Problem:    Code is indented more than necessary.
Solution:   Use an early return where it makes sense. (Yegappan Lakshmanan,
            closes #11833)

Problem:    Trying every character position for a match is inefficient.
Solution:   Use the start position of the match ignoring "\zs".

Problem:    The pattern "\_s\zs" matches at EOL.
Solution:   Make the pattern "\_s\zs" match at the start of the next line.
            (closes #11617)

Problem:    Various typos.
Solution:   Correct typos. (closes #11432)

Problem:    Evaluating "expr" options has more overhead than needed.
Solution:   Use call_simple_func() for 'foldtext', 'includeexpr', 'printexpr',
            "expr" of 'spellsuggest', 'diffexpr', 'patchexpr', 'balloonexpr',
            'formatexpr', 'indentexpr' and 'charconvert'.

Problem:    May not be able to use a pattern ad the debug prompt.
Solution:   Temporarily disable the timeout. (closes #11164)

Problem:    Cannot use a :def varargs function with substitute().
Solution:   Use has_varargs(). (closes #11146)

Problem:    Varargs does not work for replacement function of substitute().
Solution:   Check the varargs flag of the function. (closes #11142)

Problem:    A nested timout stops the previous timeout.
Solution:   Ignore any nested timeout.

Problem:    Illegal memory access when pattern starts with illegal byte.
Solution:   Do not match a character with an illegal byte.