Auto merge of #113169 - oli-obk:tait_must_be_constrained_if_in_sig, r=lcnr

Tait must be constrained if in sig

r? `@compiler-errors`

kind of reverts #62423, but that PR only removed cycles in cases that we want to forbid now anyway.

see https://rust-lang.zulipchat.com/#narrow/stream/315482-t-compiler.2Fetc.2Fopaque-types/topic/lcnr.20oli.20meeting/near/370712246 for related discussion and motivating example

a TAIT showing up in a signature doesn't just mean it can register a hidden type, but that it must.

This is the [design the types team decided upon](https://hackmd.io/QOsEaEJtQK-XDS_xN4UyQA#Proposal-preferred) for the following reasons

* avoids a hypothetical situation where getting smarter in trait solving could cause new cycle errors or new inference errors to show up, where today something is treated as opaque.
* avoids having the situation where a (minor?) change to a function body causes an error, because its TAIT usage suddenly isn't "opaque enough" anymore.
* avoids having to explain why in some cases you need to put a Tait into a tiny module together with its defining usages. Now you basically gotta always do this, the moment a Tait is in a signature that isn't in the defining scope.
* avoids false-cycle errors
* anything diverging from this pattern needs to either
    * move the TAIT declaration and defining function into their own helper sub module
    * use the attributes we'll provide in the future to explicitly opt in or out of being in the defining scope

fixes #117861

reverts #125362 cc `@Nilstrieb` `@joboet`

Author: bors

Diff for: compiler/rustc_codegen_cranelift/example/issue-72793.rs

@@ -2,20 +2,23 @@
 
 #![feature(type_alias_impl_trait)]
 
-trait T {
-    type Item;
-}
+mod helper {
+    pub trait T {
+        type Item;
+    }
 
-type Alias<'a> = impl T<Item = &'a ()>;
+    pub type Alias<'a> = impl T<Item = &'a ()>;
 
-struct S;
-impl<'a> T for &'a S {
-    type Item = &'a ();
-}
+    struct S;
+    impl<'a> T for &'a S {
+        type Item = &'a ();
+    }
 
-fn filter_positive<'a>() -> Alias<'a> {
-    &S
+    pub fn filter_positive<'a>() -> Alias<'a> {
+        &S
+    }
 }
+use helper::*;
 
 fn with_positive(fun: impl Fn(Alias<'_>)) {
     fun(filter_positive());

Diff for: compiler/rustc_data_structures/src/obligation_forest/mod.rs

@@ -146,8 +146,6 @@ pub enum ProcessResult<O, E> {
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
 struct ObligationTreeId(usize);
 
-type ObligationTreeIdGenerator = impl Iterator<Item = ObligationTreeId>;
-
 pub struct ObligationForest<O: ForestObligation> {
     /// The list of obligations. In between calls to [Self::process_obligations],
     /// this list only contains nodes in the `Pending` or `Waiting` state.
@@ -310,18 +308,25 @@ pub struct Error<O, E> {
     pub backtrace: Vec<O>,
 }
 
-impl<O: ForestObligation> ObligationForest<O> {
-    pub fn new() -> ObligationForest<O> {
-        ObligationForest {
-            nodes: vec![],
-            done_cache: Default::default(),
-            active_cache: Default::default(),
-            reused_node_vec: vec![],
-            obligation_tree_id_generator: (0..).map(ObligationTreeId),
-            error_cache: Default::default(),
+mod helper {
+    use super::*;
+    pub type ObligationTreeIdGenerator = impl Iterator<Item = ObligationTreeId>;
+    impl<O: ForestObligation> ObligationForest<O> {
+        pub fn new() -> ObligationForest<O> {
+            ObligationForest {
+                nodes: vec![],
+                done_cache: Default::default(),
+                active_cache: Default::default(),
+                reused_node_vec: vec![],
+                obligation_tree_id_generator: (0..).map(ObligationTreeId),
+                error_cache: Default::default(),
+            }
         }
     }
+}
+use helper::*;
 
+impl<O: ForestObligation> ObligationForest<O> {
     /// Returns the total number of nodes in the forest that have not
     /// yet been fully resolved.
     pub fn len(&self) -> usize {

Diff for: compiler/rustc_error_codes/src/error_codes/E0792.md

@@ -39,6 +39,8 @@ type Foo<T> = impl std::fmt::Debug;
 fn foo<U>() -> Foo<U> {
     5u32
 }
+
+fn main() {}
 ```
 
 This means that no matter the generic parameter to `foo`,
@@ -57,4 +59,6 @@ type Foo<T: Debug> = impl Debug;
 fn foo<U: Debug>() -> Foo<U> {
     Vec::<U>::new()
 }
+
+fn main() {}
 ```

Diff for: compiler/rustc_hir_analysis/messages.ftl

@@ -460,6 +460,10 @@ hir_analysis_static_specialize = cannot specialize on `'static` lifetime
 hir_analysis_tait_forward_compat = item constrains opaque type that is not in its signature
     .note = this item must mention the opaque type in its signature in order to be able to register hidden types
 
+hir_analysis_tait_forward_compat2 = item does not constrain `{$opaque_type}`, but has it in its signature
+    .note = consider moving the opaque type's declaration and defining uses into a separate module
+    .opaque = this opaque type is in the signature
+
 hir_analysis_target_feature_on_main = `main` function is not allowed to have `#[target_feature]`
 
 hir_analysis_too_large_static = extern static is too large for the current architecture

Diff for: compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -8,7 +8,7 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::{sym, ErrorGuaranteed, DUMMY_SP};
 
-use crate::errors::{TaitForwardCompat, TypeOf, UnconstrainedOpaqueType};
+use crate::errors::{TaitForwardCompat, TaitForwardCompat2, TypeOf, UnconstrainedOpaqueType};
 
 pub fn test_opaque_hidden_types(tcx: TyCtxt<'_>) -> Result<(), ErrorGuaranteed> {
     let mut res = Ok(());
@@ -229,13 +229,14 @@ impl TaitConstraintLocator<'_> {
             return;
         }
 
+        let opaque_types_defined_by = self.tcx.opaque_types_defined_by(item_def_id);
+
         let mut constrained = false;
         for (&opaque_type_key, &hidden_type) in &tables.concrete_opaque_types {
             if opaque_type_key.def_id != self.def_id {
                 continue;
             }
             constrained = true;
-            let opaque_types_defined_by = self.tcx.opaque_types_defined_by(item_def_id);
 
             if !opaque_types_defined_by.contains(&self.def_id) {
                 self.tcx.dcx().emit_err(TaitForwardCompat {
@@ -259,6 +260,16 @@ impl TaitConstraintLocator<'_> {
 
         if !constrained {
             debug!("no constraints in typeck results");
+            if opaque_types_defined_by.contains(&self.def_id) {
+                self.tcx.dcx().emit_err(TaitForwardCompat2 {
+                    span: self
+                        .tcx
+                        .def_ident_span(item_def_id)
+                        .unwrap_or_else(|| self.tcx.def_span(item_def_id)),
+                    opaque_type_span: self.tcx.def_span(self.def_id),
+                    opaque_type: self.tcx.def_path_str(self.def_id),
+                });
+            }
             return;
         };
 

Diff for: compiler/rustc_hir_analysis/src/errors.rs

@@ -390,6 +390,17 @@ pub struct TaitForwardCompat {
     pub item_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_tait_forward_compat2)]
+#[note]
+pub struct TaitForwardCompat2 {
+    #[primary_span]
+    pub span: Span,
+    #[note(hir_analysis_opaque)]
+    pub opaque_type_span: Span,
+    pub opaque_type: String,
+}
+
 pub struct MissingTypeParams {
     pub span: Span,
     pub def_span: Span,

Diff for: compiler/rustc_lint/src/opaque_hidden_inferred_bound.rs

@@ -46,6 +46,8 @@ declare_lint! {
     /// fn test() -> impl Trait<Assoc = Tait> {
     ///     42
     /// }
+    ///
+    /// fn main() {}
     /// ```
     ///
     /// {{produces}}

Diff for: compiler/rustc_middle/src/mir/terminator.rs

@@ -377,9 +377,6 @@ pub struct Terminator<'tcx> {
     pub kind: TerminatorKind<'tcx>,
 }
 
-pub type Successors<'a> = impl DoubleEndedIterator<Item = BasicBlock> + 'a;
-pub type SuccessorsMut<'a> = impl DoubleEndedIterator<Item = &'a mut BasicBlock> + 'a;
-
 impl<'tcx> Terminator<'tcx> {
     #[inline]
     pub fn successors(&self) -> Successors<'_> {
@@ -407,81 +404,95 @@ impl<'tcx> TerminatorKind<'tcx> {
     pub fn if_(cond: Operand<'tcx>, t: BasicBlock, f: BasicBlock) -> TerminatorKind<'tcx> {
         TerminatorKind::SwitchInt { discr: cond, targets: SwitchTargets::static_if(0, f, t) }
     }
+}
 
-    #[inline]
-    pub fn successors(&self) -> Successors<'_> {
-        use self::TerminatorKind::*;
-        match *self {
-            Call { target: Some(ref t), unwind: UnwindAction::Cleanup(u), .. }
-            | Yield { resume: ref t, drop: Some(u), .. }
-            | Drop { target: ref t, unwind: UnwindAction::Cleanup(u), .. }
-            | Assert { target: ref t, unwind: UnwindAction::Cleanup(u), .. }
-            | FalseUnwind { real_target: ref t, unwind: UnwindAction::Cleanup(u) } => {
-                slice::from_ref(t).into_iter().copied().chain(Some(u))
-            }
-            Goto { target: ref t }
-            | Call { target: None, unwind: UnwindAction::Cleanup(ref t), .. }
-            | Call { target: Some(ref t), unwind: _, .. }
-            | Yield { resume: ref t, drop: None, .. }
-            | Drop { target: ref t, unwind: _, .. }
-            | Assert { target: ref t, unwind: _, .. }
-            | FalseUnwind { real_target: ref t, unwind: _ } => {
-                slice::from_ref(t).into_iter().copied().chain(None)
-            }
-            UnwindResume
-            | UnwindTerminate(_)
-            | CoroutineDrop
-            | Return
-            | Unreachable
-            | Call { target: None, unwind: _, .. } => (&[]).into_iter().copied().chain(None),
-            InlineAsm { ref targets, unwind: UnwindAction::Cleanup(u), .. } => {
-                targets.iter().copied().chain(Some(u))
-            }
-            InlineAsm { ref targets, unwind: _, .. } => targets.iter().copied().chain(None),
-            SwitchInt { ref targets, .. } => targets.targets.iter().copied().chain(None),
-            FalseEdge { ref real_target, imaginary_target } => {
-                slice::from_ref(real_target).into_iter().copied().chain(Some(imaginary_target))
+pub use helper::*;
+
+mod helper {
+    use super::*;
+    pub type Successors<'a> = impl DoubleEndedIterator<Item = BasicBlock> + 'a;
+    pub type SuccessorsMut<'a> = impl DoubleEndedIterator<Item = &'a mut BasicBlock> + 'a;
+    impl<'tcx> TerminatorKind<'tcx> {
+        #[inline]
+        pub fn successors(&self) -> Successors<'_> {
+            use self::TerminatorKind::*;
+            match *self {
+                Call { target: Some(ref t), unwind: UnwindAction::Cleanup(u), .. }
+                | Yield { resume: ref t, drop: Some(u), .. }
+                | Drop { target: ref t, unwind: UnwindAction::Cleanup(u), .. }
+                | Assert { target: ref t, unwind: UnwindAction::Cleanup(u), .. }
+                | FalseUnwind { real_target: ref t, unwind: UnwindAction::Cleanup(u) } => {
+                    slice::from_ref(t).into_iter().copied().chain(Some(u))
+                }
+                Goto { target: ref t }
+                | Call { target: None, unwind: UnwindAction::Cleanup(ref t), .. }
+                | Call { target: Some(ref t), unwind: _, .. }
+                | Yield { resume: ref t, drop: None, .. }
+                | Drop { target: ref t, unwind: _, .. }
+                | Assert { target: ref t, unwind: _, .. }
+                | FalseUnwind { real_target: ref t, unwind: _ } => {
+                    slice::from_ref(t).into_iter().copied().chain(None)
+                }
+                UnwindResume
+                | UnwindTerminate(_)
+                | CoroutineDrop
+                | Return
+                | Unreachable
+                | Call { target: None, unwind: _, .. } => (&[]).into_iter().copied().chain(None),
+                InlineAsm { ref targets, unwind: UnwindAction::Cleanup(u), .. } => {
+                    targets.iter().copied().chain(Some(u))
+                }
+                InlineAsm { ref targets, unwind: _, .. } => targets.iter().copied().chain(None),
+                SwitchInt { ref targets, .. } => targets.targets.iter().copied().chain(None),
+                FalseEdge { ref real_target, imaginary_target } => {
+                    slice::from_ref(real_target).into_iter().copied().chain(Some(imaginary_target))
+                }
             }
         }
-    }
 
-    #[inline]
-    pub fn successors_mut(&mut self) -> SuccessorsMut<'_> {
-        use self::TerminatorKind::*;
-        match *self {
-            Call { target: Some(ref mut t), unwind: UnwindAction::Cleanup(ref mut u), .. }
-            | Yield { resume: ref mut t, drop: Some(ref mut u), .. }
-            | Drop { target: ref mut t, unwind: UnwindAction::Cleanup(ref mut u), .. }
-            | Assert { target: ref mut t, unwind: UnwindAction::Cleanup(ref mut u), .. }
-            | FalseUnwind { real_target: ref mut t, unwind: UnwindAction::Cleanup(ref mut u) } => {
-                slice::from_mut(t).into_iter().chain(Some(u))
-            }
-            Goto { target: ref mut t }
-            | Call { target: None, unwind: UnwindAction::Cleanup(ref mut t), .. }
-            | Call { target: Some(ref mut t), unwind: _, .. }
-            | Yield { resume: ref mut t, drop: None, .. }
-            | Drop { target: ref mut t, unwind: _, .. }
-            | Assert { target: ref mut t, unwind: _, .. }
-            | FalseUnwind { real_target: ref mut t, unwind: _ } => {
-                slice::from_mut(t).into_iter().chain(None)
-            }
-            UnwindResume
-            | UnwindTerminate(_)
-            | CoroutineDrop
-            | Return
-            | Unreachable
-            | Call { target: None, unwind: _, .. } => (&mut []).into_iter().chain(None),
-            InlineAsm { ref mut targets, unwind: UnwindAction::Cleanup(ref mut u), .. } => {
-                targets.iter_mut().chain(Some(u))
-            }
-            InlineAsm { ref mut targets, unwind: _, .. } => targets.iter_mut().chain(None),
-            SwitchInt { ref mut targets, .. } => targets.targets.iter_mut().chain(None),
-            FalseEdge { ref mut real_target, ref mut imaginary_target } => {
-                slice::from_mut(real_target).into_iter().chain(Some(imaginary_target))
+        #[inline]
+        pub fn successors_mut(&mut self) -> SuccessorsMut<'_> {
+            use self::TerminatorKind::*;
+            match *self {
+                Call {
+                    target: Some(ref mut t), unwind: UnwindAction::Cleanup(ref mut u), ..
+                }
+                | Yield { resume: ref mut t, drop: Some(ref mut u), .. }
+                | Drop { target: ref mut t, unwind: UnwindAction::Cleanup(ref mut u), .. }
+                | Assert { target: ref mut t, unwind: UnwindAction::Cleanup(ref mut u), .. }
+                | FalseUnwind {
+                    real_target: ref mut t,
+                    unwind: UnwindAction::Cleanup(ref mut u),
+                } => slice::from_mut(t).into_iter().chain(Some(u)),
+                Goto { target: ref mut t }
+                | Call { target: None, unwind: UnwindAction::Cleanup(ref mut t), .. }
+                | Call { target: Some(ref mut t), unwind: _, .. }
+                | Yield { resume: ref mut t, drop: None, .. }
+                | Drop { target: ref mut t, unwind: _, .. }
+                | Assert { target: ref mut t, unwind: _, .. }
+                | FalseUnwind { real_target: ref mut t, unwind: _ } => {
+                    slice::from_mut(t).into_iter().chain(None)
+                }
+                UnwindResume
+                | UnwindTerminate(_)
+                | CoroutineDrop
+                | Return
+                | Unreachable
+                | Call { target: None, unwind: _, .. } => (&mut []).into_iter().chain(None),
+                InlineAsm { ref mut targets, unwind: UnwindAction::Cleanup(ref mut u), .. } => {
+                    targets.iter_mut().chain(Some(u))
+                }
+                InlineAsm { ref mut targets, unwind: _, .. } => targets.iter_mut().chain(None),
+                SwitchInt { ref mut targets, .. } => targets.targets.iter_mut().chain(None),
+                FalseEdge { ref mut real_target, ref mut imaginary_target } => {
+                    slice::from_mut(real_target).into_iter().chain(Some(imaginary_target))
+                }
             }
         }
     }
+}
 
+impl<'tcx> TerminatorKind<'tcx> {
     #[inline]
     pub fn unwind(&self) -> Option<&UnwindAction> {
         match *self {

Diff for: library/core/src/internal_macros.rs

@@ -80,6 +80,47 @@ macro_rules! forward_ref_op_assign {
     }
 }
 
+/// Create a zero-size type similar to a closure type, but named.
+macro_rules! impl_fn_for_zst {
+    ($(
+        $( #[$attr: meta] )*
+        struct $Name: ident impl$( <$( $lifetime : lifetime ),+> )? Fn =
+            |$( $arg: ident: $ArgTy: ty ),*| -> $ReturnTy: ty
+            $body: block;
+    )+) => {
+        $(
+            $( #[$attr] )*
+            struct $Name;
+
+            impl $( <$( $lifetime ),+> )? Fn<($( $ArgTy, )*)> for $Name {
+                #[inline]
+                extern "rust-call" fn call(&self, ($( $arg, )*): ($( $ArgTy, )*)) -> $ReturnTy {
+                    $body
+                }
+            }
+
+            impl $( <$( $lifetime ),+> )? FnMut<($( $ArgTy, )*)> for $Name {
+                #[inline]
+                extern "rust-call" fn call_mut(
+                    &mut self,
+                    ($( $arg, )*): ($( $ArgTy, )*)
+                ) -> $ReturnTy {
+                    Fn::call(&*self, ($( $arg, )*))
+                }
+            }
+
+            impl $( <$( $lifetime ),+> )? FnOnce<($( $ArgTy, )*)> for $Name {
+                type Output = $ReturnTy;
+
+                #[inline]
+                extern "rust-call" fn call_once(self, ($( $arg, )*): ($( $ArgTy, )*)) -> $ReturnTy {
+                    Fn::call(&self, ($( $arg, )*))
+                }
+            }
+        )+
+    }
+}
+
 /// A macro for defining `#[cfg]` if-else statements.
 ///
 /// `cfg_if` is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade