Rollup merge of rust-lang#121681 - jswrenn:nix-visibility-analysis, r=compiler-errors

Safe Transmute: Revise safety analysis

This PR migrates `BikeshedIntrinsicFrom` to a simplified safety analysis (described [here](rust-lang/project-safe-transmute#15)) that does not rely on analyzing the visibility of types and fields.

The revised analysis treats primitive types as safe, and user-defined types as potentially carrying safety invariants. If Rust gains explicit (un)safe fields, this PR is structured so that it will be fairly easy to thread support for those annotations into the analysis.

Notably, this PR removes the `Context` type parameter from `BikeshedIntrinsicFrom`. Most of the files changed by this PR are just UI tests tweaked to accommodate the removed parameter.

r? `@compiler-errors`

Author: matthiaskrgr

core/src/mem/transmutability.rs

@@ -6,10 +6,10 @@ use crate::marker::ConstParamTy;
 /// any value of type `Self` are safely transmutable into a value of type `Dst`, in a given `Context`,
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
 #[unstable(feature = "transmutability", issue = "99571")]
-#[lang = "transmute_trait"]
+#[cfg_attr(not(bootstrap), lang = "transmute_trait")]
 #[rustc_deny_explicit_impl(implement_via_object = false)]
 #[rustc_coinductive]
-pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
+pub unsafe trait BikeshedIntrinsicFrom<Src, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,
 {
@@ -28,8 +28,9 @@ pub struct Assume {
     /// that violates Rust's memory model.
     pub lifetimes: bool,
 
-    /// When `true`, the compiler assumes that *you* have ensured that it is safe for you to violate the
-    /// type and field privacy of the destination type (and sometimes of the source type, too).
+    /// When `true`, the compiler assumes that *you* have ensured that no
+    /// unsoundness will arise from violating the safety invariants of the
+    /// destination type (and sometimes of the source type, too).
     pub safety: bool,
 
     /// When `true`, the compiler assumes that *you* are ensuring that the source type is actually a valid