Rollup of 8 pull requests

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -240,7 +240,7 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
                     let align = ImmTy::from_uint(target_align, args[1].layout).into();
                     let fn_abi = self.fn_abi_of_instance(instance, ty::List::empty())?;
 
-                    // We replace the entire entire function call with a "tail call".
+                    // We replace the entire function call with a "tail call".
                     // Note that this happens before the frame of the original function
                     // is pushed on the stack.
                     self.eval_fn_call(

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -676,6 +676,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         return_to_block: StackPopCleanup,
     ) -> InterpResult<'tcx> {
         trace!("body: {:#?}", body);
+        // Clobber previous return place contents, nobody is supposed to be able to see them any more
+        // This also checks dereferenceable, but not align. We rely on all constructed places being
+        // sufficiently aligned (in particular we rely on `deref_operand` checking alignment).
+        self.write_uninit(return_place)?;
         // first push a stack frame so we have access to the local substs
         let pre_frame = Frame {
             body,

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -32,11 +32,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     pub(in super::super) fn suggest_semicolon_at_end(&self, span: Span, err: &mut Diagnostic) {
+        // This suggestion is incorrect for
+        // fn foo() -> bool { match () { () => true } || match () { () => true } }
         err.span_suggestion_short(
             span.shrink_to_hi(),
             "consider using a semicolon here",
             ";",
-            Applicability::MachineApplicable,
+            Applicability::MaybeIncorrect,
         );
     }
 

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -406,7 +406,7 @@ impl<V: Clone> Clone for StateData<V> {
 /// The dataflow state for an instance of [`ValueAnalysis`].
 ///
 /// Every instance specifies a lattice that represents the possible values of a single tracked
-/// place. If we call this lattice `V` and set set of tracked places `P`, then a [`State`] is an
+/// place. If we call this lattice `V` and set of tracked places `P`, then a [`State`] is an
 /// element of `{unreachable} ∪ (P -> V)`. This again forms a lattice, where the bottom element is
 /// `unreachable` and the top element is the mapping `p ↦ ⊤`. Note that the mapping `p ↦ ⊥` is not
 /// the bottom element (because joining an unreachable and any other reachable state yields a

compiler/rustc_parse/src/parser/expr.rs

@@ -390,20 +390,11 @@ impl<'a> Parser<'a> {
             // want to keep their span info to improve diagnostics in these cases in a later stage.
             (true, Some(AssocOp::Multiply)) | // `{ 42 } *foo = bar;` or `{ 42 } * 3`
             (true, Some(AssocOp::Subtract)) | // `{ 42 } -5`
-            (true, Some(AssocOp::Add)) // `{ 42 } + 42
-            // If the next token is a keyword, then the tokens above *are* unambiguously incorrect:
-            // `if x { a } else { b } && if y { c } else { d }`
-            if !self.look_ahead(1, |t| t.is_used_keyword()) => {
-                // These cases are ambiguous and can't be identified in the parser alone.
-                let sp = self.sess.source_map().start_point(self.token.span);
-                self.sess.ambiguous_block_expr_parse.borrow_mut().insert(sp, lhs.span);
-                false
-            }
-            (true, Some(AssocOp::LAnd)) |
-            (true, Some(AssocOp::LOr)) |
-            (true, Some(AssocOp::BitOr)) => {
-                // `{ 42 } &&x` (#61475) or `{ 42 } && if x { 1 } else { 0 }`. Separated from the
-                // above due to #74233.
+            (true, Some(AssocOp::Add)) | // `{ 42 } + 42` (unary plus)
+            (true, Some(AssocOp::LAnd)) | // `{ 42 } &&x` (#61475) or `{ 42 } && if x { 1 } else { 0 }`
+            (true, Some(AssocOp::LOr)) | // `{ 42 } || 42` ("logical or" or closure)
+            (true, Some(AssocOp::BitOr)) // `{ 42 } | 42` or `{ 42 } |x| 42`
+            => {
                 // These cases are ambiguous and can't be identified in the parser alone.
                 //
                 // Bitwise AND is left out because guessing intent is hard. We can make

compiler/rustc_resolve/src/late.rs

@@ -1927,7 +1927,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                                 // We have a single lifetime => success.
                                 elision_lifetime = Elision::Param(res)
                             } else {
-                                // We have have multiple lifetimes => error.
+                                // We have multiple lifetimes => error.
                                 elision_lifetime = Elision::Err;
                             }
                         }

library/core/src/cmp.rs

@@ -29,15 +29,16 @@ use crate::marker::StructuralPartialEq;
 
 use self::Ordering::*;
 
-/// Trait for equality comparisons which are [partial equivalence
-/// relations](https://en.wikipedia.org/wiki/Partial_equivalence_relation).
+/// Trait for equality comparisons.
 ///
 /// `x.eq(y)` can also be written `x == y`, and `x.ne(y)` can be written `x != y`.
 /// We use the easier-to-read infix notation in the remainder of this documentation.
 ///
 /// This trait allows for partial equality, for types that do not have a full
 /// equivalence relation. For example, in floating point numbers `NaN != NaN`,
 /// so floating point types implement `PartialEq` but not [`trait@Eq`].
+/// Formally speaking, when `Rhs == Self`, this trait corresponds to a [partial equivalence
+/// relation](https://en.wikipedia.org/wiki/Partial_equivalence_relation).
 ///
 /// Implementations must ensure that `eq` and `ne` are consistent with each other:
 ///

library/core/src/iter/sources/repeat_n.rs

@@ -20,7 +20,7 @@ use crate::mem::ManuallyDrop;
 /// #![feature(iter_repeat_n)]
 /// use std::iter;
 ///
-/// // four of the the number four:
+/// // four of the number four:
 /// let mut four_fours = iter::repeat_n(4, 4);
 ///
 /// assert_eq!(Some(4), four_fours.next());

library/core/src/str/pattern.rs

@@ -1894,7 +1894,7 @@ unsafe fn small_slice_eq(x: &[u8], y: &[u8]) -> bool {
     // Thus, derefencing both `px` and `py` in the loop below is safe.
     //
     // Moreover, we set `pxend` and `pyend` to be 4 bytes before the actual
-    // end of of `px` and `py`. Thus, the final dereference outside of the
+    // end of `px` and `py`. Thus, the final dereference outside of the
     // loop is guaranteed to be valid. (The final comparison will overlap with
     // the last comparison done in the loop for lengths that aren't multiples
     // of four.)

library/std/src/sync/mpmc/array.rs

@@ -225,7 +225,7 @@ impl<T> Channel<T> {
             let slot = unsafe { self.buffer.get_unchecked(index) };
             let stamp = slot.stamp.load(Ordering::Acquire);
 
-            // If the the stamp is ahead of the head by 1, we may attempt to pop.
+            // If the stamp is ahead of the head by 1, we may attempt to pop.
             if head + 1 == stamp {
                 let new = if index + 1 < self.cap {
                     // Same lap, incremented index.

library/std/src/thread/scoped.rs

@@ -46,7 +46,7 @@ impl ScopeData {
         // We check for 'overflow' with usize::MAX / 2, to make sure there's no
         // chance it overflows to 0, which would result in unsoundness.
         if self.num_running_threads.fetch_add(1, Ordering::Relaxed) > usize::MAX / 2 {
-            // This can only reasonably happen by mem::forget()'ing many many ScopedJoinHandles.
+            // This can only reasonably happen by mem::forget()'ing a lot of ScopedJoinHandles.
             self.decrement_num_running_threads(false);
             panic!("too many running threads in thread scope");
         }

src/bootstrap/flags.rs

@@ -143,7 +143,7 @@ pub enum Subcommand {
         args: Vec<String>,
     },
     Setup {
-        profile: Profile,
+        profile: Option<Profile>,
     },
 }
 
@@ -628,14 +628,15 @@ Arguments:
                         |path| format!("{} is not a valid UTF8 string", path.to_string_lossy())
                     ));
 
-                    profile_string.parse().unwrap_or_else(|err| {
+                    let profile = profile_string.parse().unwrap_or_else(|err| {
                         eprintln!("error: {}", err);
                         eprintln!("help: the available profiles are:");
                         eprint!("{}", Profile::all_for_help("- "));
                         crate::detail_exit(1);
-                    })
+                    });
+                    Some(profile)
                 } else {
-                    t!(crate::setup::interactive_path())
+                    None
                 };
                 Subcommand::Setup { profile }
             }

src/bootstrap/lib.rs

@@ -542,16 +542,6 @@ impl Build {
             metrics: metrics::BuildMetrics::init(),
         };
 
-        build.verbose("finding compilers");
-        cc_detect::find(&mut build);
-        // When running `setup`, the profile is about to change, so any requirements we have now may
-        // be different on the next invocation. Don't check for them until the next time x.py is
-        // run. This is ok because `setup` never runs any build commands, so it won't fail if commands are missing.
-        if !matches!(build.config.cmd, Subcommand::Setup { .. }) {
-            build.verbose("running sanity check");
-            sanity::check(&mut build);
-        }
-
         // If local-rust is the same major.minor as the current version, then force a
         // local-rebuild
         let local_version_verbose =
@@ -567,16 +557,34 @@ impl Build {
             build.local_rebuild = true;
         }
 
-        // Make sure we update these before gathering metadata so we don't get an error about missing
-        // Cargo.toml files.
-        let rust_submodules =
-            ["src/tools/rust-installer", "src/tools/cargo", "library/backtrace", "library/stdarch"];
-        for s in rust_submodules {
-            build.update_submodule(Path::new(s));
-        }
+        build.verbose("finding compilers");
+        cc_detect::find(&mut build);
+        // When running `setup`, the profile is about to change, so any requirements we have now may
+        // be different on the next invocation. Don't check for them until the next time x.py is
+        // run. This is ok because `setup` never runs any build commands, so it won't fail if commands are missing.
+        //
+        // Similarly, for `setup` we don't actually need submodules or cargo metadata.
+        if !matches!(build.config.cmd, Subcommand::Setup { .. }) {
+            build.verbose("running sanity check");
+            sanity::check(&mut build);
+
+            // Make sure we update these before gathering metadata so we don't get an error about missing
+            // Cargo.toml files.
+            let rust_submodules = [
+                "src/tools/rust-installer",
+                "src/tools/cargo",
+                "library/backtrace",
+                "library/stdarch",
+            ];
+            for s in rust_submodules {
+                build.update_submodule(Path::new(s));
+            }
+            // Now, update all existing submodules.
+            build.update_existing_submodules();
 
-        build.verbose("learning about cargo");
-        metadata::build(&mut build);
+            build.verbose("learning about cargo");
+            metadata::build(&mut build);
+        }
 
         build
     }
@@ -668,7 +676,7 @@ impl Build {
 
     /// If any submodule has been initialized already, sync it unconditionally.
     /// This avoids contributors checking in a submodule change by accident.
-    pub fn maybe_update_submodules(&self) {
+    pub fn update_existing_submodules(&self) {
         // Avoid running git when there isn't a git checkout.
         if !self.config.submodules(&self.rust_info()) {
             return;
@@ -697,8 +705,6 @@ impl Build {
             job::setup(self);
         }
 
-        self.maybe_update_submodules();
-
         if let Subcommand::Format { check, paths } = &self.config.cmd {
             return format::format(&builder::Builder::new(&self), *check, &paths);
         }

src/bootstrap/setup.rs

@@ -1,15 +1,13 @@
+use crate::Config;
 use crate::{t, VERSION};
-use crate::{Config, TargetSelection};
 use std::env::consts::EXE_SUFFIX;
 use std::fmt::Write as _;
 use std::fs::File;
+use std::io::Write;
 use std::path::{Path, PathBuf, MAIN_SEPARATOR};
 use std::process::Command;
 use std::str::FromStr;
-use std::{
-    env, fmt, fs,
-    io::{self, Write},
-};
+use std::{fmt, fs, io};
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum Profile {
@@ -81,38 +79,10 @@ impl fmt::Display for Profile {
     }
 }
 
-pub fn setup(config: &Config, profile: Profile) {
-    let path = &config.config.clone().unwrap_or(PathBuf::from("config.toml"));
-
-    if path.exists() {
-        eprintln!(
-            "error: you asked `x.py` to setup a new config file, but one already exists at `{}`",
-            path.display()
-        );
-        eprintln!("help: try adding `profile = \"{}\"` at the top of {}", profile, path.display());
-        eprintln!(
-            "note: this will use the configuration in {}",
-            profile.include_path(&config.src).display()
-        );
-        crate::detail_exit(1);
-    }
-
-    let settings = format!(
-        "# Includes one of the default files in src/bootstrap/defaults\n\
-    profile = \"{}\"\n\
-    changelog-seen = {}\n",
-        profile, VERSION
-    );
-    t!(fs::write(path, settings));
-
-    let include_path = profile.include_path(&config.src);
-    println!("`x.py` will now use the configuration at {}", include_path.display());
-
-    let build = TargetSelection::from_user(&env!("BUILD_TRIPLE"));
+pub fn setup(config: &Config, profile: Option<Profile>) {
+    let profile = profile.unwrap_or_else(|| t!(interactive_path()));
     let stage_path =
-        ["build", build.rustc_target_arg(), "stage1"].join(&MAIN_SEPARATOR.to_string());
-
-    println!();
+        ["build", config.build.rustc_target_arg(), "stage1"].join(&MAIN_SEPARATOR.to_string());
 
     if !rustup_installed() && profile != Profile::User {
         eprintln!("`rustup` is not installed; cannot link `stage1` toolchain");
@@ -134,8 +104,6 @@ pub fn setup(config: &Config, profile: Profile) {
         Profile::User => &["dist", "build"],
     };
 
-    println!();
-
     t!(install_git_hook_maybe(&config));
 
     println!();
@@ -150,6 +118,36 @@ pub fn setup(config: &Config, profile: Profile) {
             "For more suggestions, see https://rustc-dev-guide.rust-lang.org/building/suggested.html"
         );
     }
+
+    let path = &config.config.clone().unwrap_or(PathBuf::from("config.toml"));
+    setup_config_toml(path, profile, config);
+}
+
+fn setup_config_toml(path: &PathBuf, profile: Profile, config: &Config) {
+    if path.exists() {
+        eprintln!();
+        eprintln!(
+            "error: you asked `x.py` to setup a new config file, but one already exists at `{}`",
+            path.display()
+        );
+        eprintln!("help: try adding `profile = \"{}\"` at the top of {}", profile, path.display());
+        eprintln!(
+            "note: this will use the configuration in {}",
+            profile.include_path(&config.src).display()
+        );
+        crate::detail_exit(1);
+    }
+
+    let settings = format!(
+        "# Includes one of the default files in src/bootstrap/defaults\n\
+    profile = \"{}\"\n\
+    changelog-seen = {}\n",
+        profile, VERSION
+    );
+    t!(fs::write(path, settings));
+
+    let include_path = profile.include_path(&config.src);
+    println!("`x.py` will now use the configuration at {}", include_path.display());
 }
 
 fn rustup_installed() -> bool {
@@ -303,7 +301,18 @@ pub fn interactive_path() -> io::Result<Profile> {
 
 // install a git hook to automatically run tidy --bless, if they want
 fn install_git_hook_maybe(config: &Config) -> io::Result<()> {
+    let git = t!(config.git().args(&["rev-parse", "--git-common-dir"]).output().map(|output| {
+        assert!(output.status.success(), "failed to run `git`");
+        PathBuf::from(t!(String::from_utf8(output.stdout)).trim())
+    }));
+    let dst = git.join("hooks").join("pre-push");
+    if dst.exists() {
+        // The git hook has already been set up, or the user already has a custom hook.
+        return Ok(());
+    }
+
     let mut input = String::new();
+    println!();
     println!(
         "Rust's CI will automatically fail if it doesn't pass `tidy`, the internal tool for ensuring code quality.
 If you'd like, x.py can install a git hook for you that will automatically run `tidy --bless` before
@@ -329,12 +338,6 @@ undesirable, simply delete the `pre-push` file from .git/hooks."
 
     if should_install {
         let src = config.src.join("src").join("etc").join("pre-push.sh");
-        let git =
-            t!(config.git().args(&["rev-parse", "--git-common-dir"]).output().map(|output| {
-                assert!(output.status.success(), "failed to run `git`");
-                PathBuf::from(t!(String::from_utf8(output.stdout)).trim())
-            }));
-        let dst = git.join("hooks").join("pre-push");
         match fs::hard_link(src, &dst) {
             Err(e) => eprintln!(
                 "error: could not create hook {}: do you already have the git hook installed?\n{}",

src/test/rustdoc-gui/method-margins.goml

@@ -1,3 +1,4 @@
+// This test ensures that the margins on methods are coherent inside an impl block.
 goto: "file://" + |DOC_PATH| + "/test_docs/trait_members/struct.HasTrait.html#impl-TraitMembers-for-HasTrait"
 
 assert-count: ("#trait-implementations-list > .rustdoc-toggle", 1)

src/test/ui/parser/expr-as-stmt.fixed

@@ -64,4 +64,16 @@ fn asteroids() -> impl FnOnce() -> bool {
     { foo(); } || { true } //~ ERROR E0308
 }
 
+// https://github.com/rust-lang/rust/issues/105179
+fn r#match() -> i32 {
+    (match () { () => 1 }) + match () { () => 1 } //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+// https://github.com/rust-lang/rust/issues/102171
+fn r#unsafe() -> i32 {
+    (unsafe { 1 }) + unsafe { 1 } //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
 fn main() {}

src/test/ui/parser/expr-as-stmt.rs

@@ -64,4 +64,16 @@ fn asteroids() -> impl FnOnce() -> bool {
     { foo() } || { true } //~ ERROR E0308
 }
 
+// https://github.com/rust-lang/rust/issues/105179
+fn r#match() -> i32 {
+    match () { () => 1 } + match () { () => 1 } //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
+// https://github.com/rust-lang/rust/issues/102171
+fn r#unsafe() -> i32 {
+    unsafe { 1 } + unsafe { 1 } //~ ERROR expected expression, found `+`
+    //~^ ERROR mismatched types
+}
+
 fn main() {}

src/test/ui/parser/expr-as-stmt.stderr

@@ -55,6 +55,28 @@ help: parentheses are required to parse this as an expression
 LL |     ({ true }) | { true }
    |     +        +
 
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:69:26
+   |
+LL |     match () { () => 1 } + match () { () => 1 }
+   |                          ^ expected expression
+   |
+help: parentheses are required to parse this as an expression
+   |
+LL |     (match () { () => 1 }) + match () { () => 1 }
+   |     +                    +
+
+error: expected expression, found `+`
+  --> $DIR/expr-as-stmt.rs:75:18
+   |
+LL |     unsafe { 1 } + unsafe { 1 }
+   |                  ^ expected expression
+   |
+help: parentheses are required to parse this as an expression
+   |
+LL |     (unsafe { 1 }) + unsafe { 1 }
+   |     +            +
+
 error[E0308]: mismatched types
   --> $DIR/expr-as-stmt.rs:64:7
    |
@@ -201,7 +223,26 @@ help: parentheses are required to parse this as an expression
 LL |     ({ true }) || { true }
    |     +        +
 
-error: aborting due to 18 previous errors
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:69:5
+   |
+LL |     match () { () => 1 } + match () { () => 1 }
+   |     ^^^^^^^^^^^^^^^^^^^^- help: consider using a semicolon here
+   |     |
+   |     expected `()`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/expr-as-stmt.rs:75:14
+   |
+LL |     unsafe { 1 } + unsafe { 1 }
+   |              ^ expected `()`, found integer
+   |
+help: you might have meant to return this value
+   |
+LL |     unsafe { return 1; } + unsafe { 1 }
+   |              ++++++  +
+
+error: aborting due to 22 previous errors
 
 Some errors have detailed explanations: E0308, E0600, E0614.
 For more information about an error, try `rustc --explain E0308`.

src/test/ui/rfc-1445-restrict-constants-in-patterns/match-requires-both-partialeq-and-eq.rs

@@ -5,7 +5,7 @@ struct Foo {
 
 impl PartialEq for Foo {
     fn eq(&self, _: &Foo) -> bool {
-        false // ha ha sucker!
+        false // ha ha!
     }
 }
 

src/tools/miri/rust-version

@@ -1 +1 @@
-cef44f53034eac46be3a0e3eec7b2b3d4ef5140b
+203c8765ea33c65d888febe0e8219c4bb11b0d89

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -45,7 +45,9 @@ pub struct Stacks {
 /// new pointer.
 #[derive(Copy, Clone, Hash, PartialEq, Eq)]
 enum RefKind {
-    /// `&mut` and `Box`.
+    /// `Box`.
+    Box,
+    /// `&mut`.
     Unique { two_phase: bool },
     /// `&` with or without interior mutability.
     Shared,
@@ -56,6 +58,7 @@ enum RefKind {
 impl fmt::Display for RefKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
+            RefKind::Box => write!(f, "Box"),
             RefKind::Unique { two_phase: false } => write!(f, "unique reference"),
             RefKind::Unique { two_phase: true } => write!(f, "unique reference (two-phase)"),
             RefKind::Shared => write!(f, "shared reference"),
@@ -654,15 +657,17 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
         let (perm, access) = match kind {
             RefKind::Unique { two_phase } => {
                 // Permission is Unique only if the type is `Unpin` and this is not twophase
-                let perm = if !two_phase && place.layout.ty.is_unpin(*this.tcx, this.param_env()) {
-                    Permission::Unique
+                if !two_phase && place.layout.ty.is_unpin(*this.tcx, this.param_env()) {
+                    (Permission::Unique, Some(AccessKind::Write))
                 } else {
-                    Permission::SharedReadWrite
-                };
-                // We do an access for all full borrows, even if `!Unpin`.
-                let access = if !two_phase { Some(AccessKind::Write) } else { None };
-                (perm, access)
+                    // FIXME: We emit `dereferenceable` for `!Unpin` mutable references, so we
+                    // should do fake accesses here. But then we run into
+                    // <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>, so for now
+                    // we don't do that.
+                    (Permission::SharedReadWrite, None)
+                }
             }
+            RefKind::Box => (Permission::Unique, Some(AccessKind::Write)),
             RefKind::Raw { mutable: true } => {
                 // Creating a raw ptr does not count as an access
                 (Permission::SharedReadWrite, None)
@@ -853,7 +858,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 // Boxes get a weak protectors, since they may be deallocated.
                 self.retag_place(
                     place,
-                    RefKind::Unique { two_phase: false },
+                    RefKind::Box,
                     self.retag_cause,
                     /*protector*/
                     (self.kind == RetagKind::FnEntry).then_some(ProtectorKind::WeakProtector),

src/tools/miri/tests/pass/stacked-borrows/future-self-referential.rs

@@ -0,0 +1,102 @@
+#![feature(pin_macro)]
+
+use std::future::*;
+use std::marker::PhantomPinned;
+use std::pin::*;
+use std::ptr;
+use std::task::*;
+
+struct Delay {
+    delay: usize,
+}
+
+impl Delay {
+    fn new(delay: usize) -> Self {
+        Delay { delay }
+    }
+}
+
+impl Future for Delay {
+    type Output = ();
+    fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<()> {
+        if self.delay > 0 {
+            self.delay -= 1;
+            Poll::Pending
+        } else {
+            Poll::Ready(())
+        }
+    }
+}
+
+async fn do_stuff() {
+    (&mut Delay::new(1)).await;
+}
+
+// Same thing implemented by hand
+struct DoStuff {
+    state: usize,
+    delay: Delay,
+    delay_ref: *mut Delay,
+    _marker: PhantomPinned,
+}
+
+impl DoStuff {
+    fn new() -> Self {
+        DoStuff {
+            state: 0,
+            delay: Delay::new(1),
+            delay_ref: ptr::null_mut(),
+            _marker: PhantomPinned,
+        }
+    }
+}
+
+impl Future for DoStuff {
+    type Output = ();
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+        unsafe {
+            let this = self.get_unchecked_mut();
+            match this.state {
+                0 => {
+                    // Set up self-ref.
+                    this.delay_ref = &mut this.delay;
+                    // Move to next state.
+                    this.state = 1;
+                    Poll::Pending
+                }
+                1 => {
+                    let delay = &mut *this.delay_ref;
+                    Pin::new_unchecked(delay).poll(cx)
+                }
+                _ => unreachable!(),
+            }
+        }
+    }
+}
+
+fn run_fut<T>(fut: impl Future<Output = T>) -> T {
+    use std::sync::Arc;
+
+    struct MyWaker;
+    impl Wake for MyWaker {
+        fn wake(self: Arc<Self>) {
+            unimplemented!()
+        }
+    }
+
+    let waker = Waker::from(Arc::new(MyWaker));
+    let mut context = Context::from_waker(&waker);
+
+    let mut pinned = pin!(fut);
+    loop {
+        match pinned.as_mut().poll(&mut context) {
+            Poll::Pending => continue,
+            Poll::Ready(v) => return v,
+        }
+    }
+}
+
+fn main() {
+    run_fut(do_stuff());
+    run_fut(DoStuff::new());
+}