Auto merge of #130540 - veera-sivarajan:fix-87525, r=estebank

Add a Lint for Pointer to Integer Transmutes in Consts

Fixes #87525

This PR adds a MirLint for pointer to integer transmutes in const functions and associated consts. The implementation closely follows this comment: https://github.com/rust-lang/rust/pull/85769#issuecomment-880969112. More details about the implementation can be found in the comments.

Note: This could break some sound code as mentioned by RalfJung in https://github.com/rust-lang/rust/pull/85769#issuecomment-886491680:

> ... technically const-code could transmute/cast an int to a ptr and then transmute it back and that would be correct -- so the lint will deny some sound code. Does not seem terribly likely though.

References:
1. https://doc.rust-lang.org/std/mem/fn.transmute.html
2. https://doc.rust-lang.org/reference/items/associated-items.html#associated-constants

2 files changed, 12 insertions, 6 deletions

diff --git a/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.fixed b/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.fixed
index e95054a7ccb..617d32d1fa7 100644
--- a/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.fixed
+++ b/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.fixed
@@ -84,8 +84,11 @@ fn issue_10449() {
 }
 
 // Pointers cannot be cast to integers in const contexts
+#[allow(ptr_to_integer_transmute_in_consts, reason = "This is tested in the compiler test suite")]
 const fn issue_12402<P>(ptr: *const P) {
-    unsafe { transmute::<*const i32, usize>(&42i32) };
-    unsafe { transmute::<fn(*const P), usize>(issue_12402) };
-    let _ = unsafe { transmute::<_, usize>(ptr) };
+    // This test exists even though the compiler lints against it
+    // to test that clippy's transmute lints do not trigger on this.
+    unsafe { std::mem::transmute::<*const i32, usize>(&42i32) };
+    unsafe { std::mem::transmute::<fn(*const P), usize>(issue_12402) };
+    let _ = unsafe { std::mem::transmute::<_, usize>(ptr) };
 }
diff --git a/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.rs b/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.rs
index e5fcdef7a1c..d68db3c2deb 100644
--- a/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.rs
+++ b/src/tools/clippy/tests/ui/transmutes_expressible_as_ptr_casts.rs
@@ -84,8 +84,11 @@ fn issue_10449() {
 }
 
 // Pointers cannot be cast to integers in const contexts
+#[allow(ptr_to_integer_transmute_in_consts, reason = "This is tested in the compiler test suite")]
 const fn issue_12402<P>(ptr: *const P) {
-    unsafe { transmute::<*const i32, usize>(&42i32) };
-    unsafe { transmute::<fn(*const P), usize>(issue_12402) };
-    let _ = unsafe { transmute::<_, usize>(ptr) };
+    // This test exists even though the compiler lints against it
+    // to test that clippy's transmute lints do not trigger on this.
+    unsafe { std::mem::transmute::<*const i32, usize>(&42i32) };
+    unsafe { std::mem::transmute::<fn(*const P), usize>(issue_12402) };
+    let _ = unsafe { std::mem::transmute::<_, usize>(ptr) };
 }