Rollup merge of #144706 - zachs18:fix-144661, r=RalfJung

Do not give function allocations alignment in consteval and Miri.

We do not yet have a (clear and T-lang approved) design for how `#[align(N)]` on functions should affect function pointers' addresses on various platforms, so for now do not give function pointers alignment in consteval and Miri.

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Old summary:

Not a full solution to <#144661>, but fixes the immediate issue by making function allocations all have alignment 1 in consteval, ignoring `#[rustc_align(N)]`, so the compiler doesn't know if any offset other than 0 is non-null.

A more "principlied" solution would probably be to make function pointers to `#[instruction_set(arm::t32)]` functions be at offset 1 of an align-`max(2, align attribute)` allocation instead of at offset 0 of their allocation during consteval, and on wasm to either disallow `#[align(N)]` where N > 1, or to pad the function table such that the function pointer of a `#[align(N)]` function is a multiple of `N` at runtime.

Author: Zalathar

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -937,8 +937,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         // (both global from `alloc_map` and local from `extra_fn_ptr_map`)
         if let Some(fn_val) = self.get_fn_alloc(id) {
             let align = match fn_val {
-                FnVal::Instance(instance) => {
-                    self.tcx.codegen_instance_attrs(instance.def).alignment.unwrap_or(Align::ONE)
+                FnVal::Instance(_instance) => {
+                    // FIXME: Until we have a clear design for the effects of align(N) functions
+                    // on the address of function pointers, we don't consider the align(N)
+                    // attribute on functions in the interpreter.
+                    // See <https://github.com/rust-lang/rust/issues/144661> for more context.
+                    Align::ONE
                 }
                 // Machine-specific extra functions currently do not support alignment restrictions.
                 FnVal::Other(_) => Align::ONE,