vim: Tweak how comment nesting works

Don't try to match line comments inside of a comment block. That makes
no sense and can highlight differently for people who override their
highlights.

Similarly, don't match a doc-comment inside of a comment block. It
shouldn't be highlighted differently unless it's actually a doc-comment
(and nested comments are obviously not doc comments).

Fixes #12307.

src/etc/vim/syntax/rust.vim

@@ -178,11 +178,12 @@ syn region rustGenericLifetimeCandidate display start=/\%(<\|,\s*\)\@<='/ end=/[
 syn match     rustLifetime    display "\'\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\)\%([^[:cntrl:][:punct:][:space:]]\|_\)*"
 syn match   rustCharacter   /'\([^'\\]\|\\\(.\|x\x\{2}\|u\x\{4}\|U\x\{8}\)\)'/ contains=rustSpecial,rustSpecialError
 
-syn cluster rustComment contains=rustCommentLine,rustCommentLineDoc,rustCommentBlock,rustCommentBlockDoc
-syn region rustCommentLine                                    start="//"                      end="$"   contains=rustTodo,@Spell
-syn region rustCommentLineDoc                                 start="//\%(//\@!\|!\)"         end="$"   contains=rustTodo,@Spell
-syn region rustCommentBlock    matchgroup=rustCommentBlock    start="/\*\%(!\|\*[*/]\@!\)\@!" end="\*/" contains=rustTodo,@rustComment,@Spell keepend extend
-syn region rustCommentBlockDoc matchgroup=rustCommentBlockDoc start="/\*\%(!\|\*[*/]\@!\)"    end="\*/" contains=rustTodo,@rustComment,@Spell keepend extend
+syn region rustCommentLine                                        start="//"                      end="$"   contains=rustTodo,@Spell
+syn region rustCommentLineDoc                                     start="//\%(//\@!\|!\)"         end="$"   contains=rustTodo,@Spell
+syn region rustCommentBlock    matchgroup=rustCommentBlock        start="/\*\%(!\|\*[*/]\@!\)\@!" end="\*/" contains=rustTodo,rustCommentBlockNest,@Spell
+syn region rustCommentBlockDoc matchgroup=rustCommentBlockDoc     start="/\*\%(!\|\*[*/]\@!\)"    end="\*/" contains=rustTodo,rustCommentBlockDocNest,@Spell
+syn region rustCommentBlockNest matchgroup=rustCommentBlock       start="/\*"                     end="\*/" contains=rustTodo,rustCommentBlockNest,@Spell contained transparent
+syn region rustCommentBlockDocNest matchgroup=rustCommentBlockDoc start="/\*"                     end="\*/" contains=rustTodo,rustCommentBlockDocNest,@Spell contained transparent
 " FIXME: this is a really ugly and not fully correct implementation. Most
 " importantly, a case like ``/* */*`` should have the final ``*`` not being in
 " a comment, but in practice at present it leaves comments open two levels