test: add test case for disambiguate the associated function diagnostic

Author: Young-Flash

tests/ui/methods/disambiguate-associated-function-first-arg.rs

@@ -0,0 +1,28 @@
+struct A {}
+
+fn main() {
+    let _a = A {};
+    _a.new(1);
+    //~^ ERROR no method named `new` found for struct `A` in the current scope
+}
+
+trait M {
+    fn new(_a: i32);
+}
+impl M for A {
+    fn new(_a: i32) {}
+}
+
+trait N {
+    fn new(_a: Self, _b: i32);
+}
+impl N for A {
+    fn new(_a: Self, _b: i32) {}
+}
+
+trait O {
+    fn new(_a: Self, _b: i32);
+}
+impl O for A {
+    fn new(_a: A, _b: i32) {}
+}

tests/ui/methods/disambiguate-associated-function-first-arg.stderr

@@ -0,0 +1,41 @@
+error[E0599]: no method named `new` found for struct `A` in the current scope
+  --> $DIR/disambiguate-associated-function-first-arg.rs:5:8
+   |
+LL | struct A {}
+   | -------- method `new` not found for this struct
+...
+LL |     _a.new(1);
+   |        ^^^ this is an associated function, not a method
+   |
+   = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
+note: candidate #1 is defined in the trait `M`
+  --> $DIR/disambiguate-associated-function-first-arg.rs:10:5
+   |
+LL |     fn new(_a: i32);
+   |     ^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in the trait `N`
+  --> $DIR/disambiguate-associated-function-first-arg.rs:17:5
+   |
+LL |     fn new(_a: Self, _b: i32);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #3 is defined in the trait `O`
+  --> $DIR/disambiguate-associated-function-first-arg.rs:24:5
+   |
+LL |     fn new(_a: Self, _b: i32);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: disambiguate the associated function for candidate #1
+   |
+LL |     <A as M>::new(1);
+   |     ~~~~~~~~~~~~~~~~
+help: disambiguate the associated function for candidate #2
+   |
+LL |     <A as N>::new(_a, 1);
+   |     ~~~~~~~~~~~~~~~~~~~~
+help: disambiguate the associated function for candidate #3
+   |
+LL |     <A as O>::new(_a, 1);
+   |     ~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.