Rollup merge of #109957 - fmease:fix-109905, r=petrochenkov

diagnostics: account for self type when looking for source of unsolved type variable

Fixes #109905.

When searching for the source of an unsolved infer var inside of a list of generic args, we look through the `tcx.generics_of(…).own_substs(…)` which *skips* the self type if present. However, the computed `argument_index` is later[^1] used to index into `tcx.generics_of(…).params` which may still contain the self type. In such case, we are off by one when indexing into the parameters.

From now on, we account for this immediately after calling `own_substs` which keeps things local.

This also fixes the wrong output in the preexisting UI test `inference/need_type_info/concrete-impl.rs` which was overlooked. It used to claim that the *type of type parameter `Self`* couldn't be inferred in `<Struct as Ambiguous<_>>::method()` which of course isn't true: `Self` equals `Struct` here, `A` couldn't be inferred.

`@rustbot` label A-diagnostics

[^1]: https://github.com/rust-lang/rust/blob/f98a2718141593fbb8dbad10acc537786d748156/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs#L471

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -1191,11 +1191,14 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
                 have_turbofish,
             } = args;
             let generics = tcx.generics_of(generics_def_id);
-            if let Some(argument_index) = generics
+            if let Some(mut argument_index) = generics
                 .own_substs(substs)
                 .iter()
                 .position(|&arg| self.generic_arg_contains_target(arg))
             {
+                if generics.parent.is_none() && generics.has_self {
+                    argument_index += 1;
+                }
                 let substs = self.infcx.resolve_vars_if_possible(substs);
                 let generic_args = &generics.own_substs_no_defaults(tcx, substs)
                     [generics.own_counts().lifetimes..];

tests/ui/inference/need_type_info/concrete-impl.rs

@@ -7,10 +7,13 @@ struct Two;
 struct Struct;
 
 impl Ambiguous<One> for Struct {}
+//~^ NOTE multiple `impl`s satisfying `Struct: Ambiguous<_>` found
 impl Ambiguous<Two> for Struct {}
 
 fn main() {
     <Struct as Ambiguous<_>>::method();
     //~^ ERROR type annotations needed
+    //~| NOTE cannot infer type of the type parameter `A`
     //~| ERROR type annotations needed
+    //~| NOTE infer type of the type parameter `A`
 }

tests/ui/inference/need_type_info/concrete-impl.stderr

@@ -1,20 +1,21 @@
 error[E0282]: type annotations needed
-  --> $DIR/concrete-impl.rs:13:5
+  --> $DIR/concrete-impl.rs:14:5
    |
 LL |     <Struct as Ambiguous<_>>::method();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `Self` declared on the trait `Ambiguous`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `A` declared on the trait `Ambiguous`
 
 error[E0283]: type annotations needed
-  --> $DIR/concrete-impl.rs:13:5
+  --> $DIR/concrete-impl.rs:14:5
    |
 LL |     <Struct as Ambiguous<_>>::method();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `Self` declared on the trait `Ambiguous`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `A` declared on the trait `Ambiguous`
    |
 note: multiple `impl`s satisfying `Struct: Ambiguous<_>` found
   --> $DIR/concrete-impl.rs:9:1
    |
 LL | impl Ambiguous<One> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
 LL | impl Ambiguous<Two> for Struct {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

tests/ui/inference/need_type_info/issue-109905.rs

@@ -0,0 +1,25 @@
+// Test that we show the correct type parameter that couldn't be inferred and that we don't
+// end up stating nonsense like "type parameter `'a`" which we used to do.
+
+trait Trait<'a, T> {
+    fn m(self);
+}
+
+impl<'a, A> Trait<'a, A> for () {
+    fn m(self) {}
+}
+
+fn qualified() {
+    <() as Trait<'static, _>>::m(());
+    //~^ ERROR type annotations needed
+    //~| NOTE cannot infer type of the type parameter `T`
+
+}
+
+fn unqualified() {
+    Trait::<'static, _>::m(());
+    //~^ ERROR type annotations needed
+    //~| NOTE cannot infer type of the type parameter `T`
+}
+
+fn main() {}

tests/ui/inference/need_type_info/issue-109905.stderr

@@ -0,0 +1,15 @@
+error[E0282]: type annotations needed
+  --> $DIR/issue-109905.rs:13:5
+   |
+LL |     <() as Trait<'static, _>>::m(());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the trait `Trait`
+
+error[E0282]: type annotations needed
+  --> $DIR/issue-109905.rs:20:5
+   |
+LL |     Trait::<'static, _>::m(());
+   |     ^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the trait `Trait`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.