Added proper explanation of ErrorCode-E0688

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"),
+E0688: include_str!("./error_codes/E0688.md"),
 E0689: include_str!("./error_codes/E0689.md"),
 E0690: include_str!("./error_codes/E0690.md"),
 E0691: include_str!("./error_codes/E0691.md"),
@@ -600,7 +601,6 @@ E0751: include_str!("./error_codes/E0751.md"),
 //  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
 //  E0702, // replaced with a generic attribute input check

src/librustc_error_codes/error_codes/E0688.md

@@ -0,0 +1,36 @@
+In-band lifetimes were mixed with explicit lifetime binders.
+
+Erroneous code example:
+
+```compile_fail,E0688
+#![feature(in_band_lifetimes)]
+
+fn foo<'a>(x: &'a u32, y: &'b u32) {}   // error!
+
+struct Foo<'a> { x: &'a u32 }
+
+impl Foo<'a> {
+    fn bar<'b>(x: &'a u32, y: &'b u32, z: &'c u32) {}   // error!
+}
+
+impl<'b> Foo<'a> {  // error!
+    fn baz() {}
+}
+```
+
+In-band lifetimes cannot be mixed with explicit lifetime binders.
+For example:
+
+```
+fn foo<'a, 'b>(x: &'a u32, y: &'b u32) {}   // ok!
+
+struct Foo<'a> { x: &'a u32 }
+
+impl<'a> Foo<'a> {
+    fn bar<'b,'c>(x: &'a u32, y: &'b u32, z: &'c u32) {}    // ok!
+}
+
+impl<'a> Foo<'a> {  // ok!
+    fn baz() {}
+}
+```

src/test/ui/in-band-lifetimes/E0688.stderr

@@ -24,3 +24,4 @@ LL | impl<'b> Foo<'a> {
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0688`.