Rollup merge of #128399 - mammothbane:master, r=Amanieu,tgross35

liballoc: introduce String, Vec const-slicing

This change `const`-qualifies many methods on `Vec` and `String`, notably `as_slice`, `as_str`, `len`. These changes are made behind the unstable feature flag `const_vec_string_slice`.

## Motivation
This is to support simultaneous variance over ownership and constness. I have an enum type that may contain either `String` or `&str`, and I want to produce a `&str` from it in a possibly-`const` context.

```rust
enum StrOrString<'s> {
    Str(&'s str),
    String(String),
}

impl<'s> StrOrString<'s> {
    const fn as_str(&self) -> &str {
        match self {
             // In a const-context, I really only expect to see this variant, but I can't switch the implementation
             // in some mode like #[cfg(const)] -- there has to be a single body
             Self::Str(s) => s,

             // so this is a problem, since it's not `const`
             Self::String(s) => s.as_str(),
        }
    }
}
```

Currently `String` and `Vec` don't support this, but can without functional changes. Similar logic applies for `len`, `capacity`, `is_empty`.

## Changes

The essential thing enabling this change is that `Unique::as_ptr` is `const`. This lets us convert `RawVec::ptr` -> `Vec::as_ptr` -> `Vec::as_slice` -> `String::as_str`.

I had to move the `Deref` implementations into `as_{str,slice}` because `Deref` isn't `#[const_trait]`, but I would expect this change to be invisible up to inlining. I moved the `DerefMut` implementations as well for uniformity.

4 files changed, 88 insertions, 106 deletions

diff --git a/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir b/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir
index 0ad7f5910a0..4d964b0afb7 100644
--- a/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir
+++ b/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-abort.mir
@@ -5,66 +5,61 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
     let mut _0: &[u8];
     scope 1 (inlined <Vec<u8> as Deref>::deref) {
         debug self => _1;
-        let mut _6: usize;
-        scope 2 (inlined Vec::<u8>::as_ptr) {
+        scope 2 (inlined Vec::<u8>::as_slice) {
             debug self => _1;
-            let mut _2: &alloc::raw_vec::RawVec<u8>;
-            scope 3 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
-                debug self => _2;
-                let mut _3: &alloc::raw_vec::RawVecInner;
-                scope 4 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
-                    debug self => _3;
-                    scope 5 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
+            let mut _6: usize;
+            scope 3 (inlined Vec::<u8>::as_ptr) {
+                debug self => _1;
+                let mut _2: &alloc::raw_vec::RawVec<u8>;
+                scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
+                    debug self => _2;
+                    let mut _3: &alloc::raw_vec::RawVecInner;
+                    scope 5 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
                         debug self => _3;
-                        let mut _4: std::ptr::NonNull<u8>;
-                        scope 6 (inlined Unique::<u8>::cast::<u8>) {
-                            debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                            debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                            scope 7 (inlined NonNull::<u8>::cast::<u8>) {
-                                debug self => _4;
-                                scope 8 (inlined NonNull::<u8>::as_ptr) {
+                        scope 6 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
+                            debug self => _3;
+                            let mut _4: std::ptr::NonNull<u8>;
+                            scope 7 (inlined Unique::<u8>::cast::<u8>) {
+                                debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
+                                debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
+                                scope 8 (inlined NonNull::<u8>::cast::<u8>) {
                                     debug self => _4;
-                                    let mut _5: *const u8;
+                                    scope 9 (inlined NonNull::<u8>::as_ptr) {
+                                        debug self => _4;
+                                        let mut _5: *const u8;
+                                    }
                                 }
                             }
-                        }
-                        scope 9 (inlined #[track_caller] <Unique<u8> as Into<NonNull<u8>>>::into) {
-                            debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                            debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                            scope 10 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
-                                debug ((unique: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                                debug ((unique: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                                scope 11 (inlined Unique::<u8>::as_non_null_ptr) {
-                                    debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                                    debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                                }
+                            scope 10 (inlined Unique::<u8>::as_non_null_ptr) {
+                                debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
+                                debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
                             }
                         }
+                        scope 11 (inlined NonNull::<u8>::as_ptr) {
+                            debug self => _4;
+                        }
                     }
-                    scope 12 (inlined NonNull::<u8>::as_ptr) {
-                        debug self => _4;
-                    }
-                }
-            }
-        }
-        scope 13 (inlined std::slice::from_raw_parts::<'_, u8>) {
-            debug data => _5;
-            debug len => _6;
-            let _7: *const [u8];
-            scope 14 (inlined core::ub_checks::check_language_ub) {
-                scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
                 }
             }
-            scope 16 (inlined std::mem::size_of::<u8>) {
-            }
-            scope 17 (inlined align_of::<u8>) {
-            }
-            scope 18 (inlined slice_from_raw_parts::<u8>) {
+            scope 12 (inlined std::slice::from_raw_parts::<'_, u8>) {
                 debug data => _5;
                 debug len => _6;
-                scope 19 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
-                    debug data_pointer => _5;
-                    debug metadata => _6;
+                let _7: *const [u8];
+                scope 13 (inlined core::ub_checks::check_language_ub) {
+                    scope 14 (inlined core::ub_checks::check_language_ub::runtime) {
+                    }
+                }
+                scope 15 (inlined std::mem::size_of::<u8>) {
+                }
+                scope 16 (inlined align_of::<u8>) {
+                }
+                scope 17 (inlined slice_from_raw_parts::<u8>) {
+                    debug data => _5;
+                    debug len => _6;
+                    scope 18 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
+                        debug data_pointer => _5;
+                        debug metadata => _6;
+                    }
                 }
             }
         }
diff --git a/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-unwind.mir
index 0ad7f5910a0..4d964b0afb7 100644
--- a/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-unwind.mir
+++ b/tests/mir-opt/pre-codegen/vec_deref.vec_deref_to_slice.PreCodegen.after.panic-unwind.mir
@@ -5,66 +5,61 @@ fn vec_deref_to_slice(_1: &Vec<u8>) -> &[u8] {
     let mut _0: &[u8];
     scope 1 (inlined <Vec<u8> as Deref>::deref) {
         debug self => _1;
-        let mut _6: usize;
-        scope 2 (inlined Vec::<u8>::as_ptr) {
+        scope 2 (inlined Vec::<u8>::as_slice) {
             debug self => _1;
-            let mut _2: &alloc::raw_vec::RawVec<u8>;
-            scope 3 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
-                debug self => _2;
-                let mut _3: &alloc::raw_vec::RawVecInner;
-                scope 4 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
-                    debug self => _3;
-                    scope 5 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
+            let mut _6: usize;
+            scope 3 (inlined Vec::<u8>::as_ptr) {
+                debug self => _1;
+                let mut _2: &alloc::raw_vec::RawVec<u8>;
+                scope 4 (inlined alloc::raw_vec::RawVec::<u8>::ptr) {
+                    debug self => _2;
+                    let mut _3: &alloc::raw_vec::RawVecInner;
+                    scope 5 (inlined alloc::raw_vec::RawVecInner::ptr::<u8>) {
                         debug self => _3;
-                        let mut _4: std::ptr::NonNull<u8>;
-                        scope 6 (inlined Unique::<u8>::cast::<u8>) {
-                            debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                            debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                            scope 7 (inlined NonNull::<u8>::cast::<u8>) {
-                                debug self => _4;
-                                scope 8 (inlined NonNull::<u8>::as_ptr) {
+                        scope 6 (inlined alloc::raw_vec::RawVecInner::non_null::<u8>) {
+                            debug self => _3;
+                            let mut _4: std::ptr::NonNull<u8>;
+                            scope 7 (inlined Unique::<u8>::cast::<u8>) {
+                                debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
+                                debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
+                                scope 8 (inlined NonNull::<u8>::cast::<u8>) {
                                     debug self => _4;
-                                    let mut _5: *const u8;
+                                    scope 9 (inlined NonNull::<u8>::as_ptr) {
+                                        debug self => _4;
+                                        let mut _5: *const u8;
+                                    }
                                 }
                             }
-                        }
-                        scope 9 (inlined #[track_caller] <Unique<u8> as Into<NonNull<u8>>>::into) {
-                            debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                            debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                            scope 10 (inlined <NonNull<u8> as From<Unique<u8>>>::from) {
-                                debug ((unique: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                                debug ((unique: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                                scope 11 (inlined Unique::<u8>::as_non_null_ptr) {
-                                    debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
-                                    debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
-                                }
+                            scope 10 (inlined Unique::<u8>::as_non_null_ptr) {
+                                debug ((self: Unique<u8>).0: std::ptr::NonNull<u8>) => _4;
+                                debug ((self: Unique<u8>).1: std::marker::PhantomData<u8>) => const PhantomData::<u8>;
                             }
                         }
+                        scope 11 (inlined NonNull::<u8>::as_ptr) {
+                            debug self => _4;
+                        }
                     }
-                    scope 12 (inlined NonNull::<u8>::as_ptr) {
-                        debug self => _4;
-                    }
-                }
-            }
-        }
-        scope 13 (inlined std::slice::from_raw_parts::<'_, u8>) {
-            debug data => _5;
-            debug len => _6;
-            let _7: *const [u8];
-            scope 14 (inlined core::ub_checks::check_language_ub) {
-                scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
                 }
             }
-            scope 16 (inlined std::mem::size_of::<u8>) {
-            }
-            scope 17 (inlined align_of::<u8>) {
-            }
-            scope 18 (inlined slice_from_raw_parts::<u8>) {
+            scope 12 (inlined std::slice::from_raw_parts::<'_, u8>) {
                 debug data => _5;
                 debug len => _6;
-                scope 19 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
-                    debug data_pointer => _5;
-                    debug metadata => _6;
+                let _7: *const [u8];
+                scope 13 (inlined core::ub_checks::check_language_ub) {
+                    scope 14 (inlined core::ub_checks::check_language_ub::runtime) {
+                    }
+                }
+                scope 15 (inlined std::mem::size_of::<u8>) {
+                }
+                scope 16 (inlined align_of::<u8>) {
+                }
+                scope 17 (inlined slice_from_raw_parts::<u8>) {
+                    debug data => _5;
+                    debug len => _6;
+                    scope 18 (inlined std::ptr::from_raw_parts::<[u8], u8>) {
+                        debug data_pointer => _5;
+                        debug metadata => _6;
+                    }
                 }
             }
         }
diff --git a/tests/ui/consts/issue-94675.rs b/tests/ui/consts/issue-94675.rs
index 56c4b6ea36f..2e30eebb07b 100644
--- a/tests/ui/consts/issue-94675.rs
+++ b/tests/ui/consts/issue-94675.rs
@@ -1,6 +1,6 @@
 //@ known-bug: #103507
 
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, const_vec_string_slice)]
 
 struct Foo<'a> {
     bar: &'a mut Vec<usize>,
diff --git a/tests/ui/consts/issue-94675.stderr b/tests/ui/consts/issue-94675.stderr
index ebfa09b2e5d..a85c5e10374 100644
--- a/tests/ui/consts/issue-94675.stderr
+++ b/tests/ui/consts/issue-94675.stderr
@@ -1,11 +1,3 @@
-error[E0015]: cannot call non-const fn `Vec::<u32>::len` in constant functions
-  --> $DIR/issue-94675.rs:11:27
-   |
-LL |         self.bar[0] = baz.len();
-   |                           ^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-
 error[E0015]: cannot call non-const operator in constant functions
   --> $DIR/issue-94675.rs:11:17
    |
@@ -20,6 +12,6 @@ help: add `#![feature(effects)]` to the crate attributes to enable
 LL + #![feature(effects)]
    |
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0015`.