Avoid the use of #[cfg(doc)] in std_detect (#1283)

Co-authored-by: bjorn3 <bjorn3@users.noreply.github.com>

crates/std_detect/src/detect/arch/aarch64.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: aarch64;
+    @CFG: target_arch = "aarch64";
     @MACRO_NAME: is_aarch64_feature_detected;
     @MACRO_ATTRS:
     /// This macro tests, at runtime, whether an `aarch64` feature is enabled on aarch64 platforms.

crates/std_detect/src/detect/arch/arm.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: arm;
+    @CFG: target_arch = "arm";
     @MACRO_NAME: is_arm_feature_detected;
     @MACRO_ATTRS:
     /// Checks if `arm` feature is enabled.

crates/std_detect/src/detect/arch/mips.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: mips;
+    @CFG: target_arch = "mips";
     @MACRO_NAME: is_mips_feature_detected;
     @MACRO_ATTRS:
     /// Checks if `mips` feature is enabled.

crates/std_detect/src/detect/arch/mips64.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: mips64;
+    @CFG: target_arch = "mips64";
     @MACRO_NAME: is_mips64_feature_detected;
     @MACRO_ATTRS:
     /// Checks if `mips64` feature is enabled.

crates/std_detect/src/detect/arch/mod.rs

@@ -0,0 +1,56 @@
+#![allow(dead_code)]
+
+use cfg_if::cfg_if;
+
+// Export the macros for all supported architectures.
+#[macro_use]
+mod x86;
+#[macro_use]
+mod arm;
+#[macro_use]
+mod aarch64;
+#[macro_use]
+mod riscv;
+#[macro_use]
+mod powerpc;
+#[macro_use]
+mod powerpc64;
+#[macro_use]
+mod mips;
+#[macro_use]
+mod mips64;
+
+cfg_if! {
+    if #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
+        pub use x86::*;
+    } else if #[cfg(target_arch = "arm")] {
+        pub use arm::*;
+    } else if #[cfg(target_arch = "aarch64")] {
+        pub use aarch64::*;
+    } else if #[cfg(any(target_arch = "riscv32", target_arch = "riscv64"))] {
+        pub use riscv::*;
+    } else if #[cfg(target_arch = "powerpc")] {
+        pub use powerpc::*;
+    } else if #[cfg(target_arch = "powerpc64")] {
+        pub use powerpc64::*;
+    } else if #[cfg(target_arch = "mips")] {
+        pub use mips::*;
+    } else if #[cfg(target_arch = "mips64")] {
+        pub use mips64::*;
+    } else {
+        // Unimplemented architecture:
+        #[doc(hidden)]
+        pub(crate) enum Feature {
+            Null
+        }
+        #[doc(hidden)]
+        pub mod __is_feature_detected {}
+
+        impl Feature {
+            #[doc(hidden)]
+            pub(crate) fn from_str(_s: &str) -> Result<Feature, ()> { Err(()) }
+            #[doc(hidden)]
+            pub(crate) fn to_str(self) -> &'static str { "" }
+        }
+    }
+}

crates/std_detect/src/detect/arch/powerpc.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: powerpc;
+    @CFG: target_arch = "powerpc";
     @MACRO_NAME: is_powerpc_feature_detected;
     @MACRO_ATTRS:
     /// Checks if `powerpc` feature is enabled.

crates/std_detect/src/detect/arch/powerpc64.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: powerpc64;
+    @CFG: target_arch = "powerpc64";
     @MACRO_NAME: is_powerpc64_feature_detected;
     @MACRO_ATTRS:
     /// Checks if `powerpc` feature is enabled.

crates/std_detect/src/detect/arch/riscv.rs

@@ -2,6 +2,7 @@
 
 features! {
     @TARGET: riscv;
+    @CFG: any(target_arch = "riscv32", target_arch = "riscv64");
     @MACRO_NAME: is_riscv_feature_detected;
     @MACRO_ATTRS:
     /// A macro to test at *runtime* whether instruction sets are available on

crates/std_detect/src/detect/arch/x86.rs

@@ -17,6 +17,7 @@
 
 features! {
     @TARGET: x86;
+    @CFG: any(target_arch = "x86", target_arch = "x86_64");
     @MACRO_NAME: is_x86_feature_detected;
     @MACRO_ATTRS:
     /// A macro to test at *runtime* whether a CPU feature is available on

crates/std_detect/src/detect/macros.rs

@@ -2,6 +2,7 @@
 macro_rules! features {
     (
       @TARGET: $target:ident;
+      @CFG: $cfg:meta;
       @MACRO_NAME: $macro_name:ident;
       @MACRO_ATTRS: $(#[$macro_attrs:meta])*
       $(@BIND_FEATURE_NAME: $bind_feature:tt; $feature_impl:tt; )*
@@ -11,6 +12,8 @@ macro_rules! features {
         #[macro_export]
         $(#[$macro_attrs])*
         #[allow_internal_unstable(stdsimd_internal)]
+        #[cfg($cfg)]
+        #[doc(cfg($cfg))]
         macro_rules! $macro_name {
             $(
                 ($feature_lit) => {
@@ -44,6 +47,50 @@ macro_rules! features {
             };
         }
 
+        $(#[$macro_attrs])*
+        #[macro_export]
+        #[cfg(not($cfg))]
+        #[doc(cfg($cfg))]
+        macro_rules! $macro_name {
+            $(
+                ($feature_lit) => {
+                    compile_error!(
+                        concat!(
+                            r#"This macro cannot be used on the current target.
+                            You can prevent it from being used in other architectures by
+                            guarding it behind a cfg("#,
+                            stringify!($cfg),
+                            ")."
+                        )
+                    )
+                };
+            )*
+            $(
+                ($bind_feature) => { $macro_name!($feature_impl) };
+            )*
+            $(
+                ($nort_feature) => {
+                    compile_error!(
+                        concat!(
+                            stringify!($nort_feature),
+                            " feature cannot be detected at run-time"
+                        )
+                    )
+                };
+            )*
+            ($t:tt,) => {
+                    $macro_name!($t);
+            };
+            ($t:tt) => {
+                compile_error!(
+                    concat!(
+                        concat!("unknown ", stringify!($target)),
+                        concat!(" target feature: ", $t)
+                    )
+                )
+            };
+        }
+
         /// Each variant denotes a position in a bitset for a particular feature.
         ///
         /// PLEASE: do not use this, it is an implementation detail subject
@@ -53,6 +100,7 @@ macro_rules! features {
         #[derive(Copy, Clone)]
         #[repr(u8)]
         #[unstable(feature = "stdsimd_internal", issue = "none")]
+        #[cfg($cfg)]
         pub(crate) enum Feature {
             $(
                 $(#[$feature_comment])*
@@ -63,6 +111,7 @@ macro_rules! features {
             _last
         }
 
+        #[cfg($cfg)]
         impl Feature {
             pub(crate) fn to_str(self) -> &'static str {
                 match self {
@@ -86,6 +135,7 @@ macro_rules! features {
         /// PLEASE: do not use this, it is an implementation detail subject
         /// to change.
         #[doc(hidden)]
+        #[cfg($cfg)]
         pub mod __is_feature_detected {
             $(