Auto merge of #97013 - matthiaskrgr:rollup-c1pc6pc, r=matthiaskrgr

Rollup of 5 pull requests

Successful merges:

 - #96154 (Expand core::hint::unreachable_unchecked() docs)
 - #96615 (Add a regression test for #54779)
 - #96982 (fix clippy expect_fun_call)
 - #97003 (Remove some unnecessary `rustc_allow_const_fn_unstable` attributes.)
 - #97011 (Add regression test for #28935)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -58,7 +58,9 @@ fn slice_branches<'tcx>(
     ecx: &CompileTimeEvalContext<'tcx, 'tcx>,
     place: &MPlaceTy<'tcx>,
 ) -> Option<ty::ValTree<'tcx>> {
-    let n = place.len(&ecx.tcx.tcx).expect(&format!("expected to use len of place {:?}", place));
+    let n = place
+        .len(&ecx.tcx.tcx)
+        .unwrap_or_else(|_| panic!("expected to use len of place {:?}", place));
     let branches = (0..n).map(|i| {
         let place_elem = ecx.mplace_index(place, i).unwrap();
         const_to_valtree_inner(ecx, &place_elem)

library/alloc/src/raw_vec.rs

@@ -118,7 +118,6 @@ impl<T, A: Allocator> RawVec<T, A> {
 
     /// Like `new`, but parameterized over the choice of allocator for
     /// the returned `RawVec`.
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn new_in(alloc: A) -> Self {
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr: Unique::dangling(), cap: 0, alloc }

library/core/src/hint.rs

@@ -5,27 +5,78 @@
 
 use crate::intrinsics;
 
-/// Informs the compiler that this point in the code is not reachable, enabling
-/// further optimizations.
+/// Informs the compiler that the site which is calling this function is not
+/// reachable, possibly enabling further optimizations.
 ///
 /// # Safety
 ///
-/// Reaching this function is completely *undefined behavior* (UB). In
-/// particular, the compiler assumes that all UB must never happen, and
-/// therefore will eliminate all branches that reach to a call to
-/// `unreachable_unchecked()`.
+/// Reaching this function is *Undefined Behavior*.
 ///
-/// Like all instances of UB, if this assumption turns out to be wrong, i.e., the
-/// `unreachable_unchecked()` call is actually reachable among all possible
-/// control flow, the compiler will apply the wrong optimization strategy, and
-/// may sometimes even corrupt seemingly unrelated code, causing
-/// difficult-to-debug problems.
+/// As the compiler assumes that all forms of Undefined Behavior can never
+/// happen, it will eliminate all branches in the surrounding code that it can
+/// determine will invariably lead to a call to `unreachable_unchecked()`.
 ///
-/// Use this function only when you can prove that the code will never call it.
-/// Otherwise, consider using the [`unreachable!`] macro, which does not allow
-/// optimizations but will panic when executed.
+/// If the assumptions embedded in using this function turn out to be wrong -
+/// that is, if the site which is calling `unreachable_unchecked()` is actually
+/// reachable at runtime - the compiler may have generated nonsensical machine
+/// instructions for this situation, including in seemingly unrelated code,
+/// causing difficult-to-debug problems.
 ///
-/// # Example
+/// Use this function sparingly. Consider using the [`unreachable!`] macro,
+/// which may prevent some optimizations but will safely panic in case it is
+/// actually reached at runtime. Benchmark your code to find out if using
+/// `unreachable_unchecked()` comes with a performance benefit.
+///
+/// # Examples
+///
+/// `unreachable_unchecked()` can be used in situations where the compiler
+/// can't prove invariants that were previously established. Such situations
+/// have a higher chance of occuring if those invariants are upheld by
+/// external code that the compiler can't analyze.
+/// ```
+/// fn prepare_inputs(divisors: &mut Vec<u32>) {
+///     // Note to future-self when making changes: The invariant established
+///     // here is NOT checked in `do_computation()`; if this changes, you HAVE
+///     // to change `do_computation()`.
+///     divisors.retain(|divisor| *divisor != 0)
+/// }
+///
+/// /// # Safety
+/// /// All elements of `divisor` must be non-zero.
+/// unsafe fn do_computation(i: u32, divisors: &[u32]) -> u32 {
+///     divisors.iter().fold(i, |acc, divisor| {
+///         // Convince the compiler that a division by zero can't happen here
+///         // and a check is not needed below.
+///         if *divisor == 0 {
+///             // Safety: `divisor` can't be zero because of `prepare_inputs`,
+///             // but the compiler does not know about this. We *promise*
+///             // that we always call `prepare_inputs`.
+///             std::hint::unreachable_unchecked()
+///         }
+///         // The compiler would normally introduce a check here that prevents
+///         // a division by zero. However, if `divisor` was zero, the branch
+///         // above would reach what we explicitly marked as unreachable.
+///         // The compiler concludes that `divisor` can't be zero at this point
+///         // and removes the - now proven useless - check.
+///         acc / divisor
+///     })
+/// }
+///
+/// let mut divisors = vec![2, 0, 4];
+/// prepare_inputs(&mut divisors);
+/// let result = unsafe {
+///     // Safety: prepare_inputs() guarantees that divisors is non-zero
+///     do_computation(100, &divisors)
+/// };
+/// assert_eq!(result, 12);
+///
+/// ```
+///
+/// While using `unreachable_unchecked()` is perfectly sound in the following
+/// example, the compiler is able to prove that a division by zero is not
+/// possible. Benchmarking reveals that `unreachable_unchecked()` provides
+/// no benefit over using [`unreachable!`], while the latter does not introduce
+/// the possibility of Undefined Behavior.
 ///
 /// ```
 /// fn div_1(a: u32, b: u32) -> u32 {

library/core/src/num/nonzero.rs

@@ -52,7 +52,6 @@ macro_rules! nonzero_integers {
                 #[$const_new_unchecked_stability]
                 #[must_use]
                 #[inline]
-                #[rustc_allow_const_fn_unstable(const_fn_fn_ptr_basics)] // required by assert_unsafe_precondition
                 pub const unsafe fn new_unchecked(n: $Int) -> Self {
                     // SAFETY: this is guaranteed to be safe by the caller.
                     unsafe {

library/core/src/option.rs

@@ -1486,7 +1486,6 @@ impl<T> Option<T> {
     where
         T: ~const Default,
     {
-        #[rustc_allow_const_fn_unstable(const_fn_trait_bound)]
         const fn default<T: ~const Default>() -> T {
             T::default()
         }

library/core/src/task/wake.rs

@@ -147,7 +147,6 @@ impl RawWakerVTable {
     #[rustc_promotable]
     #[stable(feature = "futures_api", since = "1.36.0")]
     #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
-    #[rustc_allow_const_fn_unstable(const_fn_fn_ptr_basics)]
     pub const fn new(
         clone: unsafe fn(*const ()) -> RawWaker,
         wake: unsafe fn(*const ()),

library/proc_macro/src/bridge/client.rs

@@ -416,7 +416,6 @@ fn run_client<A: for<'a, 's> DecodeMut<'a, 's, ()>, R: Encode<()>>(
 }
 
 impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn expand1(f: fn(crate::TokenStream) -> crate::TokenStream) -> Self {
         extern "C" fn run(
             bridge: Bridge<'_>,
@@ -429,7 +428,6 @@ impl Client<fn(crate::TokenStream) -> crate::TokenStream> {
 }
 
 impl Client<fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream> {
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn expand2(
         f: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
     ) -> Self {
@@ -474,7 +472,6 @@ impl ProcMacro {
         }
     }
 
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn custom_derive(
         trait_name: &'static str,
         attributes: &'static [&'static str],
@@ -483,7 +480,6 @@ impl ProcMacro {
         ProcMacro::CustomDerive { trait_name, attributes, client: Client::expand1(expand) }
     }
 
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn attr(
         name: &'static str,
         expand: fn(crate::TokenStream, crate::TokenStream) -> crate::TokenStream,
@@ -491,7 +487,6 @@ impl ProcMacro {
         ProcMacro::Attr { name, client: Client::expand2(expand) }
     }
 
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn bang(
         name: &'static str,
         expand: fn(crate::TokenStream) -> crate::TokenStream,

library/proc_macro/src/bridge/scoped_cell.rs

@@ -35,7 +35,6 @@ impl<'a, 'b, T: LambdaL> DerefMut for RefMutL<'a, 'b, T> {
 pub struct ScopedCell<T: LambdaL>(Cell<<T as ApplyL<'static>>::Out>);
 
 impl<T: LambdaL> ScopedCell<T> {
-    #[rustc_allow_const_fn_unstable(const_fn)]
     pub const fn new(value: <T as ApplyL<'static>>::Out) -> Self {
         ScopedCell(Cell::new(value))
     }

src/test/ui/inference/issue-28935.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+use std::cell::RefCell;
+
+pub fn f(v: Vec<RefCell<u8>>) {
+    let _t = &mut *v[0].borrow_mut();
+}
+
+fn main() {}

src/test/ui/nll/issue-54779-anon-static-lifetime.rs

@@ -0,0 +1,51 @@
+// Regression test for #54779, checks if the diagnostics are confusing.
+
+#![feature(nll)]
+
+trait DebugWith<Cx: ?Sized> {
+    fn debug_with<'me>(&'me self, cx: &'me Cx) -> DebugCxPair<'me, Self, Cx> {
+        DebugCxPair { value: self, cx }
+    }
+
+    fn fmt_with(&self, cx: &Cx, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result;
+}
+
+struct DebugCxPair<'me, Value: ?Sized, Cx: ?Sized>
+where
+    Value: DebugWith<Cx>,
+{
+    value: &'me Value,
+    cx: &'me Cx,
+}
+
+trait DebugContext {}
+
+struct Foo {
+    bar: Bar,
+}
+
+impl DebugWith<dyn DebugContext> for Foo {
+    fn fmt_with(
+        &self,
+        cx: &dyn DebugContext,
+        fmt: &mut std::fmt::Formatter<'_>,
+    ) -> std::fmt::Result {
+        let Foo { bar } = self;
+        bar.debug_with(cx); //~ ERROR: lifetime may not live long enough
+        Ok(())
+    }
+}
+
+struct Bar {}
+
+impl DebugWith<dyn DebugContext> for Bar {
+    fn fmt_with(
+        &self,
+        cx: &dyn DebugContext,
+        fmt: &mut std::fmt::Formatter<'_>,
+    ) -> std::fmt::Result {
+        Ok(())
+    }
+}
+
+fn main() {}

src/test/ui/nll/issue-54779-anon-static-lifetime.stderr

@@ -0,0 +1,11 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-54779-anon-static-lifetime.rs:34:24
+   |
+LL |         cx: &dyn DebugContext,
+   |             - let's call the lifetime of this reference `'1`
+...
+LL |         bar.debug_with(cx);
+   |                        ^^ cast requires that `'1` must outlive `'static`
+
+error: aborting due to previous error
+