Added proper explanation of ErrorCode-E0687

Author: Polkaverse

src/librustc_error_codes/error_codes.rs

@@ -380,6 +380,7 @@ E0668: include_str!("./error_codes/E0668.md"),
 E0669: include_str!("./error_codes/E0669.md"),
 E0670: include_str!("./error_codes/E0670.md"),
 E0671: include_str!("./error_codes/E0671.md"),
+E0687: include_str!("./error_codes/E0687.md"),
 E0689: include_str!("./error_codes/E0689.md"),
 E0690: include_str!("./error_codes/E0690.md"),
 E0691: include_str!("./error_codes/E0691.md"),
@@ -599,7 +600,6 @@ E0751: include_str!("./error_codes/E0751.md"),
     E0640, // infer outlives requirements
 //  E0645, // trait aliases not finished
     E0667, // `impl Trait` in projections
-    E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
     E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
 //  E0694, // an unknown tool name found in scoped attributes
     E0696, // `continue` pointing to a labeled block

src/librustc_error_codes/error_codes/E0687.md

@@ -0,0 +1,36 @@
+In-band lifetimes cannot be used in `fn`/`Fn` syntax.
+
+Erroneous code examples:
+
+```compile_fail,E0687
+#![feature(in_band_lifetimes)]
+
+fn foo(x: fn(&'a u32)) {} // error!
+
+fn bar(x: &Fn(&'a u32)) {} // error!
+
+fn baz(x: fn(&'a u32), y: &'a u32) {} // error!
+
+struct Foo<'a> { x: &'a u32 }
+
+impl Foo<'a> {
+    fn bar(&self, x: fn(&'a u32)) {} // error!
+}
+```
+
+Lifetimes used in `fn` or `Fn` syntax must be explicitly
+declared using `<...>` binders. For example:
+
+```
+fn foo<'a>(x: fn(&'a u32)) {} // ok!
+
+fn bar<'a>(x: &Fn(&'a u32)) {} // ok!
+
+fn baz<'a>(x: fn(&'a u32), y: &'a u32) {} // ok!
+
+struct Foo<'a> { x: &'a u32 }
+
+impl<'a> Foo<'a> {
+    fn bar(&self, x: fn(&'a u32)) {} // ok!
+}
+```

src/test/ui/in-band-lifetimes/E0687.stderr

@@ -24,3 +24,4 @@ LL |     fn bar(&self, x: fn(&'a u32)) {}
 
 error: aborting due to 4 previous errors
 
+For more information about this error, try `rustc --explain E0687`.

src/test/ui/in-band-lifetimes/E0687_where.stderr

@@ -12,3 +12,4 @@ LL | fn baz(x: &impl Fn(&'a u32)) {}
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0687`.