automatically fall back to the pure Rust build

There are two scenarios where compiling AVX-512 C or assembly code might
not work:

1. There might not be a C compiler installed at all. Most commonly this
   is either in cross-compiling situations, or with the Windows GNU
   target.
2. The installed C compiler might not support e.g. -mavx512f, because
   it's too old.

In both of these cases, print a relevant warning, and then automatically
fall back to using the pure Rust intrinsics build.

Note that this only affects x86 targets. Other targets always use pure
Rust, unless the "neon" feature is enabled.

1 files changed, 14 insertions, 14 deletions

diff --git a/src/platform.rs b/src/platform.rs
index 0cc8d49..d2790a6 100644
--- a/src/platform.rs
+++ b/src/platform.rs
@@ -4,10 +4,10 @@ use arrayref::{array_mut_ref, array_ref};
 cfg_if::cfg_if! {
     if #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
         cfg_if::cfg_if! {
-            if #[cfg(feature = "pure")] {
-                pub const MAX_SIMD_DEGREE: usize = 8;
-            } else {
+            if #[cfg(blake3_avx512_ffi)] {
                 pub const MAX_SIMD_DEGREE: usize = 16;
+            } else {
+                pub const MAX_SIMD_DEGREE: usize = 8;
             }
         }
     } else if #[cfg(feature = "neon")] {
@@ -24,10 +24,10 @@ cfg_if::cfg_if! {
 cfg_if::cfg_if! {
     if #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
         cfg_if::cfg_if! {
-            if #[cfg(feature = "pure")] {
-                pub const MAX_SIMD_DEGREE_OR_2: usize = 8;
-            } else {
+            if #[cfg(blake3_avx512_ffi)] {
                 pub const MAX_SIMD_DEGREE_OR_2: usize = 16;
+            } else {
+                pub const MAX_SIMD_DEGREE_OR_2: usize = 8;
             }
         }
     } else if #[cfg(feature = "neon")] {
@@ -44,7 +44,7 @@ pub enum Platform {
     SSE41,
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
     AVX2,
-    #[cfg(not(feature = "pure"))]
+    #[cfg(blake3_avx512_ffi)]
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
     AVX512,
     #[cfg(feature = "neon")]
@@ -56,7 +56,7 @@ impl Platform {
     pub fn detect() -> Self {
         #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
         {
-            #[cfg(not(feature = "pure"))]
+            #[cfg(blake3_avx512_ffi)]
             {
                 if avx512_detected() {
                     return Platform::AVX512;
@@ -85,7 +85,7 @@ impl Platform {
             Platform::SSE41 => 4,
             #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
             Platform::AVX2 => 8,
-            #[cfg(not(feature = "pure"))]
+            #[cfg(blake3_avx512_ffi)]
             #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
             Platform::AVX512 => 16,
             #[cfg(feature = "neon")]
@@ -111,7 +111,7 @@ impl Platform {
                 crate::sse41::compress_in_place(cv, block, block_len, counter, flags)
             },
             // Safe because detect() checked for platform support.
-            #[cfg(not(feature = "pure"))]
+            #[cfg(blake3_avx512_ffi)]
             #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
             Platform::AVX512 => unsafe {
                 crate::avx512::compress_in_place(cv, block, block_len, counter, flags)
@@ -138,7 +138,7 @@ impl Platform {
                 crate::sse41::compress_xof(cv, block, block_len, counter, flags)
             },
             // Safe because detect() checked for platform support.
-            #[cfg(not(feature = "pure"))]
+            #[cfg(blake3_avx512_ffi)]
             #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
             Platform::AVX512 => unsafe {
                 crate::avx512::compress_xof(cv, block, block_len, counter, flags)
@@ -210,7 +210,7 @@ impl Platform {
                 )
             },
             // Safe because detect() checked for platform support.
-            #[cfg(not(feature = "pure"))]
+            #[cfg(blake3_avx512_ffi)]
             #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
             Platform::AVX512 => unsafe {
                 crate::avx512::hash_many(
@@ -265,7 +265,7 @@ impl Platform {
         }
     }
 
-    #[cfg(not(feature = "pure"))]
+    #[cfg(blake3_avx512_ffi)]
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
     pub fn avx512() -> Option<Self> {
         if avx512_detected() {
@@ -285,7 +285,7 @@ impl Platform {
 
 // Note that AVX-512 is divided into multiple featuresets, and we use two of
 // them, F and VL.
-#[cfg(not(feature = "pure"))]
+#[cfg(blake3_avx512_ffi)]
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 #[inline(always)]
 pub fn avx512_detected() -> bool {