Add regression test

tests/ui/consts/const-fn-cycle.rs

@@ -0,0 +1,28 @@
+/// Discovered in https://github.com/rust-lang/rust/issues/112602.
+/// This caused a cycle error, which made no sense.
+/// Removing the `const` part of the `many` function would make the
+/// test pass again.
+/// The issue was that we were running const qualif checks on
+/// `const fn`s, but never using them. During const qualif checks we tend
+/// to end up revealing opaque types (the RPIT in `many`'s return type),
+/// which can quickly lead to cycles.
+
+pub struct Parser<H>(H);
+
+impl<H, T> Parser<H>
+where
+    H: for<'a> Fn(&'a str) -> T,
+{
+    pub const fn new(handler: H) -> Parser<H> {
+        Parser(handler)
+    }
+
+    pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
+        //~^ ERROR: cycle detected
+        Parser::new(|_| unimplemented!())
+    }
+}
+
+fn main() {
+    println!("Hello, world!");
+}

tests/ui/consts/const-fn-cycle.stderr

@@ -0,0 +1,34 @@
+error[E0391]: cycle detected when computing type of opaque `<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many::{opaque#0}`
+  --> $DIR/const-fn-cycle.rs:20:47
+   |
+LL |     pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
+   |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires borrow-checking `<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many`...
+  --> $DIR/const-fn-cycle.rs:20:5
+   |
+LL |     pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires promoting constants in MIR for `<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many`...
+  --> $DIR/const-fn-cycle.rs:20:5
+   |
+LL |     pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many`...
+  --> $DIR/const-fn-cycle.rs:20:5
+   |
+LL |     pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `Parser<<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many::{opaque#0}>` is freeze...
+   = note: ...which requires evaluating trait selection obligation `Parser<<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many::{opaque#0}>: core::marker::Freeze`...
+   = note: ...which again requires computing type of opaque `<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many::{opaque#0}`, completing the cycle
+note: cycle used when computing type of `<impl at $DIR/const-fn-cycle.rs:12:1: 14:33>::many::{opaque#0}`
+  --> $DIR/const-fn-cycle.rs:20:47
+   |
+LL |     pub const fn many<'s>(&'s self) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
+   |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

tests/ui/consts/const-promoted-opaque.atomic.stderr

@@ -0,0 +1,34 @@
+error[E0391]: cycle detected when computing type of opaque `helper::Foo::{opaque#0}`
+  --> $DIR/const-promoted-opaque.rs:14:20
+   |
+LL |     pub type Foo = impl Sized;
+   |                    ^^^^^^^^^^
+   |
+note: ...which requires borrow-checking `helper::FOO`...
+  --> $DIR/const-promoted-opaque.rs:21:5
+   |
+LL |     pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
+   |     ^^^^^^^^^^^^^^^^^^
+note: ...which requires promoting constants in MIR for `helper::FOO`...
+  --> $DIR/const-promoted-opaque.rs:21:5
+   |
+LL |     pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
+   |     ^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `helper::FOO`...
+  --> $DIR/const-promoted-opaque.rs:21:5
+   |
+LL |     pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
+   |     ^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `helper::Foo` is freeze...
+   = note: ...which requires evaluating trait selection obligation `helper::Foo: core::marker::Freeze`...
+   = note: ...which again requires computing type of opaque `helper::Foo::{opaque#0}`, completing the cycle
+note: cycle used when computing type of `helper::Foo::{opaque#0}`
+  --> $DIR/const-promoted-opaque.rs:14:20
+   |
+LL |     pub type Foo = impl Sized;
+   |                    ^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

tests/ui/consts/const-promoted-opaque.rs

@@ -0,0 +1,36 @@
+// revisions: string unit atomic
+#![feature(type_alias_impl_trait)]
+
+//! Check that we do not cause cycle errors when trying to
+//! obtain information about interior mutability of an opaque type.
+//! This used to happen, because when the body-analysis failed, we
+//! checked the type instead, but the constant was also defining the
+//! hidden type of the opaque type. Thus we ended up relying on the
+//! result of our analysis to compute the result of our analysis.
+
+//[unit] check-pass
+
+mod helper {
+    pub type Foo = impl Sized;
+    //[string,atomic]~^ ERROR cycle detected
+
+    #[cfg(string)]
+    pub const FOO: Foo = String::new();
+
+    #[cfg(atomic)]
+    pub const FOO: Foo = std::sync::atomic::AtomicU8::new(42);
+
+    #[cfg(unit)]
+    pub const FOO: Foo = ();
+}
+use helper::*;
+
+const BAR: () = {
+    let _: &'static _ = &FOO;
+};
+
+const BAZ: &Foo = &FOO;
+
+fn main() {
+    let _: &'static _ = &FOO;
+}

tests/ui/consts/const-promoted-opaque.string.stderr

@@ -0,0 +1,34 @@
+error[E0391]: cycle detected when computing type of opaque `helper::Foo::{opaque#0}`
+  --> $DIR/const-promoted-opaque.rs:14:20
+   |
+LL |     pub type Foo = impl Sized;
+   |                    ^^^^^^^^^^
+   |
+note: ...which requires borrow-checking `helper::FOO`...
+  --> $DIR/const-promoted-opaque.rs:18:5
+   |
+LL |     pub const FOO: Foo = String::new();
+   |     ^^^^^^^^^^^^^^^^^^
+note: ...which requires promoting constants in MIR for `helper::FOO`...
+  --> $DIR/const-promoted-opaque.rs:18:5
+   |
+LL |     pub const FOO: Foo = String::new();
+   |     ^^^^^^^^^^^^^^^^^^
+note: ...which requires const checking `helper::FOO`...
+  --> $DIR/const-promoted-opaque.rs:18:5
+   |
+LL |     pub const FOO: Foo = String::new();
+   |     ^^^^^^^^^^^^^^^^^^
+   = note: ...which requires computing whether `helper::Foo` is freeze...
+   = note: ...which requires evaluating trait selection obligation `helper::Foo: core::marker::Freeze`...
+   = note: ...which again requires computing type of opaque `helper::Foo::{opaque#0}`, completing the cycle
+note: cycle used when computing type of `helper::Foo::{opaque#0}`
+  --> $DIR/const-promoted-opaque.rs:14:20
+   |
+LL |     pub type Foo = impl Sized;
+   |                    ^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.