Auto merge of #126128 - oli-obk:method_ice, r=lcnr

Consistently use subtyping in method resolution

fixes #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`

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

@@ -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-arg2.rs

@@ -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.stderr

@@ -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/coercion/coerce-issue-49593-box-never.fallback.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.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)`
 

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

@@ -1,7 +1,5 @@
 //@ revisions: nofallback fallback
-//@ ignore-windows - the number of `Error` impls is platform-dependent
-//@[fallback] check-pass
-//@[nofallback] check-fail
+//@check-fail
 
 #![feature(never_type)]
 #![cfg_attr(fallback, feature(never_type_fallback))]
@@ -15,18 +13,21 @@ fn raw_ptr_box<T>(t: T) -> *mut T {
 }
 
 fn foo(x: !) -> Box<dyn Error> {
-    /* *mut $0 is coerced to Box<dyn Error> here */ Box::<_ /* ! */>::new(x)
-    //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
+    // Subtyping during method resolution will generate new inference vars and
+    // subtype them. Thus fallback will not fall back to `!`, but `()` instead.
+    Box::<_ /* ! */>::new(x)
+    //~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn foo_raw_ptr(x: !) -> *mut dyn Error {
-    /* *mut $0 is coerced to *mut Error here */ raw_ptr_box::<_ /* ! */>(x)
+    /* *mut $0 is coerced to *mut Error here */
+    raw_ptr_box::<_ /* ! */>(x)
     //[nofallback]~^ ERROR trait bound `(): std::error::Error` is not satisfied
 }
 
 fn no_coercion(d: *mut dyn Error) -> *mut dyn Error {
     /* an unsize coercion won't compile here, and it is indeed not used
-       because there is nothing requiring the _ to be Sized */
+    because there is nothing requiring the _ to be Sized */
     d as *mut _
 }
 
@@ -49,10 +50,9 @@ fn foo_no_never() {
         first_iter = true;
     }
 
-    let mut y : Option<S> = None;
+    let mut y: Option<S> = None;
     // assert types are equal
     mem::swap(&mut x, &mut y);
 }
 
-fn main() {
-}
+fn main() {}

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

@@ -15,7 +15,7 @@ note: required by a bound 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 type for type parameter `T` is specified
    |
-LL |     let y: Mask<_, N> = Mask::<_, _>::splat(false);
+LL |     let y: Mask<T, N> = Mask::<_, _>::splat(false);
    |          ++++++++++++
 
 error: aborting due to 1 previous error

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`
+   |     ^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `DirectAlias`
 
 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`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the type alias `DirectButWithDefaultAlias`
 
 error: aborting due to 3 previous errors
 

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

@@ -17,6 +17,7 @@ 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

@@ -10,6 +10,16 @@ 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
    |
@@ -21,21 +31,21 @@ 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:21:14
+  --> $DIR/mut-borrow-needed-by-trait.rs:22:14
    |
 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:21:14
+  --> $DIR/mut-borrow-needed-by-trait.rs:22:14
    |
 LL |     writeln!(fp, "hello world").unwrap();
    |              ^^
    = note: the following trait bounds were not satisfied:
            `&dyn std::io::Write: std::io::Write`
            which is required by `BufWriter<&dyn std::io::Write>: std::io::Write`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0277, E0599.
 For more information about an error, try `rustc --explain E0277`.