Rollup merge of #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 #61137

r? @GuillaumeGomez

1 files changed, 36 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0687.md b/src/librustc_error_codes/error_codes/E0687.md
new file mode 100644
index 00000000000..67b7db2d31f
--- /dev/null
+++ b/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!
+}
+```