Auto merge of #150214 - cuviper:beta-next, r=cuviper

[beta] backports

- attempt to fix unreachable code regression #149664
- Update to LLVM 21.1.8 #150057
- Revert #148937 -- #150096

r? cuviper

Author: bors

compiler/rustc_mir_build/src/builder/mod.rs

@@ -839,6 +839,25 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.infcx.typing_env(self.param_env),
             );
 
+            // check if the function's return type is inhabited
+            // this was added here because of this regression
+            // https://github.com/rust-lang/rust/issues/149571
+            let output_is_inhabited =
+                if matches!(self.tcx.def_kind(self.def_id), DefKind::Fn | DefKind::AssocFn) {
+                    self.tcx
+                        .fn_sig(self.def_id)
+                        .instantiate_identity()
+                        .skip_binder()
+                        .output()
+                        .is_inhabited_from(
+                            self.tcx,
+                            self.parent_module,
+                            self.infcx.typing_env(self.param_env),
+                        )
+                } else {
+                    true
+                };
+
             if !ty_is_inhabited {
                 // Unreachable code warnings are already emitted during type checking.
                 // However, during type checking, full type information is being
@@ -849,7 +868,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // uninhabited types (e.g. empty enums). The check above is used so
                 // that we do not emit the same warning twice if the uninhabited type
                 // is indeed `!`.
-                if !ty.is_never() {
+                if !ty.is_never() && output_is_inhabited {
                     lints.push((target_bb, ty, term.source_info.span));
                 }
 

library/core/src/io/borrowed_buf.rs

@@ -2,38 +2,42 @@
 
 use crate::fmt::{self, Debug, Formatter};
 use crate::mem::{self, MaybeUninit};
+use crate::{cmp, ptr};
 
-/// A borrowed buffer of initially uninitialized bytes, which is incrementally filled.
+/// A borrowed byte buffer which is incrementally filled and initialized.
 ///
-/// This type makes it safer to work with `MaybeUninit` buffers, such as to read into a buffer
-/// without having to initialize it first. It tracks the region of bytes that have been filled and
-/// the region that remains uninitialized.
+/// This type is a sort of "double cursor". It tracks three regions in the buffer: a region at the beginning of the
+/// buffer that has been logically filled with data, a region that has been initialized at some point but not yet
+/// logically filled, and a region at the end that is fully uninitialized. The filled region is guaranteed to be a
+/// subset of the initialized region.
 ///
-/// The contents of the buffer can be visualized as:
+/// In summary, the contents of the buffer can be visualized as:
 /// ```not_rust
-/// [                         capacity                            ]
-/// [ len: filled and initialized | capacity - len: uninitialized ]
+/// [             capacity              ]
+/// [ filled |         unfilled         ]
+/// [    initialized    | uninitialized ]
 /// ```
 ///
-/// Note that `BorrowedBuf` does not distinguish between uninitialized data and data that was
-/// previously initialized but no longer contains valid data.
-///
-/// A `BorrowedBuf` is created around some existing data (or capacity for data) via a unique
-/// reference (`&mut`). The `BorrowedBuf` can be configured (e.g., using `clear` or `set_len`), but
-/// cannot be directly written. To write into the buffer, use `unfilled` to create a
-/// `BorrowedCursor`. The cursor has write-only access to the unfilled portion of the buffer.
+/// A `BorrowedBuf` is created around some existing data (or capacity for data) via a unique reference
+/// (`&mut`). The `BorrowedBuf` can be configured (e.g., using `clear` or `set_init`), but cannot be
+/// directly written. To write into the buffer, use `unfilled` to create a `BorrowedCursor`. The cursor
+/// has write-only access to the unfilled portion of the buffer (you can think of it as a
+/// write-only iterator).
 ///
 /// The lifetime `'data` is a bound on the lifetime of the underlying data.
 pub struct BorrowedBuf<'data> {
     /// The buffer's underlying data.
     buf: &'data mut [MaybeUninit<u8>],
     /// The length of `self.buf` which is known to be filled.
     filled: usize,
+    /// The length of `self.buf` which is known to be initialized.
+    init: usize,
 }
 
 impl Debug for BorrowedBuf<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         f.debug_struct("BorrowedBuf")
+            .field("init", &self.init)
             .field("filled", &self.filled)
             .field("capacity", &self.capacity())
             .finish()
@@ -44,22 +48,24 @@ impl Debug for BorrowedBuf<'_> {
 impl<'data> From<&'data mut [u8]> for BorrowedBuf<'data> {
     #[inline]
     fn from(slice: &'data mut [u8]) -> BorrowedBuf<'data> {
+        let len = slice.len();
+
         BorrowedBuf {
-            // SAFETY: Always in bounds. We treat the buffer as uninitialized, even though it's
-            // already initialized.
+            // SAFETY: initialized data never becoming uninitialized is an invariant of BorrowedBuf
             buf: unsafe { (slice as *mut [u8]).as_uninit_slice_mut().unwrap() },
             filled: 0,
+            init: len,
         }
     }
 }
 
 /// Creates a new `BorrowedBuf` from an uninitialized buffer.
 ///
-/// Use `set_filled` if part of the buffer is known to be already filled.
+/// Use `set_init` if part of the buffer is known to be already initialized.
 impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data> {
     #[inline]
     fn from(buf: &'data mut [MaybeUninit<u8>]) -> BorrowedBuf<'data> {
-        BorrowedBuf { buf, filled: 0 }
+        BorrowedBuf { buf, filled: 0, init: 0 }
     }
 }
 
@@ -68,11 +74,14 @@ impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data> {
 /// Use `BorrowedCursor::with_unfilled_buf` instead for a safer alternative.
 impl<'data> From<BorrowedCursor<'data>> for BorrowedBuf<'data> {
     #[inline]
-    fn from(buf: BorrowedCursor<'data>) -> BorrowedBuf<'data> {
+    fn from(mut buf: BorrowedCursor<'data>) -> BorrowedBuf<'data> {
+        let init = buf.init_mut().len();
         BorrowedBuf {
-            // SAFETY: Always in bounds. We treat the buffer as uninitialized.
+            // SAFETY: no initialized byte is ever uninitialized as per
+            // `BorrowedBuf`'s invariant
             buf: unsafe { buf.buf.buf.get_unchecked_mut(buf.buf.filled..) },
             filled: 0,
+            init,
         }
     }
 }
@@ -90,6 +99,12 @@ impl<'data> BorrowedBuf<'data> {
         self.filled
     }
 
+    /// Returns the length of the initialized part of the buffer.
+    #[inline]
+    pub fn init_len(&self) -> usize {
+        self.init
+    }
+
     /// Returns a shared reference to the filled portion of the buffer.
     #[inline]
     pub fn filled(&self) -> &[u8] {
@@ -144,16 +159,33 @@ impl<'data> BorrowedBuf<'data> {
 
     /// Clears the buffer, resetting the filled region to empty.
     ///
-    /// The contents of the buffer are not modified.
+    /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
     #[inline]
     pub fn clear(&mut self) -> &mut Self {
         self.filled = 0;
         self
     }
+
+    /// Asserts that the first `n` bytes of the buffer are initialized.
+    ///
+    /// `BorrowedBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
+    /// bytes than are already known to be initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the first `n` unfilled bytes of the buffer have already been initialized.
+    #[inline]
+    pub unsafe fn set_init(&mut self, n: usize) -> &mut Self {
+        self.init = cmp::max(self.init, n);
+        self
+    }
 }
 
 /// A writeable view of the unfilled portion of a [`BorrowedBuf`].
 ///
+/// The unfilled portion consists of an initialized and an uninitialized part; see [`BorrowedBuf`]
+/// for details.
+///
 /// Data can be written directly to the cursor by using [`append`](BorrowedCursor::append) or
 /// indirectly by getting a slice of part or all of the cursor and writing into the slice. In the
 /// indirect case, the caller must call [`advance`](BorrowedCursor::advance) after writing to inform
@@ -206,17 +238,48 @@ impl<'a> BorrowedCursor<'a> {
         self.buf.filled
     }
 
+    /// Returns a mutable reference to the initialized portion of the cursor.
+    #[inline]
+    pub fn init_mut(&mut self) -> &mut [u8] {
+        // SAFETY: We only slice the initialized part of the buffer, which is always valid
+        unsafe {
+            let buf = self.buf.buf.get_unchecked_mut(self.buf.filled..self.buf.init);
+            buf.assume_init_mut()
+        }
+    }
+
     /// Returns a mutable reference to the whole cursor.
     ///
     /// # Safety
     ///
-    /// The caller must not uninitialize any previously initialized bytes.
+    /// The caller must not uninitialize any bytes in the initialized portion of the cursor.
     #[inline]
     pub unsafe fn as_mut(&mut self) -> &mut [MaybeUninit<u8>] {
         // SAFETY: always in bounds
         unsafe { self.buf.buf.get_unchecked_mut(self.buf.filled..) }
     }
 
+    /// Advances the cursor by asserting that `n` bytes have been filled.
+    ///
+    /// After advancing, the `n` bytes are no longer accessible via the cursor and can only be
+    /// accessed via the underlying buffer. I.e., the buffer's filled portion grows by `n` elements
+    /// and its unfilled portion (and the capacity of this cursor) shrinks by `n` elements.
+    ///
+    /// If less than `n` bytes initialized (by the cursor's point of view), `set_init` should be
+    /// called first.
+    ///
+    /// # Panics
+    ///
+    /// Panics if there are less than `n` bytes initialized.
+    #[inline]
+    pub fn advance(&mut self, n: usize) -> &mut Self {
+        // The subtraction cannot underflow by invariant of this type.
+        assert!(n <= self.buf.init - self.buf.filled);
+
+        self.buf.filled += n;
+        self
+    }
+
     /// Advances the cursor by asserting that `n` bytes have been filled.
     ///
     /// After advancing, the `n` bytes are no longer accessible via the cursor and can only be
@@ -225,11 +288,42 @@ impl<'a> BorrowedCursor<'a> {
     ///
     /// # Safety
     ///
-    /// The caller must ensure that the first `n` bytes of the cursor have been initialized. `n`
-    /// must not exceed the remaining capacity of this cursor.
+    /// The caller must ensure that the first `n` bytes of the cursor have been properly
+    /// initialised.
     #[inline]
-    pub unsafe fn advance(&mut self, n: usize) -> &mut Self {
+    pub unsafe fn advance_unchecked(&mut self, n: usize) -> &mut Self {
         self.buf.filled += n;
+        self.buf.init = cmp::max(self.buf.init, self.buf.filled);
+        self
+    }
+
+    /// Initializes all bytes in the cursor.
+    #[inline]
+    pub fn ensure_init(&mut self) -> &mut Self {
+        // SAFETY: always in bounds and we never uninitialize these bytes.
+        let uninit = unsafe { self.buf.buf.get_unchecked_mut(self.buf.init..) };
+
+        // SAFETY: 0 is a valid value for MaybeUninit<u8> and the length matches the allocation
+        // since it is comes from a slice reference.
+        unsafe {
+            ptr::write_bytes(uninit.as_mut_ptr(), 0, uninit.len());
+        }
+        self.buf.init = self.buf.capacity();
+
+        self
+    }
+
+    /// Asserts that the first `n` unfilled bytes of the cursor are initialized.
+    ///
+    /// `BorrowedBuf` assumes that bytes are never de-initialized, so this method does nothing when
+    /// called with fewer bytes than are already known to be initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the first `n` bytes of the buffer have already been initialized.
+    #[inline]
+    pub unsafe fn set_init(&mut self, n: usize) -> &mut Self {
+        self.buf.init = cmp::max(self.buf.init, self.buf.filled + n);
         self
     }
 
@@ -247,6 +341,10 @@ impl<'a> BorrowedCursor<'a> {
             self.as_mut()[..buf.len()].write_copy_of_slice(buf);
         }
 
+        // SAFETY: We just added the entire contents of buf to the filled section.
+        unsafe {
+            self.set_init(buf.len());
+        }
         self.buf.filled += buf.len();
     }
 
@@ -269,9 +367,17 @@ impl<'a> BorrowedCursor<'a> {
         // there, one could mark some bytes as initialized even though there aren't.
         assert!(core::ptr::addr_eq(prev_ptr, buf.buf));
 
-        // SAFETY: These bytes were filled in the `BorrowedBuf`, so they're filled in the cursor
-        // too, because the buffer wasn't replaced.
-        self.buf.filled += buf.filled;
+        let filled = buf.filled;
+        let init = buf.init;
+
+        // Update `init` and `filled` fields with what was written to the buffer.
+        // `self.buf.filled` was the starting length of the `BorrowedBuf`.
+        //
+        // SAFETY: These amounts of bytes were initialized/filled in the `BorrowedBuf`,
+        // and therefore they are initialized/filled in the cursor too, because the
+        // buffer wasn't replaced.
+        self.buf.init = self.buf.filled + init;
+        self.buf.filled += filled;
 
         res
     }