Fix deny(unused) of an unused import with SGX + Miri (#581)

The `deny(unused)` use that used to be here is incompatible with building
this crate with Miri. The import in question is only used in a module
that is only declared when we are not `cfg(miri)`.

I tried to fix this by grouping items into modules so that I could apply
`cfg(not(miri))` to all the SGX code. One could also make the build work
by deleting the `#[deny(unused)]`, but that felt hacky.

src/backtrace/libunwind.rs

@@ -49,7 +49,7 @@ impl Frame {
         // the address here, which could be later mapped to correct function.
         #[cfg(all(target_env = "sgx", target_vendor = "fortanix"))]
         {
-            let image_base = super::get_image_base();
+            let image_base = super::sgx_image_base::get_image_base();
             ip = usize::wrapping_sub(ip as usize, image_base as _) as _;
         }
         ip

src/backtrace/mod.rs

@@ -125,39 +125,49 @@ impl fmt::Debug for Frame {
     }
 }
 
-#[cfg(all(target_env = "sgx", target_vendor = "fortanix", not(feature = "std")))]
-mod sgx_no_std_image_base {
-    use core::ffi::c_void;
-    use core::sync::atomic::{AtomicUsize, Ordering::SeqCst};
-
-    static IMAGE_BASE: AtomicUsize = AtomicUsize::new(0);
-
-    /// Set the image base address. This is only available for Fortanix SGX
-    /// target when the `std` feature is not enabled. This can be used in the
-    /// standard library to set the correct base address.
-    #[doc(hidden)]
-    pub fn set_image_base(base_addr: *mut c_void) {
-        IMAGE_BASE.store(base_addr as _, SeqCst);
+#[cfg(all(target_env = "sgx", target_vendor = "fortanix", not(miri)))]
+mod sgx_image_base {
+
+    #[cfg(not(feature = "std"))]
+    pub(crate) mod imp {
+        use core::ffi::c_void;
+        use core::sync::atomic::{AtomicUsize, Ordering::SeqCst};
+
+        static IMAGE_BASE: AtomicUsize = AtomicUsize::new(0);
+
+        /// Set the image base address. This is only available for Fortanix SGX
+        /// target when the `std` feature is not enabled. This can be used in the
+        /// standard library to set the correct base address.
+        #[doc(hidden)]
+        pub fn set_image_base(base_addr: *mut c_void) {
+            IMAGE_BASE.store(base_addr as _, SeqCst);
+        }
+
+        pub(crate) fn get_image_base() -> *mut c_void {
+            IMAGE_BASE.load(SeqCst) as _
+        }
     }
 
-    pub(crate) fn get_image_base() -> *mut c_void {
-        IMAGE_BASE.load(SeqCst) as _
-    }
-}
-
-#[cfg(all(target_env = "sgx", target_vendor = "fortanix", not(feature = "std")))]
-pub use self::sgx_no_std_image_base::set_image_base;
+    #[cfg(feature = "std")]
+    mod imp {
+        use core::ffi::c_void;
 
-#[cfg(all(target_env = "sgx", target_vendor = "fortanix", not(feature = "std")))]
-#[deny(unused)]
-pub(crate) use self::sgx_no_std_image_base::get_image_base;
+        pub(crate) fn get_image_base() -> *mut c_void {
+            std::os::fortanix_sgx::mem::image_base() as _
+        }
+    }
 
-#[cfg(all(target_env = "sgx", target_vendor = "fortanix", feature = "std"))]
-#[deny(unused)]
-pub(crate) fn get_image_base() -> *mut c_void {
-    std::os::fortanix_sgx::mem::image_base() as _
+    pub(crate) use imp::get_image_base;
 }
 
+#[cfg(all(
+    target_env = "sgx",
+    target_vendor = "fortanix",
+    not(feature = "std"),
+    not(miri)
+))]
+pub use sgx_image_base::imp::set_image_base;
+
 cfg_if::cfg_if! {
     // This needs to come first, to ensure that
     // Miri takes priority over the host platform