Auto merge of #69309 - Dylan-DPC:rollup-gjdqx7l, r=Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - #68705 (Add LinkedList::remove())
 - #68945 (Stabilize Once::is_completed)
 - #68978 (Make integer exponentiation methods unstably const)
 - #69266 (Fix race condition when allocating source files in SourceMap)
 - #69287 (Clean up E0317 explanation)

Failed merges:

r? @ghost

src/liballoc/collections/linked_list.rs

@@ -878,6 +878,52 @@ impl<T> LinkedList<T> {
         unsafe { self.split_off_after_node(split_node, at) }
     }
 
+    /// Removes the element at the given index and returns it.
+    ///
+    /// This operation should compute in O(n) time.
+    ///
+    /// # Panics
+    /// Panics if at >= len
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(linked_list_remove)]
+    /// use std::collections::LinkedList;
+    ///
+    /// let mut d = LinkedList::new();
+    ///
+    /// d.push_front(1);
+    /// d.push_front(2);
+    /// d.push_front(3);
+    ///
+    /// assert_eq!(d.remove(1), 2);
+    /// assert_eq!(d.remove(0), 3);
+    /// assert_eq!(d.remove(0), 1);
+    /// ```
+    #[unstable(feature = "linked_list_remove", issue = "69210")]
+    pub fn remove(&mut self, at: usize) -> T {
+        let len = self.len();
+        assert!(at < len, "Cannot remove at an index outside of the list bounds");
+
+        // Below, we iterate towards the node at the given index, either from
+        // the start or the end, depending on which would be faster.
+        let offset_from_end = len - at - 1;
+        if at <= offset_from_end {
+            let mut cursor = self.cursor_front_mut();
+            for _ in 0..at {
+                cursor.move_next();
+            }
+            cursor.remove_current().unwrap()
+        } else {
+            let mut cursor = self.cursor_back_mut();
+            for _ in 0..offset_from_end {
+                cursor.move_prev();
+            }
+            cursor.remove_current().unwrap()
+        }
+    }
+
     /// Creates an iterator which uses a closure to determine if an element should be removed.
     ///
     /// If the closure returns true, then the element is removed and yielded.

src/libcore/lib.rs

@@ -73,11 +73,14 @@
 #![feature(const_ascii_ctype_on_intrinsics)]
 #![feature(const_alloc_layout)]
 #![feature(const_if_match)]
+#![feature(const_loop)]
 #![feature(const_checked_int_methods)]
 #![feature(const_euclidean_int_methods)]
 #![feature(const_overflowing_int_methods)]
 #![feature(const_saturating_int_methods)]
 #![feature(const_int_unchecked_arith)]
+#![feature(const_int_pow)]
+#![feature(constctlz)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]

src/libcore/num/mod.rs

@@ -8,9 +8,18 @@ use crate::convert::Infallible;
 use crate::fmt;
 use crate::intrinsics;
 use crate::mem;
-use crate::ops;
 use crate::str::FromStr;
 
+// Used because the `?` operator is not allowed in a const context.
+macro_rules! try_opt {
+    ($e:expr) => {
+        match $e {
+            Some(x) => x,
+            None => return None,
+        }
+    };
+}
+
 macro_rules! impl_nonzero_fmt {
     ( #[$stability: meta] ( $( $Trait: ident ),+ ) for $Ty: ident ) => {
         $(
@@ -993,26 +1002,27 @@ $EndFeature, "
 ```"),
 
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn checked_pow(self, mut exp: u32) -> Option<Self> {
+            pub const fn checked_pow(self, mut exp: u32) -> Option<Self> {
                 let mut base = self;
                 let mut acc: Self = 1;
 
                 while exp > 1 {
                     if (exp & 1) == 1 {
-                        acc = acc.checked_mul(base)?;
+                        acc = try_opt!(acc.checked_mul(base));
                     }
                     exp /= 2;
-                    base = base.checked_mul(base)?;
+                    base = try_opt!(base.checked_mul(base));
                 }
 
                 // Deal with the final bit of the exponent separately, since
                 // squaring the base afterwards is not necessary and may cause a
                 // needless overflow.
                 if exp == 1 {
-                    acc = acc.checked_mul(base)?;
+                    acc = try_opt!(acc.checked_mul(base));
                 }
 
                 Some(acc)
@@ -1180,10 +1190,11 @@ assert_eq!(", stringify!($SelfT), "::MIN.saturating_pow(3), ", stringify!($SelfT
 $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn saturating_pow(self, exp: u32) -> Self {
+            pub const fn saturating_pow(self, exp: u32) -> Self {
                 match self.checked_pow(exp) {
                     Some(x) => x,
                     None if self < 0 && exp % 2 == 1 => Self::min_value(),
@@ -1523,10 +1534,11 @@ assert_eq!(3i8.wrapping_pow(6), -39);",
 $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn wrapping_pow(self, mut exp: u32) -> Self {
+            pub const fn wrapping_pow(self, mut exp: u32) -> Self {
                 let mut base = self;
                 let mut acc: Self = 1;
 
@@ -1900,10 +1912,11 @@ assert_eq!(3i8.overflowing_pow(5), (-13, true));",
 $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
+            pub const fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
                 let mut base = self;
                 let mut acc: Self = 1;
                 let mut overflown = false;
@@ -1949,11 +1962,12 @@ assert_eq!(x.pow(5), 32);",
 $EndFeature, "
 ```"),
             #[stable(feature = "rust1", since = "1.0.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
             #[rustc_inherit_overflow_checks]
-            pub fn pow(self, mut exp: u32) -> Self {
+            pub const fn pow(self, mut exp: u32) -> Self {
                 let mut base = self;
                 let mut acc = 1;
 
@@ -3119,26 +3133,27 @@ Basic usage:
 assert_eq!(", stringify!($SelfT), "::max_value().checked_pow(2), None);", $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn checked_pow(self, mut exp: u32) -> Option<Self> {
+            pub const fn checked_pow(self, mut exp: u32) -> Option<Self> {
                 let mut base = self;
                 let mut acc: Self = 1;
 
                 while exp > 1 {
                     if (exp & 1) == 1 {
-                        acc = acc.checked_mul(base)?;
+                        acc = try_opt!(acc.checked_mul(base));
                     }
                     exp /= 2;
-                    base = base.checked_mul(base)?;
+                    base = try_opt!(base.checked_mul(base));
                 }
 
                 // Deal with the final bit of the exponent separately, since
                 // squaring the base afterwards is not necessary and may cause a
                 // needless overflow.
                 if exp == 1 {
-                    acc = acc.checked_mul(base)?;
+                    acc = try_opt!(acc.checked_mul(base));
                 }
 
                 Some(acc)
@@ -3234,10 +3249,11 @@ assert_eq!(", stringify!($SelfT), "::MAX.saturating_pow(2), ", stringify!($SelfT
 $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn saturating_pow(self, exp: u32) -> Self {
+            pub const fn saturating_pow(self, exp: u32) -> Self {
                 match self.checked_pow(exp) {
                     Some(x) => x,
                     None => Self::max_value(),
@@ -3527,10 +3543,11 @@ Basic usage:
 assert_eq!(3u8.wrapping_pow(6), 217);", $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn wrapping_pow(self, mut exp: u32) -> Self {
+            pub const fn wrapping_pow(self, mut exp: u32) -> Self {
                 let mut base = self;
                 let mut acc: Self = 1;
 
@@ -3853,10 +3870,11 @@ Basic usage:
 assert_eq!(3u8.overflowing_pow(6), (217, true));", $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_pow", since = "1.34.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
             #[inline]
-            pub fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
+            pub const fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
                 let mut base = self;
                 let mut acc: Self = 1;
                 let mut overflown = false;
@@ -3899,11 +3917,12 @@ Basic usage:
 ", $Feature, "assert_eq!(2", stringify!($SelfT), ".pow(5), 32);", $EndFeature, "
 ```"),
         #[stable(feature = "rust1", since = "1.0.0")]
+        #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
         #[must_use = "this returns the result of the operation, \
                           without modifying the original"]
         #[inline]
         #[rustc_inherit_overflow_checks]
-        pub fn pow(self, mut exp: u32) -> Self {
+        pub const fn pow(self, mut exp: u32) -> Self {
             let mut base = self;
             let mut acc = 1;
 
@@ -4014,7 +4033,8 @@ assert!(!10", stringify!($SelfT), ".is_power_of_two());", $EndFeature, "
         // overflow cases it instead ends up returning the maximum value
         // of the type, and can return 0 for 0.
         #[inline]
-        fn one_less_than_next_power_of_two(self) -> Self {
+        #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
+        const fn one_less_than_next_power_of_two(self) -> Self {
             if self <= 1 { return 0; }
 
             let p = self - 1;
@@ -4042,10 +4062,11 @@ Basic usage:
 assert_eq!(3", stringify!($SelfT), ".next_power_of_two(), 4);", $EndFeature, "
 ```"),
             #[stable(feature = "rust1", since = "1.0.0")]
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
             #[inline]
-            pub fn next_power_of_two(self) -> Self {
-                // Call the trait to get overflow checks
-                ops::Add::add(self.one_less_than_next_power_of_two(), 1)
+            #[rustc_inherit_overflow_checks]
+            pub const fn next_power_of_two(self) -> Self {
+                self.one_less_than_next_power_of_two() + 1
             }
         }
 
@@ -4067,7 +4088,8 @@ $EndFeature, "
 ```"),
             #[inline]
             #[stable(feature = "rust1", since = "1.0.0")]
-            pub fn checked_next_power_of_two(self) -> Option<Self> {
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
+            pub const fn checked_next_power_of_two(self) -> Option<Self> {
                 self.one_less_than_next_power_of_two().checked_add(1)
             }
         }
@@ -4091,7 +4113,8 @@ $EndFeature, "
 ```"),
             #[unstable(feature = "wrapping_next_power_of_two", issue = "32463",
                        reason = "needs decision on wrapping behaviour")]
-            pub fn wrapping_next_power_of_two(self) -> Self {
+            #[rustc_const_unstable(feature = "const_int_pow", issue = "53718")]
+            pub const fn wrapping_next_power_of_two(self) -> Self {
                 self.one_less_than_next_power_of_two().wrapping_add(1)
             }
         }

src/librustc_error_codes/error_codes/E0317.md

@@ -1,14 +1,30 @@
-This error occurs when an `if` expression without an `else` block is used in a
-context where a type other than `()` is expected, for example a `let`
-expression:
+An `if` expression is missing an `else` block.
+
+Erroneous code example:
 
 ```compile_fail,E0317
-fn main() {
-    let x = 5;
-    let a = if x == 5 { 1 };
-}
+let x = 5;
+let a = if x == 5 {
+    1
+};
 ```
 
+This error occurs when an `if` expression without an `else` block is used in a
+context where a type other than `()` is expected. In the previous code example,
+the `let` expression was expecting a value but since there was no `else`, no
+value was returned.
+
 An `if` expression without an `else` block has the type `()`, so this is a type
 error. To resolve it, add an `else` block having the same type as the `if`
 block.
+
+So to fix the previous code example:
+
+```
+let x = 5;
+let a = if x == 5 {
+    1
+} else {
+    2
+};
+```

src/librustc_span/lib.rs

@@ -1075,7 +1075,7 @@ impl SourceFile {
         unmapped_path: FileName,
         mut src: String,
         start_pos: BytePos,
-    ) -> Result<SourceFile, OffsetOverflowError> {
+    ) -> Self {
         let normalized_pos = normalize_src(&mut src, start_pos);
 
         let src_hash = {
@@ -1089,14 +1089,12 @@ impl SourceFile {
             hasher.finish::<u128>()
         };
         let end_pos = start_pos.to_usize() + src.len();
-        if end_pos > u32::max_value() as usize {
-            return Err(OffsetOverflowError);
-        }
+        assert!(end_pos <= u32::max_value() as usize);
 
         let (lines, multibyte_chars, non_narrow_chars) =
             analyze_source_file::analyze_source_file(&src[..], start_pos);
 
-        Ok(SourceFile {
+        SourceFile {
             name,
             name_was_remapped,
             unmapped_path: Some(unmapped_path),
@@ -1111,7 +1109,7 @@ impl SourceFile {
             non_narrow_chars,
             normalized_pos,
             name_hash,
-        })
+        }
     }
 
     /// Returns the `BytePos` of the beginning of the current line.