Auto merge of #86866 - nikomatsakis:issue-84841, r=oli-obk

Hack: Ignore inference variables in certain queries

Fixes #84841
Fixes #86753

Some queries are not built to accept types with inference variables, which can lead to ICEs. These queries probably ought to be converted to canonical form, but as a quick workaround, we can return conservative results in the case that inference variables are found.

We should file a follow-up issue (and update the FIXMEs...) to do the proper refactoring.

cc `@arora-aman`

r? `@oli-obk`

src/test/ui/async-await/issue-84841.rs

@@ -0,0 +1,16 @@
+// edition:2018
+
+fn main() {
+
+}
+
+async fn foo() {
+    // Adding an .await here avoids the ICE
+    test()?;
+    //~^ ERROR the `?` operator can only be applied to values that implement `Try`
+    //~| ERROR the `?` operator can only be used in an async function that returns
+}
+
+// Removing the const generic parameter here avoids the ICE
+async fn test<const N: usize>() {
+}

src/test/ui/async-await/issue-84841.stderr

@@ -0,0 +1,28 @@
+error[E0277]: the `?` operator can only be applied to values that implement `Try`
+  --> $DIR/issue-84841.rs:9:5
+   |
+LL |     test()?;
+   |     ^^^^^^^ the `?` operator cannot be applied to type `impl Future`
+   |
+   = help: the trait `Try` is not implemented for `impl Future`
+   = note: required by `branch`
+
+error[E0277]: the `?` operator can only be used in an async function that returns `Result` or `Option` (or another type that implements `FromResidual`)
+  --> $DIR/issue-84841.rs:9:11
+   |
+LL |   async fn foo() {
+   |  ________________-
+LL | |     // Adding an .await here avoids the ICE
+LL | |     test()?;
+   | |           ^ cannot use the `?` operator in an async function that returns `()`
+LL | |
+LL | |
+LL | | }
+   | |_- this function should return `Result` or `Option` to accept `?`
+   |
+   = help: the trait `FromResidual<_>` is not implemented for `()`
+   = note: required by `from_residual`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/closures/2229_closure_analysis/migrations/issue-86753.rs

@@ -0,0 +1,34 @@
+// edition:2018
+// check-pass
+
+#![warn(rust_2021_compatibility)]
+
+use std::future::Future;
+
+struct Runtime;
+
+impl Runtime {
+    pub fn block_on<F: Future>(&self, _future: F) -> F::Output {
+        unimplemented!()
+    }
+}
+
+pub fn http<F, Fut>(_func: F)
+where
+    F: Fn() -> Fut,
+    Fut: Future<Output = ()>,
+{
+    let rt = Runtime {};
+    let srv = rt.block_on(async move { serve(move || async move { unimplemented!() }) });
+    let _ = || rt.block_on(async { srv });
+}
+
+pub struct Server<S> {
+    _marker: std::marker::PhantomData<S>,
+}
+
+pub fn serve<S>(_new_service: S) -> Server<S> {
+    unimplemented!()
+}
+
+fn main() { }