Auto merge of rust-lang#116915 - bend-n:unwet, r=<try>

Use `.get().unwrap()` in `[T]::get_unchecked`

Fixes rust-lang#116878

library/core/src/slice/mod.rs

@@ -662,7 +662,7 @@ impl<T> [T] {
         // SAFETY: the caller must uphold most of the safety requirements for `get_unchecked`;
         // the slice is dereferenceable because `self` is a safe reference.
         // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
-        unsafe { &*index.get_unchecked(self) }
+        unsafe { &*index.get(self).unwrap_unchecked() }
     }
 
     /// Returns a mutable reference to an element or subslice, without doing

src/tools/miri/tests/fail/stacked_borrows/zst_slice.rs

@@ -1,5 +1,5 @@
 //@compile-flags: -Zmiri-strict-provenance
-//@error-in-other-file: /retag .* tag does not exist in the borrow stack/
+//@error-in-other-file: unreachable
 
 fn main() {
     unsafe {

src/tools/miri/tests/fail/stacked_borrows/zst_slice.stderr

@@ -1,20 +1,12 @@
-error: Undefined Behavior: trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
+error: Undefined Behavior: entering unreachable code
   --> RUSTLIB/core/src/slice/mod.rs:LL:CC
    |
-LL |         unsafe { &*index.get_unchecked(self) }
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |                  |
-   |                  trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
-   |                  this error occurs as part of retag at ALLOC[0x4..0x8]
+LL |         unsafe { &*index.get(self).unwrap_unchecked() }
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code
    |
-   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
-   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
-help: <TAG> would have been created here, but this is a zero-size retag ([0x0..0x0]) so the tag in question does not exist anywhere
-  --> $DIR/zst_slice.rs:LL:CC
-   |
-LL |         assert_eq!(*s.get_unchecked(1), 2);
-   |                     ^^^^^^^^^^^^^^^^^^
-   = note: BACKTRACE (of the first span):
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
    = note: inside `core::slice::<impl [i32]>::get_unchecked::<usize>` at RUSTLIB/core/src/slice/mod.rs:LL:CC
 note: inside `main`
   --> $DIR/zst_slice.rs:LL:CC

src/tools/miri/tests/fail/uninit_byte_read.rs

@@ -1,7 +1,7 @@
 //@compile-flags: -Zmiri-disable-stacked-borrows
 fn main() {
     let v: Vec<u8> = Vec::with_capacity(10);
-    let undef = unsafe { *v.get_unchecked(5) }; //~ ERROR: uninitialized
+    let undef = unsafe { *v.as_ptr().add(5) }; //~ ERROR: uninitialized
     let x = undef + 1;
     panic!("this should never print: {}", x);
 }

src/tools/miri/tests/fail/uninit_byte_read.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> $DIR/uninit_byte_read.rs:LL:CC
    |
-LL |     let undef = unsafe { *v.get_unchecked(5) };
-   |                          ^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+LL |     let undef = unsafe { *v.as_ptr().add(5) };
+   |                          ^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

tests/codegen/issues/issue-116878.rs

@@ -0,0 +1,13 @@
+// no-system-llvm
+// compile-flags: -O
+// ignore-debug: the debug assertions get in the way
+#![crate_type = "lib"]
+
+/// Make sure no bounds checks are emitted after a `get_unchecked`.
+// CHECK-LABEL: @unchecked_slice_no_bounds_check
+#[no_mangle]
+pub unsafe fn unchecked_slice_no_bounds_check(s: &[u8]) -> u8 {
+    let a = *s.get_unchecked(1);
+    // CHECK-NOT: panic_bounds_check
+    a + s[0]
+}