Auto merge of #116915 - bend-n:unwet, r=saethlin

Add an assume that the index is inbounds to slice::get_unchecked

Fixes #116878

Author: bors

tests/fail/stacked_borrows/zst_slice.rs

@@ -1,11 +1,10 @@
 //@compile-flags: -Zmiri-strict-provenance
-//@error-in-other-file: /retag .* tag does not exist in the borrow stack/
 
 fn main() {
     unsafe {
         let a = [1, 2, 3];
         let s = &a[0..0];
         assert_eq!(s.len(), 0);
-        assert_eq!(*s.get_unchecked(1), 2);
+        assert_eq!(*s.as_ptr().add(1), 2); //~ ERROR: /retag .* tag does not exist in the borrow stack/
     }
 }

tests/fail/stacked_borrows/zst_slice.stderr

@@ -1,26 +1,22 @@
 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
-  --> RUSTLIB/core/src/slice/mod.rs:LL:CC
+  --> $DIR/zst_slice.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 |         assert_eq!(*s.as_ptr().add(1), 2);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         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]
    |
    = 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);
-   |                     ^^^^^^^^^^^^^^^^^^
+LL |         assert_eq!(*s.as_ptr().add(1), 2);
+   |                     ^^^^^^^^^^
    = note: BACKTRACE (of the first span):
-   = 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
-   |
-LL |         assert_eq!(*s.get_unchecked(1), 2);
-   |                     ^^^^^^^^^^^^^^^^^^
+   = note: inside `main` at RUSTLIB/core/src/macros/mod.rs:LL:CC
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail/uninit/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);
 }

tests/fail/uninit/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/pass/float_nan.rs

@@ -20,8 +20,8 @@ use NaNKind::*;
 #[track_caller]
 fn check_all_outcomes<T: Eq + Hash + fmt::Display>(expected: HashSet<T>, generate: impl Fn() -> T) {
     let mut seen = HashSet::new();
-    // Let's give it 8x as many tries as we are expecting values.
-    let tries = expected.len() * 8;
+    // Let's give it sixteen times as many tries as we are expecting values.
+    let tries = expected.len() * 16;
     for _ in 0..tries {
         let val = generate();
         assert!(expected.contains(&val), "got an unexpected value: {val}");