Add regression test

src/test/ui/issues/issue-75777.nll.stderr

@@ -0,0 +1,13 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-75777.rs:13:5
+   |
+LL | fn inject<'a, Env: 'a, A: 'a + Send>(v: A) -> Box<dyn FnOnce(&'a Env) -> BoxFuture<'a, A>> {
+   |           -- lifetime `'a` defined here
+LL |     let fut: BoxFuture<'a, A> = Box::pin(future::ready(v));
+LL |     Box::new(move |_| fut)
+   |     ^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
+   |
+   = help: consider replacing `'a` with `'static`
+
+error: aborting due to previous error
+

src/test/ui/issues/issue-75777.rs

@@ -0,0 +1,17 @@
+// Regression test for #75777.
+// Checks that a boxed future can be properly constructed.
+
+#![feature(future_readiness_fns)]
+
+use std::future::{self, Future};
+use std::pin::Pin;
+
+type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + 'a + Send>>;
+
+fn inject<'a, Env: 'a, A: 'a + Send>(v: A) -> Box<dyn FnOnce(&'a Env) -> BoxFuture<'a, A>> {
+    let fut: BoxFuture<'a, A> = Box::pin(future::ready(v));
+    Box::new(move |_| fut)
+    //~^ ERROR: cannot infer an appropriate lifetime
+}
+
+fn main() {}

src/test/ui/issues/issue-75777.stderr

@@ -0,0 +1,30 @@
+error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
+  --> $DIR/issue-75777.rs:13:14
+   |
+LL |     Box::new(move |_| fut)
+   |              ^^^^^^^^^^^^
+   |
+note: first, the lifetime cannot outlive the lifetime `'a` as defined on the function body at 11:11...
+  --> $DIR/issue-75777.rs:11:11
+   |
+LL | fn inject<'a, Env: 'a, A: 'a + Send>(v: A) -> Box<dyn FnOnce(&'a Env) -> BoxFuture<'a, A>> {
+   |           ^^
+note: ...so that the types are compatible
+  --> $DIR/issue-75777.rs:13:14
+   |
+LL |     Box::new(move |_| fut)
+   |              ^^^^^^^^^^^^
+   = note: expected `(Pin<Box<dyn Future<Output = A> + Send>>,)`
+              found `(Pin<Box<(dyn Future<Output = A> + Send + 'a)>>,)`
+   = note: but, the lifetime must be valid for the static lifetime...
+note: ...so that the expression is assignable
+  --> $DIR/issue-75777.rs:13:5
+   |
+LL |     Box::new(move |_| fut)
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected `Box<(dyn FnOnce(&'a Env) -> Pin<Box<(dyn Future<Output = A> + Send + 'a)>> + 'static)>`
+              found `Box<dyn FnOnce(&'a Env) -> Pin<Box<(dyn Future<Output = A> + Send + 'a)>>>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0495`.