doc_nested_refdefs: do not falsely report checkboxes as refdefs (#15146)

changelog: [`doc_nested_refdefs`]: fix false positive where [task
lists](http://pulldown-cmark.github.io/pulldown-cmark/third_party/gfm_tasklist.html)
are reported as refdefs

Author: samueltardieu

clippy_lints/src/doc/mod.rs

@@ -1249,7 +1249,9 @@ fn looks_like_refdef(doc: &str, range: Range<usize>) -> Option<Range<usize>> {
             b'[' => {
                 start = Some(i + offset);
             },
-            b']' if let Some(start) = start => {
+            b']' if let Some(start) = start
+                && doc.as_bytes().get(i + offset + 1) == Some(&b':') =>
+            {
                 return Some(start..i + offset + 1);
             },
             _ => {},

tests/ui/doc/doc_nested_refdef_list_item.fixed

@@ -72,8 +72,6 @@ pub struct NotEmptyTight;
 
 /// ## Heading
 ///
-/// - [x][] - Done
-//~^ ERROR: link reference defined in list item
-/// - [ ][] - Not Done
-//~^ ERROR: link reference defined in list item
+/// - [x] - Done
+/// - [ ] - Not Done
 pub struct GithubCheckboxes;

tests/ui/doc/doc_nested_refdef_list_item.rs

@@ -73,7 +73,5 @@ pub struct NotEmptyTight;
 /// ## Heading
 ///
 /// - [x] - Done
-//~^ ERROR: link reference defined in list item
 /// - [ ] - Not Done
-//~^ ERROR: link reference defined in list item
 pub struct GithubCheckboxes;

tests/ui/doc/doc_nested_refdef_list_item.stderr

@@ -144,29 +144,5 @@ help: for an intra-doc link, add `[]` between the label and the colon
 LL | /// - [link][]: def "title"
    |             ++
 
-error: link reference defined in list item
-  --> tests/ui/doc/doc_nested_refdef_list_item.rs:75:7
-   |
-LL | /// - [x] - Done
-   |       ^^^
-   |
-   = help: link definitions are not shown in rendered documentation
-help: for an intra-doc link, add `[]` between the label and the colon
-   |
-LL | /// - [x][] - Done
-   |          ++
-
-error: link reference defined in list item
-  --> tests/ui/doc/doc_nested_refdef_list_item.rs:77:7
-   |
-LL | /// - [ ] - Not Done
-   |       ^^^
-   |
-   = help: link definitions are not shown in rendered documentation
-help: for an intra-doc link, add `[]` between the label and the colon
-   |
-LL | /// - [ ][] - Not Done
-   |          ++
-
-error: aborting due to 14 previous errors
+error: aborting due to 12 previous errors