Add explanation for E0726

compiler/rustc_error_codes/src/error_codes.rs

@@ -429,6 +429,7 @@ E0720: include_str!("./error_codes/E0720.md"),
 E0722: include_str!("./error_codes/E0722.md"),
 E0724: include_str!("./error_codes/E0724.md"),
 E0725: include_str!("./error_codes/E0725.md"),
+E0726: include_str!("./error_codes/E0726.md"),
 E0727: include_str!("./error_codes/E0727.md"),
 E0728: include_str!("./error_codes/E0728.md"),
 E0729: include_str!("./error_codes/E0729.md"),
@@ -641,6 +642,5 @@ E0787: include_str!("./error_codes/E0787.md"),
     E0717, // rustc_promotable without stability attribute
 //  E0721, // `await` keyword
 //  E0723, // unstable feature in `const` context
-    E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
 }

compiler/rustc_error_codes/src/error_codes/E0726.md

@@ -0,0 +1,46 @@
+An argument lifetime was elided in an async function.
+
+Erroneous code example:
+
+When a struct or a type is bound/declared with a lifetime it is important for
+the Rust compiler to know, on usage, the lifespan of the type. When the
+lifetime is not explicitly mentioned and the Rust Compiler cannot determine
+the lifetime of your type, the following error occurs.
+
+```compile_fail,E0726
+use futures::executor::block_on;
+struct Content<'a> {
+    title: &'a str,
+    body: &'a str,
+}
+async fn create(content: Content) { // error: implicit elided
+                                    // lifetime not allowed here
+    println!("title: {}", content.title);
+    println!("body: {}", content.body);
+}
+let content = Content { title: "Rust", body: "is great!" };
+let future = create(content);
+block_on(future);
+```
+
+Specify desired lifetime of parameter `content` or indicate the anonymous
+lifetime like `content: Content<'_>`. The anonymous lifetime tells the Rust
+compiler that `content` is only needed until create function is done with
+it's execution.
+
+The `implicit elision` meaning the omission of suggested lifetime that is
+`pub async fn create<'a>(content: Content<'a>) {}` is not allowed here as
+lifetime of the `content` can differ from current context:
+
+```ignore (needs futures dependency)
+async fn create(content: Content<'_>) { // ok!
+    println!("title: {}", content.title);
+    println!("body: {}", content.body);
+}
+```
+
+Know more about lifetime elision in this [chapter][lifetime-elision] and a
+chapter on lifetimes can be found [here][lifetimes].
+
+[lifetime-elision]: https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html#lifetime-elision
+[lifetimes]: https://doc.rust-lang.org/rust-by-example/scope/lifetime.html