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.
a5c333c3ed