Auto merge of #53933 - GuillaumeGomez:codeblock-error-display, r=QuietMisdreavus

Improve error display for codeblocks in rustdoc

Part of #53919.

r? @QuietMisdreavus

src/test/rustdoc-ui/invalid-syntax.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+// compile-flags: --error-format=human
+
+/// ```
+/// \__________pkt->size___________/          \_result->size_/ \__pkt->size__/
+/// ```
+pub fn foo() {}

src/test/rustdoc-ui/invalid-syntax.stderr

@@ -0,0 +1,10 @@
+Output from rustc:
+error: unknown start of token: /
+ --> <stdin>:1:1
+  |
+1 | /__________pkt->size___________/          /_result->size_/ /__pkt->size__/
+  | ^
+
+warning: Invalid doc comment starting with: `/__________pkt->size___________/          /_result->size_/ /__pkt->size__/`
+(Ignoring this codeblock)
+