Rollup merge of rust-lang#72071 - PankajChaudhary5:ErrorCode-E0687, r=davidtwco

Added detailed error code explanation for issue E0687 in Rust compiler.

Added proper error explanation for issue E0687 in the Rust compiler.
Error Code E0687

Sub Part of Issue rust-lang#61137

r? @GuillaumeGomez

Author: Manishearth

src/librustc_error_codes/error_codes.rs

@@ -382,6 +382,7 @@ 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"),
@@ -613,7 +614,6 @@ E0766: include_str!("./error_codes/E0766.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
 //  E0702, // replaced with a generic attribute input check

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`.