MIPS32R6
+	default y if (BR2_mips64 || BR2_mips64el) && !BR2_MIPS_CPU_MIPS64R6 \
+		&& BR2_MIPS_NABI64

The glibc dependency should be re-added here.

Quoted 10 lines

package/rustc/Config.in.host +3 -0

+	depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
+
 config BR2_PACKAGE_HOST_RUSTC_ARCH
 	string
 	default "armv7"  if BR2_ARM_CPU_ARMV7A

@@ -66,9 +79,11 @@ config BR2_PACKAGE_HOST_RUST_BIN
 
 endchoice
 
+endif

package/rustc/Config.in.host config BR2_PACKAGE_HOST_RUST_BIN · line 89

Quoted 10 lines

package/rustc/Config.in.host +4 -3

+
 config BR2_PACKAGE_PROVIDES_HOST_RUSTC
 	string
 	default "host-rust" if BR2_PACKAGE_HOST_RUST
-	default "host-rust-bin" if BR2_PACKAGE_HOST_RUST_BIN
-
-endif
+	# Default to host-rust-bin as long as host arch supports it
+	default "host-rust-bin" if !BR2_PACKAGE_HOST_RUST
+	depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS

However, even with those changes, if I do:

$ cat > .config <<EOF
BR2_nios2=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-nios2-full-2018.05.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_6=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_16=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM_GLIBC=y
# BR2_TOOLCHAIN_EXTERNAL_HAS_SSP is not set
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_INIT_NONE=y
BR2_SYSTEM_BIN_SH_NONE=y
# BR2_PACKAGE_BUSYBOX is not set
# BR2_TARGET_ROOTFS_TAR is not set
EOF
$ make olddefconfig
$ make host-rustc

Then I see:

Quoted 1 line

Quoted 1 line
Quoted 1 line
host-rust-bin 1.27.1 Extracting

xzcat /home/thomas/dl/rust-bin/rustc-1.27.1-x86_64-unknown-linux-gnu.tar.xz | tar --strip-components=1 -C /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1 -xf -
mkdir -p /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1/std
xzcat /home/thomas/dl/rust-bin/rust-std-1.27.1-x86_64-unknown-linux-gnu.tar.xz | tar -C /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1/std -xf -
cd /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1/rustc/lib/rustlib/x86_64-unknown-linux-gnu; ln -sf ../../../../std/rust-std-1.27.1-x86_64-unknown-linux-gnu/rust-std-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib

Quoted 4 lines

Quoted 4 lines
Quoted 4 lines
host-rust-bin 1.27.1 Patching
host-rust-bin 1.27.1 Configuring
host-rust-bin 1.27.1 Building
host-rust-bin 1.27.1 Installing to host directory

And nothing gets installed in output/host. This is because BR2_PACKAGE_HOST_RUST_BIN is false, and therefore rust-bin.mk doesn't do much. I am not sure why rust-bin.mk has this condition:

ifeq ($(BR2_PACKAGE_HOST_RUST_BIN),y)

This condition shouldn't be necessary. So it should be removed, and instead HOST_RUST_BIN_INSTALL_LIBSTD_TARGET should be put within ifeq ($(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS),y).

With this fixed, it does install a Rust compiler and host standard library, and I can successfully build and run a host Rust program, even if the NIOSII target architecture is not supported by Rust.

Could you fix this issue and send an updated patch ?

Note: I think your current patch is correct that the "host-rustc" package should *NOT* appear in menuconfig when the target architecture is not supported by Rust. Indeed, in such a case, Rust can only build host programs, and it's therefore a mere build dependency of some other package.

Please add a comment above the definitions of BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS that all packages that use host-rustc to build host Rust programs should depend on this option. And a comment above BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS to say that all packages that use host-rustc to build target Rust programs should depend on this option.

For the record, the current state of the patch I had after some fixes is at http://code.bulix.org/pqww33-400669.

Thanks!

Thomas

Signature · 3 lines

Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

Samuel Voss

author

2018-08-22 19:04 UTC #4 lore raw

PATCH 1/1 rust: Modify Rust to be usable as host-tool only

Hey Thomas!

Thanks for putting some time into this, I was afraid it was going to sit forever because it's kind of a pain to test these kinds of changes.

Quoted 12 lines · Thomas Petazzoni

Hello Sam,
Thanks for taking the lead on this!
Quoted 6 lines · sam.voss at gmail.com
From: Sam Voss <[email protected]>
Modify host-rust virtual package to default to host-rust-bin when no other selection has been made, as long as the host supports rust. This allows host only tools to still use rust when the target architecture does not support it.

[snip]

Quoted 6 lines

Quoted 6 lines
ifeq ($(BR2_PACKAGE_JEMALLOC),y)
HOST_RUST_DEPENDENCIES += jemalloc
HOST_RUST_JEMALLOC_ENABLED = true
package/rustc/Config.in.host +0 -1
index 2ae8f89d3f..d7f0efcd3f 100644
--- a/package/rustc/Config.in.host

Quoted 8 lines

package/rustc/Config.in.host config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS · line 15
Quoted 4 lines
+++ b/package/rustc/Config.in.host
package/rustc/Config.in.host +0 -0
@@ -14,6 +14,19 @@ config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
      depends on BR2_TOOLCHAIN_USES_GLIBC
      depends on BR2_HOSTARCH = "x86_64" || BR2_HOSTARCH = "x86"
I think you need to remove all the architecture dependencies here:
host-rust is always available, as long as the host architecture is x86-64 or x86.

Ah, defaulting -y instead of depending makes sense. Thanks for pointing this out.

Quoted 94 lines

The glibc dependency should also be removed, as we don't have any way of expressing a "we depend on a host machine with glibc" dependency today.
package/rustc/Config.in.host config BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS · line 27
Quoted 11 lines
package/rustc/Config.in.host +11 -0
+config BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
+     bool
+     default y if BR2_i386
+     default y if BR2_x86_64
+     default y if BR2_aarch64
+     default y if BR2_arm && !BR2_ARM_CPU_ARMV4 && !BR2_ARM_CPU_ARMV5 \
+             && !(BR2_ARM_CPU_ARMV7A && BR2_ARM_EABI)
+     default y if BR2_powerpc || BR2_powerpc64 || BR2_powerpc64le
+     default y if (BR2_mips || BR2_mipsel) && !BR2_MIPS_CPU_MIPS32R6
+     default y if (BR2_mips64 || BR2_mips64el) && !BR2_MIPS_CPU_MIPS64R6 \
+             && BR2_MIPS_NABI64
The glibc dependency should be re-added here.
Quoted 10 lines
package/rustc/Config.in.host +3 -0
+     depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
+
 config BR2_PACKAGE_HOST_RUSTC_ARCH
      string
      default "armv7"  if BR2_ARM_CPU_ARMV7A
@@ -66,9 +79,11 @@ config BR2_PACKAGE_HOST_RUST_BIN
 
 endchoice
 
+endif
package/rustc/Config.in.host config BR2_PACKAGE_HOST_RUST_BIN · line 89
Quoted 10 lines
package/rustc/Config.in.host +4 -3
+
 config BR2_PACKAGE_PROVIDES_HOST_RUSTC
      string
      default "host-rust" if BR2_PACKAGE_HOST_RUST
-     default "host-rust-bin" if BR2_PACKAGE_HOST_RUST_BIN
-
-endif
+     # Default to host-rust-bin as long as host arch supports it
+     default "host-rust-bin" if !BR2_PACKAGE_HOST_RUST
+     depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
However, even with those changes, if I do:
$ cat > .config <<EOF
BR2_nios2=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-nios2-full-2018.05.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_6=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_16=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM_GLIBC=y
# BR2_TOOLCHAIN_EXTERNAL_HAS_SSP is not set
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_INIT_NONE=y
BR2_SYSTEM_BIN_SH_NONE=y
# BR2_PACKAGE_BUSYBOX is not set
# BR2_TARGET_ROOTFS_TAR is not set
EOF
$ make olddefconfig
$ make host-rustc
Then I see:
Quoted 1 line
Quoted 1 line
Quoted 1 line
host-rust-bin 1.27.1 Extracting
xzcat /home/thomas/dl/rust-bin/rustc-1.27.1-x86_64-unknown-linux-gnu.tar.xz | tar --strip-components=1 -C /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1 -xf -
mkdir -p /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1/std
xzcat /home/thomas/dl/rust-bin/rust-std-1.27.1-x86_64-unknown-linux-gnu.tar.xz | tar -C /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1/std -xf -
cd /home/thomas/projets/buildroot/output/build/host-rust-bin-1.27.1/rustc/lib/rustlib/x86_64-unknown-linux-gnu; ln -sf ../../../../std/rust-std-1.27.1-x86_64-unknown-linux-gnu/rust-std-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib
Quoted 4 lines
Quoted 4 lines
Quoted 4 lines
host-rust-bin 1.27.1 Patching
host-rust-bin 1.27.1 Configuring
host-rust-bin 1.27.1 Building
host-rust-bin 1.27.1 Installing to host directory
And nothing gets installed in output/host. This is because BR2_PACKAGE_HOST_RUST_BIN is false, and therefore rust-bin.mk doesn't do much. I am not sure why rust-bin.mk has this condition:
ifeq ($(BR2_PACKAGE_HOST_RUST_BIN),y)
This condition shouldn't be necessary. So it should be removed, and instead HOST_RUST_BIN_INSTALL_LIBSTD_TARGET should be put within ifeq ($(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS),y).
With this fixed, it does install a Rust compiler and host standard library, and I can successfully build and run a host Rust program, even if the NIOSII target architecture is not supported by Rust.
Could you fix this issue and send an updated patch ?
Note: I think your current patch is correct that the "host-rustc" package should *NOT* appear in menuconfig when the target architecture is not supported by Rust. Indeed, in such a case, Rust can only build host programs, and it's therefore a mere build dependency of some other package.

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Please add a comment above the definitions of BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS that all packages that use host-rustc to build host Rust programs should depend on this option. And a comment above BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS to say that all packages that use host-rustc to build target Rust programs should depend on this option.

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For the record, the current state of the patch I had after some fixes is at http://code.bulix.org/pqww33-400669.

Fantastic, testing now and will send a rev 2 shortly

Thanks!

Sam