Do not repeat idx

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1156,14 +1156,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // ```
         // which includes the replacement of the first two `()` for the correct type, and the
         // removal of the last `()`.
-        let mut prev = -1;
+        let mut idx = -1;
         for (expected_idx, provided_idx) in matched_inputs.iter_enumerated() {
             // We want to point not at the *current* argument expression index, but rather at the
             // index position where it *should have been*, which is *after* the previous one.
-            if let Some(provided_idx) = provided_idx {
+            idx = match provided_idx {
-                prev = provided_idx.index() as i64;
+                Some(provided_idx) => provided_idx.index() as i64 + 1,
-            }
+                None => idx + 1,
-            let idx = ProvidedIdx::from_usize((prev + 1) as usize);
+            };
+            let idx = ProvidedIdx::from_usize(idx as usize);
             if let None = provided_idx
                 && let Some((_, arg_span)) = provided_arg_tys.get(idx)
             {

tests/ui/argument-suggestions/109831.rs

@@ -0,0 +1,9 @@
+struct A;
+struct B;
+
+fn f(b1: B, b2: B, a2: C) {} //~ ERROR E0412
+
+fn main() {
+    f(A, A, B, C); //~ ERROR E0425
+    //~^ ERROR E0061
+}

tests/ui/argument-suggestions/109831.stderr

@@ -0,0 +1,51 @@
+error[E0412]: cannot find type `C` in this scope
+  --> $DIR/109831.rs:4:24
+   |
+LL | struct A;
+   | --------- similarly named struct `A` defined here
+...
+LL | fn f(b1: B, b2: B, a2: C) {}
+   |                        ^
+   |
+help: a struct with a similar name exists
+   |
+LL | fn f(b1: B, b2: B, a2: A) {}
+   |                        ~
+help: you might be missing a type parameter
+   |
+LL | fn f<C>(b1: B, b2: B, a2: C) {}
+   |     +++
+
+error[E0425]: cannot find value `C` in this scope
+  --> $DIR/109831.rs:7:16
+   |
+LL | struct A;
+   | --------- similarly named unit struct `A` defined here
+...
+LL |     f(A, A, B, C);
+   |                ^ help: a unit struct with a similar name exists: `A`
+
+error[E0061]: this function takes 3 arguments but 4 arguments were supplied
+  --> $DIR/109831.rs:7:5
+   |
+LL |     f(A, A, B, C);
+   |     ^ -  -     - unexpected argument
+   |       |  |
+   |       |  expected `B`, found `A`
+   |       expected `B`, found `A`
+   |
+note: function defined here
+  --> $DIR/109831.rs:4:4
+   |
+LL | fn f(b1: B, b2: B, a2: C) {}
+   |    ^ -----  -----  -----
+help: remove the extra argument
+   |
+LL -     f(A, A, B, C);
+LL +     f(/* B */, /* B */, B);
+   |
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0061, E0412, E0425.
+For more information about an error, try `rustc --explain E0061`.