[simonw]

If you're curious how Git knows the syntax of different languages in order to support this kind of feature, take a look in https://github.com/git/git/blob/master/userdiff.c

Here's how support for Python and Ruby are defined:

    PATTERNS("python",
        "^[ \t]*((class|(async[ \t]+)?def)[ \t].*)$",
        /* -- */
        "[a-zA-Z_][a-zA-Z0-9_]*"
        "|[-+0-9.e]+[jJlL]?|0[xX]?[0-9a-fA-F]+[lL]?"
        "|[-+*/<>%&^|=!]=|//=?|<<=?|>>=?|\\*\\*=?"),
        /* -- */
    PATTERNS("ruby",
        "^[ \t]*((class|module|def)[ \t].*)$",
        /* -- */
        "(@|@@|\\$)?[a-zA-Z_][a-zA-Z0-9_]*"
        "|[-+0-9.e]+|0[xXbB]?[0-9a-fA-F]+|\\?(\\\\C-)?(\\\\M-)?."
        "|//=?|[-+*/<>%&^|=!]=|<<=?|>>=?|===|\\.{1,3}|::|[!=]~"),

[tessierashpool]

it's a fantastic feature in theory. in practice, it's imprecise and error-prone, and I believe these regular expressions are probably why. I hadn't looked at the implementation before, but I approached it from the other end: I set up a bunch of test cases, and I was pretty disappointed.

there were two disappointments. first, `git log -L` seems to prioritize tracking blocks of code over lines of code. that's just a design choice I disagree with, so it wasn't a big deal. but it also lost track of lines of code for me quite often, and produced a number of false positives to boot.

to be fair, I haven't tried using `diff=LANG` (per a comment below), and that might get more reliable results.

[Groxx]

Yeah, this has been my experience too. Easily confused by common constructions in some codebases, and that can make it almost completely useless. I would happily sacrifice a lot of speed to get a difftastic level of precision.

[anitil]

I've attempted something similar to your ast-search tool, but it instead iterates through git history, pulls out the relevant text and then provides the diff to the user.

It's a tricky problem because it sits somewhere between text, where a function name could get renamed and it's obvious because it is textually similar, and an AST where 'similarity' is a difficult concept.

I struggled to make it usable, but of course there's a module to do half of it that I didn't find initially - https://pypi.org/project/pyastsim/

[lloydatkinson]

Interesting. Also I am surprised at how short the list is!