Rollup merge of rust-lang#67422 - GuillaumeGomez:cleanup-err-codes, r=Dylan-DPC

Cleanup err codes

r? @Dylan-DPC

Author: Centril

src/librustc_error_codes/error_codes/E0120.md

@@ -1,5 +1,7 @@
-An attempt was made to implement Drop on a trait, which is not allowed: only
-structs and enums can implement Drop. An example causing this error:
+Drop was implemented on a trait, which is not allowed: only structs and
+enums can implement Drop.
+
+Erroneous code example:
 
 ```compile_fail,E0120
 trait MyTrait {}
@@ -10,7 +12,7 @@ impl Drop for MyTrait {
 ```
 
 A workaround for this problem is to wrap the trait up in a struct, and implement
-Drop on that. An example is shown below:
+Drop on that:
 
 ```
 trait MyTrait {}
@@ -22,7 +24,7 @@ impl <T: MyTrait> Drop for MyWrapper<T> {
 
 ```
 
-Alternatively, wrapping trait objects requires something like the following:
+Alternatively, wrapping trait objects requires something:
 
 ```
 trait MyTrait {}

src/librustc_error_codes/error_codes/E0121.md

@@ -1,10 +1,24 @@
-In order to be consistent with Rust's lack of global type inference,
-type and const placeholders are disallowed by design in item signatures.
+The type placeholder `_` was used within a type on an item's signature.
 
-Examples of this error include:
+Erroneous code example:
 
 ```compile_fail,E0121
-fn foo() -> _ { 5 } // error, explicitly write out the return type instead
+fn foo() -> _ { 5 } // error
 
-static BAR: _ = "test"; // error, explicitly write out the type instead
+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.
 ```