fix for Self not respecting tuple Ctor privacy

This fixes #111220 by checking the privacy of tuple constructors using `Self`, so the following code now errors

```rust
mod my {
    pub struct Foo(&'static str);
}

impl AsRef<str> for my::Foo {
    fn as_ref(&self) -> &str {
        let Self(s) = self; // previously compiled, now errors correctly
        s
    }
}
```

compiler/rustc_hir_typeck/messages.ftl

@@ -25,6 +25,8 @@ hir_typeck_const_select_must_be_fn = this argument must be a function item
 
 hir_typeck_convert_to_str = try converting the passed type into a `&str`
 
+hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
+
 hir_typeck_expected_default_return_type = expected `()` because of default return type
 
 hir_typeck_expected_return_type = expected `{$expected}` because of return type

compiler/rustc_hir_typeck/src/errors.rs

@@ -319,3 +319,11 @@ pub struct CandidateTraitNote {
     pub item_name: Ident,
     pub action_or_ty: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_typeck_ctor_is_private, code = "E0603")]
+pub struct CtorIsPrivate {
+    #[primary_span]
+    pub span: Span,
+    pub def: String,
+}

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1,4 +1,5 @@
 use crate::callee::{self, DeferredCallResolution};
+use crate::errors::CtorIsPrivate;
 use crate::method::{self, MethodCallee, SelfSource};
 use crate::rvalue_scopes;
 use crate::{BreakableCtxt, Diverges, Expectation, FnCtxt, LocalTy, RawTy};
@@ -1207,6 +1208,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             match ty.normalized.ty_adt_def() {
                 Some(adt_def) if adt_def.has_ctor() => {
                     let (ctor_kind, ctor_def_id) = adt_def.non_enum_variant().ctor.unwrap();
+                    // Check the visibility of the ctor.
+                    let vis = tcx.visibility(ctor_def_id);
+                    if !vis.is_accessible_from(tcx.parent_module(hir_id).to_def_id(), tcx) {
+                        tcx.sess
+                            .emit_err(CtorIsPrivate { span, def: tcx.def_path_str(adt_def.did()) });
+                    }
                     let new_res = Res::Def(DefKind::Ctor(CtorOf::Struct, ctor_kind), ctor_def_id);
                     let user_substs = Self::user_substs_for_adt(ty);
                     user_self_ty = user_substs.user_self_ty;

tests/ui/privacy/issue-111220-2-tuple-struct-fields-projection.rs

@@ -0,0 +1,33 @@
+mod b {
+    pub struct A(u32);
+}
+
+trait Id {
+    type Assoc;
+}
+impl Id for b::A {
+    type Assoc = b::A;
+}
+impl Id for u32 {
+    type Assoc = u32;
+}
+
+
+trait Trait<T> {
+    fn method(&self)
+    where
+        T: Id<Assoc = b::A>;
+}
+
+impl<T: Id> Trait<T> for <T as Id>::Assoc {
+    fn method(&self)
+    where
+        T: Id<Assoc = b::A>,
+    {
+        let Self(a) = self;
+        //~^ ERROR: tuple struct constructor `A` is private
+        println!("{a}");
+    }
+}
+
+fn main() {}

tests/ui/privacy/issue-111220-2-tuple-struct-fields-projection.stderr

@@ -0,0 +1,9 @@
+error[E0603]: tuple struct constructor `A` is private
+  --> $DIR/issue-111220-2-tuple-struct-fields-projection.rs:27:13
+   |
+LL |         let Self(a) = self;
+   |             ^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0603`.

tests/ui/privacy/issue-111220-tuple-struct-fields.rs

@@ -0,0 +1,46 @@
+mod b {
+    #[derive(Default)]
+    pub struct A(u32);
+}
+
+impl b::A {
+    fn inherent_bypass(&self) {
+        let Self(x) = self;
+        //~^ ERROR: tuple struct constructor `A` is private
+        println!("{x}");
+    }
+}
+
+pub trait B {
+    fn f(&self);
+}
+
+impl B for b::A {
+    fn f(&self) {
+        let Self(a) = self;
+        //~^ ERROR: tuple struct constructor `A` is private
+        println!("{}", a);
+    }
+}
+
+pub trait Projector {
+    type P;
+}
+
+impl Projector for () {
+    type P = b::A;
+}
+
+pub trait Bypass2 {
+    fn f2(&self);
+}
+
+impl Bypass2 for <() as Projector>::P {
+    fn f2(&self) {
+        let Self(a) = self;
+        //~^ ERROR: tuple struct constructor `A` is private
+        println!("{}", a);
+    }
+}
+
+fn main() {}

tests/ui/privacy/issue-111220-tuple-struct-fields.stderr

@@ -0,0 +1,21 @@
+error[E0603]: tuple struct constructor `A` is private
+  --> $DIR/issue-111220-tuple-struct-fields.rs:8:13
+   |
+LL |         let Self(x) = self;
+   |             ^^^^^^^
+
+error[E0603]: tuple struct constructor `A` is private
+  --> $DIR/issue-111220-tuple-struct-fields.rs:20:13
+   |
+LL |         let Self(a) = self;
+   |             ^^^^^^^
+
+error[E0603]: tuple struct constructor `A` is private
+  --> $DIR/issue-111220-tuple-struct-fields.rs:40:13
+   |
+LL |         let Self(a) = self;
+   |             ^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0603`.