Auto merge of #129073 - compiler-errors:receiver-variance, r=lcnr

Relate receiver invariantly in method probe for `Mode::Path`

Effectively reverts part of #126128
Fixes #126227

This PR changes method probing to use equality for fully path-based method lookup, and subtyping for receiver `.` method lookup.

r? lcnr

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1388,10 +1388,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // This also occurs for an enum variant on a type alias.
             let impl_ty = self.normalize(span, tcx.type_of(impl_def_id).instantiate(tcx, args));
             let self_ty = self.normalize(span, self_ty);
-            match self.at(&self.misc(span), self.param_env).sub(
+            match self.at(&self.misc(span), self.param_env).eq(
                 DefineOpaqueTypes::Yes,
-                self_ty,
                 impl_ty,
+                self_ty,
             ) {
                 Ok(ok) => self.register_infer_ok_obligations(ok),
                 Err(_) => {

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -621,6 +621,16 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         self.unsatisfied_predicates.borrow_mut().clear();
     }
 
+    /// When we're looking up a method by path (UFCS), we relate the receiver
+    /// types invariantly. When we are looking up a method by the `.` operator,
+    /// we relate them covariantly.
+    fn variance(&self) -> ty::Variance {
+        match self.mode {
+            Mode::MethodCall => ty::Covariant,
+            Mode::Path => ty::Invariant,
+        }
+    }
+
     ///////////////////////////////////////////////////////////////////////////
     // CANDIDATE ASSEMBLY
 
@@ -1443,7 +1453,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     (xform_self_ty, xform_ret_ty) =
                         self.xform_self_ty(probe.item, impl_ty, impl_args);
                     xform_self_ty = ocx.normalize(cause, self.param_env, xform_self_ty);
-                    match ocx.sup(cause, self.param_env, xform_self_ty, self_ty) {
+                    match ocx.relate(cause, self.param_env, self.variance(), self_ty, xform_self_ty)
+                    {
                         Ok(()) => {}
                         Err(err) => {
                             debug!("--> cannot relate self-types {:?}", err);
@@ -1514,7 +1525,13 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                         {
                             return ProbeResult::NoMatch;
                         }
-                        _ => match ocx.sup(cause, self.param_env, xform_self_ty, self_ty) {
+                        _ => match ocx.relate(
+                            cause,
+                            self.param_env,
+                            self.variance(),
+                            self_ty,
+                            xform_self_ty,
+                        ) {
                             Ok(()) => {}
                             Err(err) => {
                                 debug!("--> cannot relate self-types {:?}", err);
@@ -1560,7 +1577,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     (xform_self_ty, xform_ret_ty) =
                         self.xform_self_ty(probe.item, trait_ref.self_ty(), trait_ref.args);
                     xform_self_ty = ocx.normalize(cause, self.param_env, xform_self_ty);
-                    match ocx.sup(cause, self.param_env, xform_self_ty, self_ty) {
+                    match ocx.relate(cause, self.param_env, self.variance(), self_ty, xform_self_ty)
+                    {
                         Ok(()) => {}
                         Err(err) => {
                             debug!("--> cannot relate self-types {:?}", err);
@@ -1603,7 +1621,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 }
 
                 debug!("comparing return_ty {:?} with xform ret ty {:?}", return_ty, xform_ret_ty);
-                match ocx.sup(cause, self.param_env, return_ty, xform_ret_ty) {
+                match ocx.relate(cause, self.param_env, self.variance(), xform_ret_ty, return_ty) {
                     Ok(()) => {}
                     Err(_) => {
                         result = ProbeResult::BadReturnType;

tests/ui/associated-consts/wrong-projection-self-ty-invalid-bivariant-arg.rs

@@ -7,4 +7,5 @@ impl Fail<i32> {
 
 fn main() {
     Fail::<()>::C
+    //~^ ERROR no associated item named `C` found for struct `Fail<()>` in the current scope
 }

tests/ui/associated-consts/wrong-projection-self-ty-invalid-bivariant-arg.stderr

@@ -7,6 +7,19 @@ LL | struct Fail<T>;
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
    = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
 
-error: aborting due to 1 previous error
+error[E0599]: no associated item named `C` found for struct `Fail<()>` in the current scope
+  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg.rs:9:17
+   |
+LL | struct Fail<T>;
+   | -------------- associated item `C` not found for this struct
+...
+LL |     Fail::<()>::C
+   |                 ^ associated item not found in `Fail<()>`
+   |
+   = note: the associated item was found for
+           - `Fail<i32>`
 
-For more information about this error, try `rustc --explain E0392`.
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0392, E0599.
+For more information about an error, try `rustc --explain E0392`.

tests/ui/associated-consts/wrong-projection-self-ty-invalid-bivariant-arg2.rs

@@ -14,4 +14,5 @@ impl Fail<i32, i32> {
 fn main() {
     Fail::<i32, u32>::C
     //~^ ERROR: type mismatch
+    //~| ERROR no associated item named `C` found for struct `Fail<i32, u32>` in the current scope
 }

tests/ui/associated-consts/wrong-projection-self-ty-invalid-bivariant-arg2.stderr

@@ -1,3 +1,15 @@
+error[E0599]: no associated item named `C` found for struct `Fail<i32, u32>` in the current scope
+  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg2.rs:15:23
+   |
+LL | struct Fail<T: Proj<Assoc = U>, U>(T);
+   | ---------------------------------- associated item `C` not found for this struct
+...
+LL |     Fail::<i32, u32>::C
+   |                       ^ associated item not found in `Fail<i32, u32>`
+   |
+   = note: the associated item was found for
+           - `Fail<i32, i32>`
+
 error[E0271]: type mismatch resolving `<i32 as Proj>::Assoc == u32`
   --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg2.rs:15:5
    |
@@ -15,6 +27,7 @@ note: required by a bound in `Fail`
 LL | struct Fail<T: Proj<Assoc = U>, U>(T);
    |                     ^^^^^^^^^ required by this bound in `Fail`
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0271`.
+Some errors have detailed explanations: E0271, E0599.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/coercion/coerce-issue-49593-box-never.fallback.stderr

@@ -1,11 +0,0 @@
-error[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:18:5
-   |
-LL |     Box::<_ /* ! */>::new(x)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
-   |
-   = note: required for the cast from `Box<()>` to `Box<(dyn std::error::Error + 'static)>`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0277`.

tests/ui/coercion/coerce-issue-49593-box-never.rs

@@ -1,5 +1,5 @@
 //@ revisions: nofallback fallback
-//@check-fail
+//@[fallback] check-pass
 
 #![feature(never_type)]
 #![cfg_attr(fallback, feature(never_type_fallback))]
@@ -13,10 +13,10 @@ fn raw_ptr_box<T>(t: T) -> *mut T {
 }
 
 fn foo(x: !) -> Box<dyn Error> {
-    // Subtyping during method resolution will generate new inference vars and
+    // Method resolution will generate new inference vars and relate them.
-    // subtype them. Thus fallback will not fall back to `!`, but `()` instead.
+    // Thus fallback will not fall back to `!`, but `()` instead.
     Box::<_ /* ! */>::new(x)
-    //~^ ERROR trait bound `(): std::error::Error` is not satisfied
+    //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn foo_raw_ptr(x: !) -> *mut dyn Error {

tests/ui/const-generics/generic_arg_infer/issue-91614.stderr

@@ -8,7 +8,7 @@ note: required by a const generic parameter in `Mask::<T, N>::splat`
   --> $SRC_DIR/core/src/../../portable-simd/crates/core_simd/src/masks.rs:LL:COL
 help: consider giving `y` an explicit type, where the value of const parameter `N` is specified
    |
-LL |     let y: Mask<T, N> = Mask::<_, _>::splat(false);
+LL |     let y: Mask<_, N> = Mask::<_, _>::splat(false);
    |          ++++++++++++
 
 error[E0284]: type annotations needed for `Mask<_, _>`
@@ -21,7 +21,7 @@ note: required by a const generic parameter in `Mask`
   --> $SRC_DIR/core/src/../../portable-simd/crates/core_simd/src/masks.rs:LL:COL
 help: consider giving `y` an explicit type, where the value of const parameter `N` is specified
    |
-LL |     let y: Mask<T, N> = Mask::<_, _>::splat(false);
+LL |     let y: Mask<_, N> = Mask::<_, _>::splat(false);
    |          ++++++++++++
 
 error: aborting due to 2 previous errors

tests/ui/impl-trait/issues/issue-62742.rs

@@ -2,7 +2,8 @@ use std::marker::PhantomData;
 
 fn a() {
     WrongImpl::foo(0i32);
-    //~^ ERROR overflow assigning `_` to `[_]`
+    //~^ ERROR the function or associated item `foo` exists for struct `SafeImpl<_, RawImpl<_>>`, but its trait bounds were not satisfied
+    //~| ERROR the trait bound `RawImpl<_>: Raw<_>` is not satisfied
 }
 
 fn b() {

tests/ui/impl-trait/issues/issue-62742.stderr

@@ -1,11 +1,43 @@
-error[E0275]: overflow assigning `_` to `[_]`
+error[E0599]: the function or associated item `foo` exists for struct `SafeImpl<_, RawImpl<_>>`, but its trait bounds were not satisfied
   --> $DIR/issue-62742.rs:4:16
    |
 LL |     WrongImpl::foo(0i32);
-   |                ^^^
+   |                ^^^ function or associated item cannot be called on `SafeImpl<_, RawImpl<_>>` due to unsatisfied trait bounds
+...
+LL | pub struct RawImpl<T>(PhantomData<T>);
+   | --------------------- doesn't satisfy `RawImpl<_>: Raw<_>`
+...
+LL | pub struct SafeImpl<T: ?Sized, A: Raw<T>>(PhantomData<(A, T)>);
+   | ----------------------------------------- function or associated item `foo` not found for this struct
+   |
+note: trait bound `RawImpl<_>: Raw<_>` was not satisfied
+  --> $DIR/issue-62742.rs:35:20
+   |
+LL | impl<T: ?Sized, A: Raw<T>> SafeImpl<T, A> {
+   |                    ^^^^^^  --------------
+   |                    |
+   |                    unsatisfied trait bound introduced here
+note: the trait `Raw` must be implemented
+  --> $DIR/issue-62742.rs:19:1
+   |
+LL | pub trait Raw<T: ?Sized> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the trait bound `RawImpl<_>: Raw<_>` is not satisfied
+  --> $DIR/issue-62742.rs:4:5
+   |
+LL |     WrongImpl::foo(0i32);
+   |     ^^^^^^^^^ the trait `Raw<_>` is not implemented for `RawImpl<_>`
+   |
+   = help: the trait `Raw<[_]>` is implemented for `RawImpl<_>`
+note: required by a bound in `SafeImpl`
+  --> $DIR/issue-62742.rs:33:35
+   |
+LL | pub struct SafeImpl<T: ?Sized, A: Raw<T>>(PhantomData<(A, T)>);
+   |                                   ^^^^^^ required by this bound in `SafeImpl`
 
 error[E0599]: the function or associated item `foo` exists for struct `SafeImpl<(), RawImpl<()>>`, but its trait bounds were not satisfied
-  --> $DIR/issue-62742.rs:9:22
+  --> $DIR/issue-62742.rs:10:22
    |
 LL |     WrongImpl::<()>::foo(0i32);
    |                      ^^^ function or associated item cannot be called on `SafeImpl<(), RawImpl<()>>` due to unsatisfied trait bounds
@@ -17,20 +49,20 @@ LL | pub struct SafeImpl<T: ?Sized, A: Raw<T>>(PhantomData<(A, T)>);
    | ----------------------------------------- function or associated item `foo` not found for this struct
    |
 note: trait bound `RawImpl<()>: Raw<()>` was not satisfied
-  --> $DIR/issue-62742.rs:34:20
+  --> $DIR/issue-62742.rs:35:20
    |
 LL | impl<T: ?Sized, A: Raw<T>> SafeImpl<T, A> {
    |                    ^^^^^^  --------------
    |                    |
    |                    unsatisfied trait bound introduced here
 note: the trait `Raw` must be implemented
-  --> $DIR/issue-62742.rs:18:1
+  --> $DIR/issue-62742.rs:19:1
    |
 LL | pub trait Raw<T: ?Sized> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `RawImpl<()>: Raw<()>` is not satisfied
-  --> $DIR/issue-62742.rs:9:5
+  --> $DIR/issue-62742.rs:10:5
    |
 LL |     WrongImpl::<()>::foo(0i32);
    |     ^^^^^^^^^^^^^^^ the trait `Raw<()>` is not implemented for `RawImpl<()>`
@@ -38,12 +70,12 @@ LL |     WrongImpl::<()>::foo(0i32);
    = help: the trait `Raw<[()]>` is implemented for `RawImpl<()>`
    = help: for that trait implementation, expected `[()]`, found `()`
 note: required by a bound in `SafeImpl`
-  --> $DIR/issue-62742.rs:32:35
+  --> $DIR/issue-62742.rs:33:35
    |
 LL | pub struct SafeImpl<T: ?Sized, A: Raw<T>>(PhantomData<(A, T)>);
    |                                   ^^^^^^ required by this bound in `SafeImpl`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0275, E0277, E0599.
+Some errors have detailed explanations: E0277, E0599.
-For more information about an error, try `rustc --explain E0275`.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/inference/need_type_info/type-alias-indirect.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/type-alias-indirect.rs:14:5
    |
 LL |     IndirectAlias::new();
-   |     ^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `IndirectAlias`
+   |     ^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `IndirectAlias`
 
 error: aborting due to 1 previous error
 

tests/ui/inference/need_type_info/type-alias.stderr

@@ -2,19 +2,19 @@ error[E0282]: type annotations needed
   --> $DIR/type-alias.rs:12:5
    |
 LL |     DirectAlias::new()
-   |     ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `DirectAlias`
+   |     ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T`
 
 error[E0282]: type annotations needed
   --> $DIR/type-alias.rs:18:5
    |
 LL |     IndirectAlias::new();
-   |     ^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `IndirectAlias`
+   |     ^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `IndirectAlias`
 
 error[E0282]: type annotations needed
   --> $DIR/type-alias.rs:32:5
    |
 LL |     DirectButWithDefaultAlias::new();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `DirectButWithDefaultAlias`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T`
 
 error: aborting due to 3 previous errors
 

tests/ui/methods/receiver-equality.rs

@@ -0,0 +1,16 @@
+// Tests that we probe receivers invariantly when using path-based method lookup.
+
+struct B<T>(T);
+
+impl B<fn(&'static ())> {
+    fn method(self) {
+        println!("hey");
+    }
+}
+
+fn foo(y: B<fn(&'static ())>) {
+    B::<for<'a> fn(&'a ())>::method(y);
+    //~^ ERROR no function or associated item named `method` found
+}
+
+fn main() {}

tests/ui/methods/receiver-equality.stderr

@@ -0,0 +1,12 @@
+error[E0599]: no function or associated item named `method` found for struct `B<for<'a> fn(&'a ())>` in the current scope
+  --> $DIR/receiver-equality.rs:12:30
+   |
+LL | struct B<T>(T);
+   | ----------- function or associated item `method` not found for this struct
+...
+LL |     B::<for<'a> fn(&'a ())>::method(y);
+   |                              ^^^^^^ function or associated item not found in `B<fn(&())>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.

tests/ui/suggestions/mut-borrow-needed-by-trait.rs

@@ -17,7 +17,6 @@ fn main() {
     let fp = BufWriter::new(fp);
     //~^ ERROR the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
     //~| ERROR the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
-    //~| ERROR the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
 
     writeln!(fp, "hello world").unwrap(); //~ ERROR the method
 }

tests/ui/suggestions/mut-borrow-needed-by-trait.stderr

@@ -10,16 +10,6 @@ LL |     let fp = BufWriter::new(fp);
 note: required by a bound in `BufWriter::<W>::new`
   --> $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
 
-error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
-  --> $DIR/mut-borrow-needed-by-trait.rs:17:14
-   |
-LL |     let fp = BufWriter::new(fp);
-   |              ^^^^^^^^^ the trait `std::io::Write` is not implemented for `&dyn std::io::Write`
-   |
-   = note: `std::io::Write` is implemented for `&mut dyn std::io::Write`, but not for `&dyn std::io::Write`
-note: required by a bound in `BufWriter`
-  --> $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
-
 error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satisfied
   --> $DIR/mut-borrow-needed-by-trait.rs:17:14
    |
@@ -31,13 +21,13 @@ note: required by a bound in `BufWriter`
   --> $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
 
 error[E0599]: the method `write_fmt` exists for struct `BufWriter<&dyn Write>`, but its trait bounds were not satisfied
-  --> $DIR/mut-borrow-needed-by-trait.rs:22:14
+  --> $DIR/mut-borrow-needed-by-trait.rs:21:14
    |
 LL |     writeln!(fp, "hello world").unwrap();
    |     ---------^^---------------- method cannot be called on `BufWriter<&dyn Write>` due to unsatisfied trait bounds
    |
 note: must implement `io::Write`, `fmt::Write`, or have a `write_fmt` method
-  --> $DIR/mut-borrow-needed-by-trait.rs:22:14
+  --> $DIR/mut-borrow-needed-by-trait.rs:21:14
    |
 LL |     writeln!(fp, "hello world").unwrap();
    |              ^^
@@ -45,7 +35,7 @@ LL |     writeln!(fp, "hello world").unwrap();
            `&dyn std::io::Write: std::io::Write`
            which is required by `BufWriter<&dyn std::io::Write>: std::io::Write`
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0277, E0599.
 For more information about an error, try `rustc --explain E0277`.