properly calculate spans for intra-doc link resolution errors

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -527,7 +527,7 @@ fn resolution_failure(
                 doc_comment_padding +
                     // Each subsequent leading whitespace and `///`
                     code_dox.lines().skip(1).take(line_offset - 1).fold(0, |sum, line| {
-                        sum + doc_comment_padding + line.len() - line.trim().len()
+                        sum + doc_comment_padding + line.len() - line.trim_start().len()
                     })
             };
 

src/test/rustdoc-ui/intra-link-span-ice-55723.rs

@@ -0,0 +1,24 @@
+// 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.
+
+// ignore-tidy-end-whitespace
+
+#![deny(intra_doc_link_resolution_failure)]
+
+// An error in calculating spans while reporting intra-doc link resolution errors caused rustdoc to
+// attempt to slice in the middle of a multibyte character. See
+// https://github.com/rust-lang/rust/issues/55723
+
+/// ## For example:
+///  
+/// （arr[i]）
+pub fn test_ice() {
+    unimplemented!();
+}

src/test/rustdoc-ui/intra-link-span-ice-55723.stderr

@@ -0,0 +1,13 @@
+error: `[i]` cannot be resolved, ignoring it...
+  --> $DIR/intra-link-span-ice-55723.rs:21:10
+   |
+LL | /// （arr[i]）
+   |           ^ cannot be resolved, ignoring
+   |
+note: lint level defined here
+  --> $DIR/intra-link-span-ice-55723.rs:13:9
+   |
+LL | #![deny(intra_doc_link_resolution_failure)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: to escape `[` and `]` characters, just add '/' before them like `/[` or `/]`
+