Improve test for FQS tuple struct pat/expr

1. Better explain what the test tests
2. Test slightly more cases

Author: WaffleLapkin

tests/ui/associated-types/associated-type-tuple-struct-construction.rs

@@ -0,0 +1,46 @@
+// Check that fully qualified syntax can **not** be used in tuple struct expressions (calls) and
+// patterns. Both tuple struct expressions and patterns are resolved in value namespace and thus
+// can't be resolved through associated *types*.
+
+#![feature(more_qualified_paths)]
+
+fn main() {
+    let <T<0> as Trait>::Assoc() = <T<0> as Trait>::Assoc();
+    //~^ error: expected method or associated constant, found associated type
+    //~| error: expected method or associated constant, found associated type
+    let <T<1> as Trait>::Assoc(_a) = <T<1> as Trait>::Assoc(0);
+    //~^ error: expected method or associated constant, found associated type
+    //~| error: expected method or associated constant, found associated type
+    let <T<2> as Trait>::Assoc(_a, _b) = <T<2> as Trait>::Assoc(0, 1);
+    //~^ error: expected method or associated constant, found associated type
+    //~| error: expected method or associated constant, found associated type
+    let <T<3> as Trait>::Assoc(ref _a, ref mut _b, mut _c) = <T<3> as Trait>::Assoc(0, 1, 2);
+    //~^ error: expected method or associated constant, found associated type
+    //~| error: expected method or associated constant, found associated type
+}
+
+
+struct T<const N: usize>;
+
+struct T0();
+struct T1(u8);
+struct T2(u8, u8);
+struct T3(u8, u8, u8);
+
+trait Trait {
+    type Assoc;
+}
+
+impl Trait for T<0> {
+    type Assoc = T0;
+}
+
+impl Trait for T<1> {
+    type Assoc = T1;
+}
+impl Trait for T<2> {
+    type Assoc = T2;
+}
+impl Trait for T<3> {
+    type Assoc = T3;
+}

tests/ui/associated-types/associated-type-tuple-struct-construction.stderr

@@ -0,0 +1,67 @@
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:8:36
+   |
+LL |     let <T<0> as Trait>::Assoc() = <T<0> as Trait>::Assoc();
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:8:9
+   |
+LL |     let <T<0> as Trait>::Assoc() = <T<0> as Trait>::Assoc();
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:11:38
+   |
+LL |     let <T<1> as Trait>::Assoc(_a) = <T<1> as Trait>::Assoc(0);
+   |                                      ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:11:9
+   |
+LL |     let <T<1> as Trait>::Assoc(_a) = <T<1> as Trait>::Assoc(0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:14:42
+   |
+LL |     let <T<2> as Trait>::Assoc(_a, _b) = <T<2> as Trait>::Assoc(0, 1);
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:14:9
+   |
+LL |     let <T<2> as Trait>::Assoc(_a, _b) = <T<2> as Trait>::Assoc(0, 1);
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:17:62
+   |
+LL |     let <T<3> as Trait>::Assoc(ref _a, ref mut _b, mut _c) = <T<3> as Trait>::Assoc(0, 1, 2);
+   |                                                              ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error[E0575]: expected method or associated constant, found associated type `Trait::Assoc`
+  --> $DIR/tuple-struct-expr-pat.rs:17:9
+   |
+LL |     let <T<3> as Trait>::Assoc(ref _a, ref mut _b, mut _c) = <T<3> as Trait>::Assoc(0, 1, 2);
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: can't use a type alias as a constructor
+
+error: aborting due to 8 previous errors
+
+For more information about this error, try `rustc --explain E0575`.