Rollup merge of rust-lang#126128 - oli-obk:method_ice, r=lcnr

Consistently use subtyping in method resolution

fixes rust-lang#126062

An earlier version of this PR modified how we compute variance, but the root cause was an inconsistency between the usage of `eq` and `sub`, where we assumed that the latter passing implies the former will pass.

r? `@compiler-errors`

Author: GuillaumeGomez

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -750,16 +750,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// Resolves an associated value path into a base type and associated constant, or method
     /// resolution. The newly resolved definition is written into `type_dependent_defs`.
+    #[instrument(level = "trace", skip(self), ret)]
     pub fn resolve_ty_and_res_fully_qualified_call(
         &self,
         qpath: &'tcx QPath<'tcx>,
         hir_id: HirId,
         span: Span,
     ) -> (Res, Option<LoweredTy<'tcx>>, &'tcx [hir::PathSegment<'tcx>]) {
-        debug!(
-            "resolve_ty_and_res_fully_qualified_call: qpath={:?} hir_id={:?} span={:?}",
-            qpath, hir_id, span
-        );
         let (ty, qself, item_segment) = match *qpath {
             QPath::Resolved(ref opt_qself, path) => {
                 return (
@@ -1417,10 +1414,10 @@ 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).eq(
+            match self.at(&self.misc(span), self.param_env).sub(
                 DefineOpaqueTypes::Yes,
-                impl_ty,
                 self_ty,
+                impl_ty,
             ) {
                 Ok(ok) => self.register_infer_ok_obligations(ok),
                 Err(_) => {

tests/crashes/126062.rs

@@ -0,0 +1,10 @@
+struct Fail<T>;
+//~^ ERROR: type parameter `T` is never used
+
+impl Fail<i32> {
+    const C: () = ();
+}
+
+fn main() {
+    Fail::<()>::C
+}

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

@@ -0,0 +1,12 @@
+error[E0392]: type parameter `T` is never used
+  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg.rs:1:13
+   |
+LL | struct Fail<T>;
+   |             ^ unused type parameter
+   |
+   = 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
+
+For more information about this error, try `rustc --explain E0392`.

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

@@ -0,0 +1,17 @@
+trait Proj {
+    type Assoc;
+}
+impl<T> Proj for T {
+    type Assoc = T;
+}
+
+struct Fail<T: Proj<Assoc = U>, U>(T);
+
+impl Fail<i32, i32> {
+    const C: () = ();
+}
+
+fn main() {
+    Fail::<i32, u32>::C
+    //~^ ERROR: type mismatch
+}

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

@@ -0,0 +1,20 @@
+error[E0271]: type mismatch resolving `<i32 as Proj>::Assoc == u32`
+  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg2.rs:15:5
+   |
+LL |     Fail::<i32, u32>::C
+   |     ^^^^^^^^^^^^^^^^ type mismatch resolving `<i32 as Proj>::Assoc == u32`
+   |
+note: expected this to be `u32`
+  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg2.rs:5:18
+   |
+LL |     type Assoc = T;
+   |                  ^
+note: required by a bound in `Fail`
+  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg2.rs:8:21
+   |
+LL | struct Fail<T: Proj<Assoc = U>, U>(T);
+   |                     ^^^^^^^^^ required by this bound in `Fail`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0271`.

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

@@ -0,0 +1,11 @@
+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-windows.nofallback.stderr

@@ -1,16 +1,16 @@
 error[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:18:53
+  --> $DIR/coerce-issue-49593-box-never.rs:18:5
    |
-LL |     /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
-   |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+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[E0277]: the trait bound `(): std::error::Error` is not satisfied
-  --> $DIR/coerce-issue-49593-box-never.rs:23:49
+  --> $DIR/coerce-issue-49593-box-never.rs:24:5
    |
-LL |     /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
-   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
+LL |     raw_ptr_box::<_ /* ! */>(x)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `()`
    |
    = note: required for the cast from `*mut ()` to `*mut (dyn std::error::Error + 'static)`