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

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

Fixes rust-lang#116878

Author: bors

library/core/src/slice/index.rs

@@ -233,7 +233,10 @@ unsafe impl<T> SliceIndex<[T]> for usize {
         // cannot be longer than `isize::MAX`. They also guarantee that
         // `self` is in bounds of `slice` so `self` cannot overflow an `isize`,
         // so the call to `add` is safe.
-        unsafe { slice.as_ptr().add(self) }
+        unsafe {
+            crate::intrinsics::assume(self < slice.len());
+            slice.as_ptr().add(self)
+        }
     }
 
     #[inline]

src/tools/miri/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/
     }
 }

src/tools/miri/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
 

src/tools/miri/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);
 }

src/tools/miri/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

src/tools/miri/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}");

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]
+}