use transmute for ptr::invalid

.github/workflows/rust.yml

@@ -32,3 +32,15 @@ jobs:
       run: cargo build --verbose
     - name: Run tests
       run: cargo test --verbose
+
+  miri:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Miri
+        run: |
+          rustup toolchain install nightly --component miri
+          rustup override set nightly
+          cargo miri setup
+      - name: Test with Miri
+        run: cargo miri test

src/lib.rs

@@ -383,7 +383,15 @@
 #[must_use]
 pub const fn invalid<T>(addr: usize) -> *const T {
     // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
-    addr as *const T
+    // We use transmute rather than a cast so tools like Miri can tell that this
+    // is *not* the same as from_exposed_addr.
+    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
+    // pointer).
+    #[cfg(miri)]
+    return unsafe { core::mem::transmute(addr) };
+    // Outside Miri we keep using casts, so that we can be a `const fn` on old Rust (pre-1.56).
+    #[cfg(not(miri))]
+    return addr as *const T;
 }
 
 /// Creates an invalid mutable pointer with the given address.
@@ -408,7 +416,15 @@ pub const fn invalid<T>(addr: usize) -> *const T {
 #[must_use]
 pub const fn invalid_mut<T>(addr: usize) -> *mut T {
     // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
-    addr as *mut T
+    // We use transmute rather than a cast so tools like Miri can tell that this
+    // is *not* the same as from_exposed_addr.
+    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
+    // pointer).
+    #[cfg(miri)]
+    return unsafe { core::mem::transmute(addr) };
+    // Outside Miri we keep using casts, so that we can be a `const fn` on old Rust (pre-1.56).
+    #[cfg(not(miri))]
+    return addr as *mut T;
 }
 
 /// Convert an address back to a pointer, picking up a previously 'exposed' provenance.