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+The type placeholder `_` was used within a type on an item's signature.
+
+Erroneous code example:
+
+```compile_fail,E0121
+fn foo() -> _ { 5 } // error
+
+static BAR: _ = "test"; // error
+```
+
+In those cases, you need to provide the type explicitly:
+
+```
+fn foo() -> i32 { 5 } // ok!
+
+static BAR: &str = "test"; // ok!
+```
+
+The type placeholder `_` can be used outside item's signature as follows:
+
+```
+let x = "a4a".split('4')
+    .collect::<Vec<_>>(); // No need to precise the Vec's generic type.
+```