From a5c333c3ed98d539fcadcc723e992f5295d22d5c Mon Sep 17 00:00:00 2001 From: Aleksey Kladov Date: Sat, 8 Sep 2018 19:10:20 +0300 Subject: Fix yet another parser infinite loop 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. --- crates/libsyntax2/src/grammar/expressions/atom.rs | 6 ++---- crates/libsyntax2/src/grammar/patterns.rs | 12 ++++++++---- 2 files changed, 10 insertions(+), 8 deletions(-) (limited to 'crates/libsyntax2/src/grammar') diff --git a/crates/libsyntax2/src/grammar/expressions/atom.rs b/crates/libsyntax2/src/grammar/expressions/atom.rs index fdb4718ba..8335c700f 100644 --- a/crates/libsyntax2/src/grammar/expressions/atom.rs +++ b/crates/libsyntax2/src/grammar/expressions/atom.rs @@ -323,11 +323,9 @@ fn match_arm_list(p: &mut Parser) { // } fn match_arm(p: &mut Parser) -> BlockLike { let m = p.start(); - loop { + patterns::pattern_r(p, TokenSet::EMPTY); + while p.eat(PIPE) { patterns::pattern(p); - if !p.eat(PIPE) { - break; - } } if p.eat(IF_KW) { expr_no_struct(p); diff --git a/crates/libsyntax2/src/grammar/patterns.rs b/crates/libsyntax2/src/grammar/patterns.rs index 6dd3ab2fa..29a55cb46 100644 --- a/crates/libsyntax2/src/grammar/patterns.rs +++ b/crates/libsyntax2/src/grammar/patterns.rs @@ -8,7 +8,11 @@ pub(super) const PATTERN_FIRST: TokenSet = ]; pub(super) fn pattern(p: &mut Parser) { - if let Some(lhs) = atom_pat(p) { + pattern_r(p, PAT_RECOVERY_SET) +} + +pub(super) fn pattern_r(p: &mut Parser, recovery_set: TokenSet) { + if let Some(lhs) = atom_pat(p, recovery_set) { // test range_pat // fn main() { // match 92 { 0 ... 100 => () } @@ -16,7 +20,7 @@ pub(super) fn pattern(p: &mut Parser) { if p.at(DOTDOTDOT) { let m = lhs.precede(p); p.bump(); - atom_pat(p); + atom_pat(p, recovery_set); m.complete(p, RANGE_PAT); } } @@ -26,7 +30,7 @@ const PAT_RECOVERY_SET: TokenSet = token_set![LET_KW, IF_KW, WHILE_KW, LOOP_KW, MATCH_KW, R_PAREN, COMMA]; -fn atom_pat(p: &mut Parser) -> Option { +fn atom_pat(p: &mut Parser, recovery_set: TokenSet) -> Option { let la0 = p.nth(0); let la1 = p.nth(1); if la0 == REF_KW || la0 == MUT_KW @@ -56,7 +60,7 @@ fn atom_pat(p: &mut Parser) -> Option { L_PAREN => tuple_pat(p), L_BRACK => slice_pat(p), _ => { - p.err_recover("expected pattern", PAT_RECOVERY_SET); + p.err_recover("expected pattern", recovery_set); return None; } }; -- cgit v1.2.3