Auto merge of #60944 - taiki-e:arbitrary_self_types-lifetime-elision, r=<try>

Allow lifetime elision in arbitrary_self_types

Currently, `self` except `&Self` and `&mut Self` is skipped. By this, other `self`s with lifetime is also ignored.
This PR changes it to only skip `Self`, `&Self` and `&mut Self`, and to handle other `self`s like normal arguments.

Closes #52675

Author: bors

src/librustc/middle/resolve_lifetime.rs

@@ -2121,6 +2121,10 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             _ => false,
         };
 
+        // Only skip `Self`, `&Self` and `&mut Self`,
+        // and to handle other `self`s like normal arguments.
+        let mut skip = 0;
+
         // In accordance with the rules for lifetime elision, we can determine
         // what region to use for elision in the output type in two ways.
         // First (determined here), if `self` is by-reference, then the
@@ -2154,19 +2158,26 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                 false
             };
 
-            if let hir::TyKind::Rptr(lifetime_ref, ref mt) = inputs[0].node {
-                if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.node {
-                    if is_self_ty(path.res) {
-                        if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
-                            let scope = Scope::Elision {
-                                elide: Elide::Exact(lifetime),
-                                s: self.scope,
-                            };
-                            self.with(scope, |_, this| this.visit_ty(output));
-                            return;
+            match inputs[0].node {
+                hir::TyKind::Rptr(lifetime_ref, ref mt) => {
+                    if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.node {
+                        if is_self_ty(path.res) {
+                            if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
+                                let scope = Scope::Elision {
+                                    elide: Elide::Exact(lifetime),
+                                    s: self.scope,
+                                };
+                                self.with(scope, |_, this| this.visit_ty(output));
+                                return;
+                            }
+                            skip = 1;
                         }
                     }
                 }
+                hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) if is_self_ty(path.res) => {
+                    skip = 1;
+                }
+                _ => {}
             }
         }
 
@@ -2178,7 +2189,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         let arg_lifetimes = inputs
             .iter()
             .enumerate()
-            .skip(has_self as usize)
+            .skip(skip)
             .map(|(i, input)| {
                 let mut gather = GatherLifetimes {
                     map: self.map,

src/test/ui/self/arbitrary_self_types_inexact_lifetime.rs

@@ -0,0 +1,14 @@
+#![feature(arbitrary_self_types)]
+
+#[derive(Debug)]
+struct Foo;
+
+impl Foo {
+    fn a(self: &Box<Foo>, f: &Foo) -> &Foo { f } //~ ERROR E0106
+
+    fn b(self: &Box<Foo>, f: &Foo) -> &Box<Foo> { self } //~ ERROR E0106
+
+    fn c(this: &Box<Foo>, f: &Foo) -> &Foo { f } //~ ERROR E0106
+}
+
+fn main() {}

src/test/ui/self/arbitrary_self_types_inexact_lifetime.stderr

@@ -0,0 +1,27 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/arbitrary_self_types_inexact_lifetime.rs:7:39
+   |
+LL |     fn a(self: &Box<Foo>, f: &Foo) -> &Foo { f }
+   |                                       ^ expected lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `self` or `f`
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/arbitrary_self_types_inexact_lifetime.rs:9:39
+   |
+LL |     fn b(self: &Box<Foo>, f: &Foo) -> &Box<Foo> { self }
+   |                                       ^ expected lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `self` or `f`
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/arbitrary_self_types_inexact_lifetime.rs:11:39
+   |
+LL |     fn c(this: &Box<Foo>, f: &Foo) -> &Foo { f }
+   |                                       ^ expected lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `this` or `f`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/self/arbitrary_self_types_lifetime.rs

@@ -0,0 +1,80 @@
+#![feature(arbitrary_self_types)]
+
+use std::pin::Pin;
+
+#[derive(Debug)]
+struct Foo;
+#[derive(Debug)]
+struct Bar<'a>(&'a Foo);
+
+impl std::ops::Deref for Bar<'_> {
+    type Target = Foo;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl Foo {
+    fn a(&self) -> Bar<'_> {
+        Bar(self)
+    }
+
+    fn b(c: &Self) -> Bar<'_> {
+        Bar(c)
+    }
+
+    fn c(self: Bar<'_>) -> Bar<'_> {
+        self
+    }
+
+    fn d(e: Bar<'_>) -> Bar<'_> {
+        e
+    }
+
+    fn e(self: &Self) -> Bar<'_> {
+        Bar(self)
+    }
+
+    fn f(self: Bar<'_>) -> impl std::fmt::Debug {
+        self
+        //~^ ERROR cannot infer an appropriate lifetime
+    }
+
+    fn g(self: Bar<'_>) -> impl std::fmt::Debug + '_ {
+        self
+    }
+}
+
+impl<'a> Bar<'a> {
+    fn a(self: Bar<'a>, f: &Foo) -> (Bar<'a>, &Foo) { (self, f) }
+    fn b(self: Self, f: &Foo) -> (Bar<'a>, &Foo) { (self, f) }
+    fn c(self: Bar<'a>, f: &Foo) -> (Self, &Foo) { (self, f) }
+}
+
+impl Bar<'_> {
+    fn e(self: Self, f: &Foo) -> (Self, &Foo) { (self, f) }
+}
+
+struct Baz<T: Unpin> {
+    field: T,
+}
+
+impl<T: Unpin> Baz<T> {
+    fn field(self: Pin<&mut Self>) -> Pin<&mut T> {
+        let this = Pin::get_mut(self);
+        Pin::new(&mut this.field)
+    }
+}
+
+fn main() {
+    let foo = Foo;
+    { foo.a() };
+    { Foo::b(&foo) };
+    { Bar(&foo).c() };
+    { Foo::d(Bar(&foo)) };
+    { foo.e() };
+    { Bar(&foo).f() };
+    { Bar(&foo).g() };
+    let mut baz = Baz { field: 0u8 };
+    { Pin::new(&mut baz).field() };
+}

src/test/ui/self/arbitrary_self_types_lifetime.stderr

@@ -0,0 +1,23 @@
+error: cannot infer an appropriate lifetime
+  --> $DIR/arbitrary_self_types_lifetime.rs:39:9
+   |
+LL |     fn f(self: Bar<'_>) -> impl std::fmt::Debug {
+   |                            -------------------- this return type evaluates to the `'static` lifetime...
+LL |         self
+   |         ^^^^ ...but this borrow...
+   |
+note: ...can't outlive the anonymous lifetime #1 defined on the method body at 38:5
+  --> $DIR/arbitrary_self_types_lifetime.rs:38:5
+   |
+LL | /     fn f(self: Bar<'_>) -> impl std::fmt::Debug {
+LL | |         self
+LL | |
+LL | |     }
+   | |_____^
+help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the method body at 38:5
+   |
+LL |     fn f(self: Bar<'_>) -> impl std::fmt::Debug + '_ {
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+