Rollup merge of #146217 - lukas-code:suggest-new-ice, r=compiler-errors

fix ICE when suggesting `::new`

fixes #146174

This code suggests to write `Foo::new(...)` when the user writes `Foo(...)` or `Foo { ... }` and the constructor is private, where `new` is some associated function that returns `Self`.

When checking that the return type of `new` is `Self`, we need to instantiate the parameters of `new` with infer vars, so we don't end up with a type like `Box<$param(0)>` in a context that doesn't have any parameters. But then we can't use `normalize_erasing_late_bound_regions` anymore because that goes though a query that can't deal with infer vars.

Since this is diagnostic-only code that is supposed to check for exactly `-> Self`, I think it's fine to just skip normalizing here, especially since The Correct Way<sup>TM</sup> would involve a probe and make this code even more complicated.

Also, the code here does almost the same thing, and these suggestions can probably be unified in the future: https://github.com/rust-lang/rust/blob/4ca8078d37c53ee4ff8fb32b4453b915116f25b8/compiler/rustc_hir_typeck/src/method/suggest.rs#L2123-L2129

r? ````@compiler-errors````
cc ````@Qelxiros```` -- this should unblock #144420

Author: tgross35

compiler/rustc_hir_typeck/src/expr.rs

@@ -2585,12 +2585,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .filter(|item| item.is_fn() && !item.is_method())
                 .filter_map(|item| {
                     // Only assoc fns that return `Self`
-                    let fn_sig = self.tcx.fn_sig(item.def_id).skip_binder();
-                    let ret_ty = fn_sig.output();
-                    let ret_ty = self.tcx.normalize_erasing_late_bound_regions(
-                        self.typing_env(self.param_env),
-                        ret_ty,
-                    );
+                    let fn_sig = self
+                        .tcx
+                        .fn_sig(item.def_id)
+                        .instantiate(self.tcx, self.fresh_args_for_item(span, item.def_id));
+                    let ret_ty = self.tcx.instantiate_bound_regions_with_erased(fn_sig.output());
                     if !self.can_eq(self.param_env, ret_ty, adt_ty) {
                         return None;
                     }

tests/ui/privacy/suggest-box-new.rs

@@ -16,4 +16,7 @@ fn main() {
     let _ = std::collections::HashMap {};
     //~^ ERROR cannot construct `HashMap<_, _, _>` with struct literal syntax due to private fields
     let _ = Box {}; //~ ERROR cannot construct `Box<_, _>` with struct literal syntax due to private fields
+
+    // test that we properly instantiate the parameter of `Box::<T>::new` with an inference variable
+    let _ = Box::<i32> {}; //~ ERROR cannot construct `Box<i32>` with struct literal syntax due to private fields
 }

tests/ui/privacy/suggest-box-new.stderr

@@ -118,13 +118,41 @@ LL +     let _ = Box::new_zeroed();
 LL -     let _ = Box {};
 LL +     let _ = Box::new_in(_, _);
    |
-   = and 12 other candidates
+   = and 13 other candidates
 help: consider using the `Default` trait
    |
 LL -     let _ = Box {};
 LL +     let _ = <Box as std::default::Default>::default();
    |
 
-error: aborting due to 4 previous errors
+error: cannot construct `Box<i32>` with struct literal syntax due to private fields
+  --> $DIR/suggest-box-new.rs:21:13
+   |
+LL |     let _ = Box::<i32> {};
+   |             ^^^^^^^^^^
+   |
+   = note: private fields `0` and `1` that were not provided
+help: you might have meant to use an associated function to build this type
+   |
+LL -     let _ = Box::<i32> {};
+LL +     let _ = Box::<i32>::new(_);
+   |
+LL -     let _ = Box::<i32> {};
+LL +     let _ = Box::<i32>::new_in(_, _);
+   |
+LL -     let _ = Box::<i32> {};
+LL +     let _ = Box::<i32>::into_inner(_);
+   |
+LL -     let _ = Box::<i32> {};
+LL +     let _ = Box::<i32>::write(_, _);
+   |
+   = and 4 other candidates
+help: consider using the `Default` trait
+   |
+LL -     let _ = Box::<i32> {};
+LL +     let _ = <Box::<i32> as std::default::Default>::default();
+   |
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0423`.

tests/ui/privacy/suggest-new-projection-ice.rs

@@ -0,0 +1,19 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/146174>.
+//! Ensure that we don't ICE when an associated function returns an associated type.
+
+mod m {
+    pub trait Project {
+        type Assoc;
+    }
+    pub struct Foo {
+        _priv: (),
+    }
+    impl Foo {
+        fn new<T: Project>() -> T::Assoc {
+            todo!()
+        }
+    }
+}
+fn main() {
+    let _ = m::Foo {}; //~ ERROR: cannot construct `Foo`
+}

tests/ui/privacy/suggest-new-projection-ice.stderr

@@ -0,0 +1,10 @@
+error: cannot construct `Foo` with struct literal syntax due to private fields
+  --> $DIR/suggest-new-projection-ice.rs:18:13
+   |
+LL |     let _ = m::Foo {};
+   |             ^^^^^^
+   |
+   = note: private field `_priv` that was not provided
+
+error: aborting due to 1 previous error
+

tests/ui/suggestions/multi-suggestion.ascii.stderr

@@ -118,7 +118,7 @@ LL +     let _ = Box::new_zeroed();
 LL -     let _ = Box {};
 LL +     let _ = Box::new_in(_, _);
    |
-   = and 12 other candidates
+   = and 13 other candidates
 help: consider using the `Default` trait
    |
 LL -     let _ = Box {};

tests/ui/suggestions/multi-suggestion.unicode.stderr

@@ -118,7 +118,7 @@ LL +     let _ = Box::new_zeroed();
 LL -     let _ = Box {};
 LL +     let _ = Box::new_in(_, _);
    │
-   ╰ and 12 other candidates
+   ╰ and 13 other candidates
 help: consider using the `Default` trait
    ╭╴
 LL -     let _ = Box {};