This commit is an example of fixing a common parser error: infinite
loop due to error recovery.
This error typically happens when we parse a list of items and fail to
parse a specific item at the current position.
One choices is to skip a token and try to parse a list item at the
next position. This is a good, but not universal, default. When
parsing a list of arguments in a function call, you, for example,
don't want to skip over `fn`, because it's most likely that it is a
function declaration, and not a mistyped arg:
```
fn foo() {
quux(1, 2
fn bar() {
}
```
Another choice is to bail out of the loop immediately, but it isn't
perfect either: sometimes skipping over garbage helps:
```
quux(1, foo:, 92) // should skip over `:`, b/c that's part of `foo::bar`
```
In general, parser tries to balance these two cases, though we don't
have a definitive strategy yet.
However, if the parser accidentally neither skips over a token, nor
breaks out of the loop, then it becomes stuck in the loop infinitely
(there's an internal counter to self-check this situation and panic
though), and that's exactly what is demonstrated by the test.
To fix such situation, first of all, add the test case to tests/data/parser/{err,fuzz-failures}.
Then, run
```
RUST_BACKTRACE=short cargo test --package libsyntax2
````
to verify that parser indeed panics, and to get an idea what grammar
production is the culprit (look for `_list` functions!).
In this case, I see
```
10: libsyntax2::grammar::expressions::atom::match_arm_list
at crates/libsyntax2/src/grammar/expressions/atom.rs:309
```
and that's look like it might be a culprit. I verify it by adding
`eprintln!("loopy {:?}", p.current());` and indeed I see that this is
printed repeatedly.
Diagnosing this a bit shows that the problem is that
`pattern::pattern` function does not consume anything if the next
token is `let`. That is a good default to make cases like
```
let
let foo = 92;
```
where the user hasn't typed the pattern yet, to parse in a reasonable
they correctly.
For match arms, pretty much the single thing we expect is a pattern,
so, for a fix, I introduce a special variant of pattern that does not
do recovery.
63: Add trivial fuzzer for parser r=matklad a=killercup
As described in #61, fuzz testing some parts of this would be ~~fun~~
helpful. So, I started with the most trivial fuzzer I could think of:
Put random stuff into File::parse and see what happens.
To speed things up, I also did
cp src/**/*.rs fuzz/corpus/parser/
in the `crates/libsyntax2/` directory (running the fuzzer once will
generate the necessary directories).
Co-authored-by: Pascal Hertleif <killercup@gmail.com>
As described in #61, fuzz testing some parts of this would be ~~fun~~
helpful. So, I started with the most trivial fuzzer I could think of:
Put random stuff into File::parse and see what happens.
To speed things up, I also did
cp src/**/*.rs fuzz/corpus/parser/
in the `crates/libsyntax2/` directory (running the fuzzer once will
generate the necessary directories).
59: Moved TokenSet into it's own file. r=matklad a=Plasticcaz
As discussed in Issue #11, the only thing left in that issue that hasn't been fixed appears to be that TokenSet is not in it's own file. This pull request pulls TokenSet, it's macros and it's test into it's own file.
Co-authored-by: Zac Winter <plasticcaz@gmail.com>
