Rollup merge of #133681 - RalfJung:niches, r=wesleywiser

improve TagEncoding::Niche docs, sanity check, and UB checks

Turns out the `niche_variants` range can actually contain the `untagged_variant`. We should report this as UB in Miri, so this PR implements that.

Also rename `partially_check_layout` to `layout_sanity_check` for better consistency with how similar functions are called in other parts of the compiler.

Turns out my adjustments to the transmutation logic also fix https://github.com/rust-lang/rust/issues/126267.

3 files changed, 39 insertions, 31 deletions

diff --git a/tests/crashes/126267.rs b/tests/crashes/126267.rs
deleted file mode 100644
index 728578179ed..00000000000
--- a/tests/crashes/126267.rs
+++ /dev/null
@@ -1,30 +0,0 @@
-//@ known-bug: rust-lang/rust#126267
-
-#![feature(transmutability)]
-#![crate_type = "lib"]
-
-pub enum ApiError {}
-pub struct TokioError {
-    b: bool,
-}
-pub enum Error {
-    Api { source: ApiError },
-    Ethereum,
-    Tokio { source: TokioError },
-}
-
-mod assert {
-    use std::mem::TransmuteFrom;
-
-    pub fn is_transmutable<Src, Dst>()
-    where
-        Dst: TransmuteFrom<Src>, // safety is NOT assumed
-    {
-    }
-}
-
-fn test() {
-    struct Src;
-    type Dst = Error;
-    assert::is_transmutable::<Src, Dst>();
-}
diff --git a/tests/ui/transmutability/uninhabited.rs b/tests/ui/transmutability/uninhabited.rs
index 74f7a1a2e89..274104ffb39 100644
--- a/tests/ui/transmutability/uninhabited.rs
+++ b/tests/ui/transmutability/uninhabited.rs
@@ -91,3 +91,19 @@ fn distant_void() {
     assert::is_maybe_transmutable::<DistantVoid, &'static Void>();
     assert::is_maybe_transmutable::<u128, DistantVoid>(); //~ ERROR: cannot be safely transmuted
 }
+
+fn issue_126267() {
+    pub enum ApiError {}
+    pub struct TokioError {
+        b: bool,
+    }
+    pub enum Error {
+        Api { source: ApiError }, // this variant is uninhabited
+        Ethereum,
+        Tokio { source: TokioError },
+    }
+
+    struct Src;
+    type Dst = Error;
+    assert::is_maybe_transmutable::<Src, Dst>(); //~ERROR: cannot be safely transmuted
+}
diff --git a/tests/ui/transmutability/uninhabited.stderr b/tests/ui/transmutability/uninhabited.stderr
index 3fa02f0867c..f112d2fbe44 100644
--- a/tests/ui/transmutability/uninhabited.stderr
+++ b/tests/ui/transmutability/uninhabited.stderr
@@ -110,7 +110,29 @@ LL | |             }
 LL | |         }>
    | |__________^ required by this bound in `is_maybe_transmutable`
 
-error: aborting due to 7 previous errors
+error[E0277]: `Src` cannot be safely transmuted into `issue_126267::Error`
+  --> $DIR/uninhabited.rs:108:42
+   |
+LL |     assert::is_maybe_transmutable::<Src, Dst>();
+   |                                          ^^^ the size of `Src` is smaller than the size of `issue_126267::Error`
+   |
+note: required by a bound in `is_maybe_transmutable`
+  --> $DIR/uninhabited.rs:10:14
+   |
+LL |       pub fn is_maybe_transmutable<Src, Dst>()
+   |              --------------------- required by a bound in this function
+LL |       where
+LL |           Dst: TransmuteFrom<Src, {
+   |  ______________^
+LL | |             Assume {
+LL | |                 alignment: true,
+LL | |                 lifetimes: true,
+...  |
+LL | |             }
+LL | |         }>
+   | |__________^ required by this bound in `is_maybe_transmutable`
+
+error: aborting due to 8 previous errors
 
 Some errors have detailed explanations: E0080, E0277.
 For more information about an error, try `rustc --explain E0080`.