Rollup merge of rust-lang#57950 - QuietMisdreavus:lifetime-err-desc, …

…r=estebank

Extend E0106, E0261

This is a reopening of rust-lang#57310 with review comments addressed because the original author has since deleted their fork.

From the author (@purple-ice):

> Added an example that points out hardly obvious mistake one could make when writing impl for a new type.

r? @rust-lang/docs

src/librustc/diagnostics.rs

@@ -362,6 +362,10 @@ struct Foo1 { x: &bool }
               // ^ expected lifetime parameter
 struct Foo2<'a> { x: &'a bool } // correct
 
+impl Foo2 {}
+  // ^^^^ expected lifetime parameter
+impl<'a> Foo2<'a> {} // correct
+
 struct Bar1 { x: Foo2 }
               // ^^^^ expected lifetime parameter
 struct Bar2<'a> { x: Foo2<'a> } // correct
@@ -766,11 +770,40 @@ struct Foo {
 These can be fixed by declaring lifetime parameters:
 
 ```
+struct Foo<'a> {
+    x: &'a str,
+}
+
 fn foo<'a>(x: &'a str) {}
+```
 
+Impl blocks declare lifetime parameters separately. You need to add lifetime
+parameters to an impl block if you're implementing a type that has a lifetime
+parameter of its own.
+For example:
+
+```compile_fail,E0261
 struct Foo<'a> {
     x: &'a str,
 }
+
+// error,  use of undeclared lifetime name `'a`
+impl Foo<'a> {
+    fn foo<'a>(x: &'a str) {}
+}
+```
+
+This is fixed by declaring the impl block like this:
+
+```
+struct Foo<'a> {
+    x: &'a str,
+}
+
+// correct
+impl<'a> Foo<'a> {
+    fn foo(x: &'a str) {}
+}
 ```
 "##,