Rollup merge of #148506 - estebank:issue-41966, r=davidtwco

Special case detecting `'static` lifetime requirement coming from `-> Box<dyn Trait>`

```
error[E0310]: the parameter type `R` may not live long enough
  --> $DIR/implicit-static-lifetime-in-dyn-trait-return-type.rs:10:5
   |
LL | fn bb<R>(r: R) -> Box<dyn Foo> {
   |                       ------- this `dyn Trait` has an implicit `'static` lifetime bound
LL |     Box::new(Bar(r))
   |     ^^^^^^^^^^^^^^^^
   |     |
   |     the parameter type `R` must be valid for the static lifetime...
   |     ...so that the type `R` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound
   |
LL | fn bb<R: 'static>(r: R) -> Box<dyn Foo> {
   |        +++++++++
```

Partly address #41966 and #54753. #103849, which shows a case where there's an intermediary binding, is not addressed at all, as aren't cases *other* than `Box<dyn Trait>` return type.

Author: Zalathar

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -3592,8 +3592,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                         && (lt.kind == MissingLifetimeKind::Ampersand
                             || lt.kind == MissingLifetimeKind::Underscore)
                     {
-                        let pre = if lt.kind == MissingLifetimeKind::Ampersand
-                            && let Some((kind, _span)) = self.diag_metadata.current_function
+                        let pre = if let Some((kind, _span)) = self.diag_metadata.current_function
                             && let FnKind::Fn(_, _, ast::Fn { sig, .. }) = kind
                             && !sig.decl.inputs.is_empty()
                             && let sugg = sig
@@ -3623,10 +3622,12 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                             } else {
                                 ("one of the", "s")
                             };
+                            let dotdotdot =
+                                if lt.kind == MissingLifetimeKind::Ampersand { "..." } else { "" };
                             err.multipart_suggestion_verbose(
                                 format!(
                                     "instead, you are more likely to want to change {the} \
-                                     argument{s} to be borrowed...",
+                                     argument{s} to be borrowed{dotdotdot}",
                                 ),
                                 sugg,
                                 Applicability::MaybeIncorrect,

compiler/rustc_trait_selection/src/error_reporting/infer/region.rs

@@ -864,6 +864,42 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             }
         }
 
+        if sub.kind() == ty::ReStatic
+            && let Some(node) = self.tcx.hir_get_if_local(generic_param_scope.into())
+            && let hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Fn { sig, body, has_body: true, .. },
+                ..
+            })
+            | hir::Node::TraitItem(hir::TraitItem {
+                kind: hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body)),
+                ..
+            })
+            | hir::Node::ImplItem(hir::ImplItem {
+                kind: hir::ImplItemKind::Fn(sig, body), ..
+            }) = node
+            && let hir::Node::Expr(expr) = self.tcx.hir_node(body.hir_id)
+            && let hir::ExprKind::Block(block, _) = expr.kind
+            && let Some(tail) = block.expr
+            && tail.span == span
+            && let hir::FnRetTy::Return(ty) = sig.decl.output
+            && let hir::TyKind::Path(path) = ty.kind
+            && let hir::QPath::Resolved(None, path) = path
+            && let hir::def::Res::Def(_, def_id) = path.res
+            && Some(def_id) == self.tcx.lang_items().owned_box()
+            && let [segment] = path.segments
+            && let Some(args) = segment.args
+            && let [hir::GenericArg::Type(ty)] = args.args
+            && let hir::TyKind::TraitObject(_, tagged_ref) = ty.kind
+            && let hir::LifetimeKind::ImplicitObjectLifetimeDefault = tagged_ref.pointer().kind
+        {
+            // Explicitly look for `-> Box<dyn Trait>` to point at it as the *likely* source of
+            // the `'static` lifetime requirement.
+            err.span_label(
+                ty.span,
+                format!("this `dyn Trait` has an implicit `'static` lifetime bound"),
+            );
+        }
+
         err
     }
 

tests/ui/impl-trait/in-trait/missing-static-bound-from-impl.stderr

@@ -1,6 +1,8 @@
 error[E0310]: the associated type `impl Fn()` may not live long enough
   --> $DIR/missing-static-bound-from-impl.rs:11:9
    |
+LL |     fn f(&self) -> Box<dyn Fn()> {
+   |                        -------- this `dyn Trait` has an implicit `'static` lifetime bound
 LL |         Box::new(<T as Original>::f())
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |         |

tests/ui/suggestions/lifetimes/implicit-static-lifetime-in-dyn-trait-return-type.rs

@@ -0,0 +1,35 @@
+// #41966
+trait Foo {}
+
+struct Bar<R>(R);
+
+impl<R> Foo for Bar<R> {
+}
+
+fn bb<R>(r: R) -> Box<dyn Foo> {
+    Box::new(Bar(r)) //~ ERROR the parameter type `R` may not live long enough
+}
+
+fn cc<R>(r: R) -> Box<dyn Foo + '_> { //~ ERROR missing lifetime specifier
+    Box::new(Bar(r))
+}
+
+// #54753
+pub struct Qux<T>(T);
+
+pub struct Bazzzz<T>(T);
+
+pub trait Baz {}
+impl<T> Baz for Bazzzz<T> {}
+
+impl<T> Qux<T> {
+    fn baz(self) -> Box<dyn Baz> {
+        Box::new(Bazzzz(self.0)) //~ ERROR the parameter type `T` may not live long enough
+    }
+}
+
+fn main() {
+    let a = 10;
+    let _b = bb(&a);
+    let _c = cc(&a);
+}

tests/ui/suggestions/lifetimes/implicit-static-lifetime-in-dyn-trait-return-type.stderr

@@ -0,0 +1,53 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/implicit-static-lifetime-in-dyn-trait-return-type.rs:13:33
+   |
+LL | fn cc<R>(r: R) -> Box<dyn Foo + '_> {
+   |                                 ^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`, or if you will only have owned values
+   |
+LL - fn cc<R>(r: R) -> Box<dyn Foo + '_> {
+LL + fn cc<R>(r: R) -> Box<dyn Foo + 'static> {
+   |
+help: instead, you are more likely to want to change the argument to be borrowed
+   |
+LL | fn cc<R>(r: &R) -> Box<dyn Foo + '_> {
+   |             +
+
+error[E0310]: the parameter type `R` may not live long enough
+  --> $DIR/implicit-static-lifetime-in-dyn-trait-return-type.rs:10:5
+   |
+LL | fn bb<R>(r: R) -> Box<dyn Foo> {
+   |                       ------- this `dyn Trait` has an implicit `'static` lifetime bound
+LL |     Box::new(Bar(r))
+   |     ^^^^^^^^^^^^^^^^
+   |     |
+   |     the parameter type `R` must be valid for the static lifetime...
+   |     ...so that the type `R` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | fn bb<R: 'static>(r: R) -> Box<dyn Foo> {
+   |        +++++++++
+
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/implicit-static-lifetime-in-dyn-trait-return-type.rs:27:9
+   |
+LL |     fn baz(self) -> Box<dyn Baz> {
+   |                         ------- this `dyn Trait` has an implicit `'static` lifetime bound
+LL |         Box::new(Bazzzz(self.0))
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         the parameter type `T` must be valid for the static lifetime...
+   |         ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | impl<T: 'static> Qux<T> {
+   |       +++++++++
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0106, E0310.
+For more information about an error, try `rustc --explain E0106`.

tests/ui/suggestions/wrap-dyn-in-suggestion-issue-120223.stderr

@@ -38,6 +38,8 @@ LL | fn without_sized<T: Fn() -> &'static (dyn std::fmt::Debug) + ?Sized>() {}
 error[E0310]: the parameter type `impl FnOnce(T) -> dyn Future<Output = ()>` may not live long enough
   --> $DIR/wrap-dyn-in-suggestion-issue-120223.rs:6:5
    |
+LL | ) -> Box<dyn FnOnce(T) -> dyn Future<Output = ()>> {
+   |          ---------------------------------------- this `dyn Trait` has an implicit `'static` lifetime bound
 LL |     Box::new(executor)
    |     ^^^^^^^^^^^^^^^^^^
    |     |