parse: improve recovery for assoc eq constraints.

src/test/ui/parser/recover-assoc-const-constraint.rs

@@ -0,0 +1,7 @@
+#[cfg(FALSE)]
+fn syntax() {
+    bar::<Item = 42>(); //~ ERROR cannot constrain an associated constant to a value
+    bar::<Item = { 42 }>(); //~ ERROR cannot constrain an associated constant to a value
+}
+
+fn main() {}

src/test/ui/parser/recover-assoc-const-constraint.stderr

@@ -0,0 +1,20 @@
+error: cannot constrain an associated constant to a value
+  --> $DIR/recover-assoc-const-constraint.rs:3:11
+   |
+LL |     bar::<Item = 42>();
+   |           ----^^^--
+   |           |      |
+   |           |      the value is given in this expression
+   |           the value constrains this associated constant
+
+error: cannot constrain an associated constant to a value
+  --> $DIR/recover-assoc-const-constraint.rs:4:11
+   |
+LL |     bar::<Item = { 42 }>();
+   |           ----^^^------
+   |           |      |
+   |           |      the value is given in this expression
+   |           the value constrains this associated constant
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/recover-assoc-eq-missing-term.rs

@@ -0,0 +1,6 @@
+#[cfg(FALSE)]
+fn syntax() {
+    bar::<Item = >(); //~ ERROR missing type to the right of `=`
+}
+
+fn main() {}

src/test/ui/parser/recover-assoc-eq-missing-term.stderr

@@ -0,0 +1,17 @@
+error: missing type to the right of `=`
+  --> $DIR/recover-assoc-eq-missing-term.rs:3:11
+   |
+LL |     bar::<Item = >();
+   |           ^^^^^^
+   |
+help: to constrain the associated type, add a type after `=`
+   |
+LL |     bar::<Item = TheType >();
+   |           ^^^^^^^^^^^^^^
+help: remove the `=` if `Item` is a type
+   |
+LL |     bar::<Item  >();
+   |               --
+
+error: aborting due to previous error
+

src/test/ui/parser/recover-assoc-lifetime-constraint.rs

@@ -0,0 +1,6 @@
+#[cfg(FALSE)]
+fn syntax() {
+    bar::<Item = 'a>(); //~ ERROR associated lifetimes are not supported
+}
+
+fn main() {}

src/test/ui/parser/recover-assoc-lifetime-constraint.stderr

@@ -0,0 +1,12 @@
+error: associated lifetimes are not supported
+  --> $DIR/recover-assoc-lifetime-constraint.rs:3:11
+   |
+LL |     bar::<Item = 'a>();
+   |           ^^^^^^^--
+   |                  |
+   |                  the lifetime is given here
+   |
+   = help: if you meant to specify a trait object, write `dyn Trait + 'lifetime`
+
+error: aborting due to previous error
+