Auto merge of #130847 - matthiaskrgr:rollup-f0n80bw, r=matthiaskrgr

Rollup of 6 pull requests

Successful merges:

 - #130735 (Simple validation for unsize coercion in MIR validation)
 - #130781 (Fix up setting strip = true in Cargo.toml makes build scripts fail in…)
 - #130811 (add link from random() helper fn to extensive DefaultRandomSource docs)
 - #130819 (Add `must_use` attribute to `len_utf8` and `len_utf16`.)
 - #130832 (fix some cfg logic around optimize_for_size and 16-bit targets)
 - #130842 (Add tracking issue for io_error_inprogress)

r? `@ghost`
`@rustbot` modify labels: rollup

2 files changed, 26 insertions, 5 deletions

diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 892dfb91201..69693230ce0 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1087,16 +1087,17 @@ fn link_natively(
     let strip = sess.opts.cg.strip;
 
     if sess.target.is_like_osx {
+        let stripcmd = "/usr/bin/strip";
         match (strip, crate_type) {
             (Strip::Debuginfo, _) => {
-                strip_symbols_with_external_utility(sess, "strip", out_filename, Some("-S"))
+                strip_symbols_with_external_utility(sess, stripcmd, out_filename, Some("-S"))
             }
             // Per the manpage, `-x` is the maximum safe strip level for dynamic libraries. (#93988)
             (Strip::Symbols, CrateType::Dylib | CrateType::Cdylib | CrateType::ProcMacro) => {
-                strip_symbols_with_external_utility(sess, "strip", out_filename, Some("-x"))
+                strip_symbols_with_external_utility(sess, stripcmd, out_filename, Some("-x"))
             }
             (Strip::Symbols, _) => {
-                strip_symbols_with_external_utility(sess, "strip", out_filename, None)
+                strip_symbols_with_external_utility(sess, stripcmd, out_filename, None)
             }
             (Strip::None, _) => {}
         }
diff --git a/compiler/rustc_codegen_ssa/src/base.rs b/compiler/rustc_codegen_ssa/src/base.rs
index fcf48d3e4a3..5c67600e4ee 100644
--- a/compiler/rustc_codegen_ssa/src/base.rs
+++ b/compiler/rustc_codegen_ssa/src/base.rs
@@ -125,8 +125,28 @@ fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
             if data_a.principal_def_id() == data_b.principal_def_id() {
-                // A NOP cast that doesn't actually change anything, should be allowed even with
-                // invalid vtables.
+                // Codegen takes advantage of the additional assumption, where if the
+                // principal trait def id of what's being casted doesn't change,
+                // then we don't need to adjust the vtable at all. This
+                // corresponds to the fact that `dyn Tr<A>: Unsize<dyn Tr<B>>`
+                // requires that `A = B`; we don't allow *upcasting* objects
+                // between the same trait with different args. If we, for
+                // some reason, were to relax the `Unsize` trait, it could become
+                // unsound, so let's assert here that the trait refs are *equal*.
+                //
+                // We can use `assert_eq` because the binders should have been anonymized,
+                // and because higher-ranked equality now requires the binders are equal.
+                debug_assert_eq!(
+                    data_a.principal(),
+                    data_b.principal(),
+                    "NOP unsize vtable changed principal trait ref: {data_a} -> {data_b}"
+                );
+
+                // A NOP cast that doesn't actually change anything, let's avoid any
+                // unnecessary work. This relies on the assumption that if the principal
+                // traits are equal, then the associated type bounds (`dyn Trait<Assoc=T>`)
+                // are also equal, which is ensured by the fact that normalization is
+                // a function and we do not allow overlapping impls.
                 return old_info;
             }