Rollup merge of #89710 - sireliah:e0482, r=GuillaumeGomez

Add long explanation for error E0482

This is longer explanation for error E0482 in the #61137.

Please take a look and leave some feedback!

compiler/rustc_error_codes/src/error_codes/E0482.md

@@ -0,0 +1,73 @@
+A lifetime of a returned value does not outlive the function call.
+
+Erroneous code example:
+
+```compile_fail,E0482
+fn prefix<'a>(
+    words: impl Iterator<Item = &'a str>
+) -> impl Iterator<Item = String> { // error!
+    words.map(|v| format!("foo-{}", v))
+}
+```
+
+To fix this error, make the lifetime of the returned value explicit:
+
+```
+fn prefix<'a>(
+    words: impl Iterator<Item = &'a str> + 'a
+) -> impl Iterator<Item = String> + 'a { // ok!
+    words.map(|v| format!("foo-{}", v))
+}
+```
+
+The [`impl Trait`] feature in this example uses an implicit `'static` lifetime
+restriction in the returned type. However the type implementing the `Iterator`
+passed to the function lives just as long as `'a`, which is not long enough.
+
+The solution involves adding lifetime bound to both function argument and
+the return value to make sure that the values inside the iterator
+are not dropped when the function goes out of the scope.
+
+An alternative solution would be to guarantee that the `Item` references
+in the iterator are alive for the whole lifetime of the program.
+
+```
+fn prefix(
+    words: impl Iterator<Item = &'static str>
+) -> impl Iterator<Item = String> {  // ok!
+    words.map(|v| format!("foo-{}", v))
+}
+```
+
+A similar lifetime problem might arise when returning closures:
+
+```compile_fail,E0482
+fn foo(
+    x: &mut Vec<i32>
+) -> impl FnMut(&mut Vec<i32>) -> &[i32] { // error!
+    |y| {
+        y.append(x);
+        y
+    }
+}
+```
+
+Analogically, a solution here is to use explicit return lifetime
+and move the ownership of the variable to the closure.
+
+```
+fn foo<'a>(
+    x: &'a mut Vec<i32>
+) -> impl FnMut(&mut Vec<i32>) -> &[i32] + 'a { // ok!
+    move |y| {
+        y.append(x);
+        y
+    }
+}
+```
+
+To better understand the lifetime treatment in the [`impl Trait`],
+please see the [RFC 1951].
+
+[`impl Trait`]: https://doc.rust-lang.org/reference/types/impl-trait.html
+[RFC 1951]: https://rust-lang.github.io/rfcs/1951-expand-impl-trait.html