use .get().unwrap() in [T]::get_unchecked

Author: bend-n

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.stderr

@@ -1,26 +1,17 @@
-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
+error: Undefined Behavior: entering unreachable code
+##[error]  --> /checkout/library/core/src/slice/mod.rs:665:20
    |
-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]
-   |
-   = 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):
-   = note: inside `core::slice::<impl [i32]>::get_unchecked::<usize>` at RUSTLIB/core/src/slice/mod.rs:LL:CC
+LL |         unsafe { &*index.get(self).unwrap_unchecked() }
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code
+   = 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 /checkout/library/core/src/slice/mod.rs:665:20: 665:54
+   = note: inside `core::slice::<impl [i32]>::get_unchecked::<usize>` at /checkout/library/core/src/slice/mod.rs:665:20: 665:54
 note: inside `main`
-  --> $DIR/zst_slice.rs:LL:CC
+  --> tests/fail/stacked_borrows/zst_slice.rs:9:21
    |
 LL |         assert_eq!(*s.get_unchecked(1), 2);
-   |                     ^^^^^^^^^^^^^^^^^^
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

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.get_unchecked(5) }; //~ ERROR: unreachable
     let x = undef + 1;
     panic!("this should never print: {}", x);
 }

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

@@ -1,13 +1,17 @@
-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
+error: Undefined Behavior: entering unreachable code
+##[error]  --> /checkout/library/core/src/slice/mod.rs:665:20
    |
+LL |         unsafe { &*index.get(self).unwrap_unchecked() }
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code
    = 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 `main` at $DIR/uninit_byte_read.rs:LL:CC
+   = note: inside `core::slice::<impl [u8]>::get_unchecked::<usize>` at /checkout/library/core/src/slice/mod.rs:665:20: 665:54
+   = note: inside `core::slice::<impl [u8]>::get_unchecked::<usize>` at /checkout/library/core/src/slice/mod.rs:665:20: 665:54
+note: inside `main`
+  --> tests/fail/uninit_byte_read.rs:4:27
+   |
+LL |     let undef = unsafe { *v.get_unchecked(5) };
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

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]
+}