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: #85769 (comment). More details about the implementation can be found in the comments.

Note: This could break some sound code as mentioned by RalfJung in #85769 (comment):

> ... 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

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) };
 }

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) };
 }