---

yaml
---
r: 125946
b: refs/heads/try
c: 769dae0
h: refs/heads/master
v: v3

Author: bors

[refs]

@@ -2,7 +2,7 @@
 refs/heads/master: f2fa55903e378368ed9173560f03a0ef16e371c2
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: 9fc8394d3bce22ab483f98842434c84c396212ae
-refs/heads/try: 28d543a40ffa872622025bcb4f112c890ca59760
+refs/heads/try: 769dae0a00f04bae5ffba6c9f3c0cf2478782be1
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/ndm: f3868061cd7988080c30d6d5bf352a5a5fe2460b
 refs/heads/try2: 147ecfdd8221e4a4d4e090486829a06da1e0ca3c

branches/try/src/grammar/raw-string-literal-ambiguity.md

@@ -1,7 +1,11 @@
 Rust's lexical grammar is not context-free. Raw string literals are the source
 of the problem. Informally, a raw string literal is an `r`, followed by `N`
 hashes (where N can be zero), a quote, any characters, then a quote followed
-by `N` hashes. This grammar describes this as best possible:
+by `N` hashes. Critically, once inside the first pair of quotes,
+another quote cannot be followed by `N` consecutive hashes. e.g.
+`r###""###"###` is invalid.
+
+This grammar describes this as best possible:
 
     R -> 'r' S
     S -> '"' B '"'
@@ -22,8 +26,39 @@ accepted as one by the above grammar, using the derivation:
 (Where `T : U` means the rule `T` is applied, and `U` is the remainder of the
 string.) The difficulty arises from the fact that it is fundamentally
 context-sensitive. In particular, the context needed is the number of hashes.
-I know of no way to resolve this, but also have not come up with a proof that
-it is not context sensitive. Such a proof would probably use the pumping lemma
-for context-free languages, but I (cmr) could not come up with a proof after
-spending a few hours on it, and decided my time best spent elsewhere. Pull
-request welcome!
+
+To prove that Rust's string literals are not context-free, we will use
+the fact that context-free languages are closed under intersection with
+regular languages, and the
+[pumping lemma for context-free languages](https://en.wikipedia.org/wiki/Pumping_lemma_for_context-free_languages).
+
+Consider the regular language `R = r#+""#*"#+`. If Rust's raw string literals are
+context-free, then their intersection with `R`, `R'`, should also be context-free.
+Therefore, to prove that raw string literals are not context-free,
+it is sufficient to prove that `R'` is not context-free.
+
+The language `R'` is `{r#^n""#^m"#^n | m < n}`.
+
+Assume `R'` *is* context-free. Then `R'` has some pumping length `p > 0` for which
+the pumping lemma applies. Consider the following string `s` in `R'`:
+
+`r#^p""#^{p-1}"#^p`
+
+e.g. for `p = 2`: `s = r##""#"##`
+
+Then `s = uvwxy` for some choice of `uvwxy` such that `vx` is non-empty,
+`|vwx| < p+1`, and `uv^iwx^iy` is in `R'` for all `i >= 0`.
+
+Neither `v` nor `x` can contain a `"` or `r`, as the number of these characters
+in any string in `R'` is fixed. So `v` and `x` contain only hashes.
+Consequently, of the three sequences of hashes, `v` and `x` combined
+can only pump two of them.
+If we ever choose the central sequence of hashes, then one of the outer sequences
+will not grow when we pump, leading to an imbalance between the outer sequences.
+Therefore, we must pump both outer sequences of hashes. However,
+there are `p+2` characters between these two sequences of hashes, and `|vwx|` must
+be less than `p+1`. Therefore we have a contradiction, and `R'` must not be
+context-free.
+
+Since `R'` is not context-free, it follows that the Rust's raw string literals
+must not be context-free.

branches/try/src/librustdoc/html/render.rs

@@ -845,8 +845,12 @@ impl DocFolder for Cache {
                     }
                     _ => (None, Some(self.stack.as_slice()))
                 };
+                let hidden_field = match item.inner {
+                    clean::StructFieldItem(clean::HiddenStructField) => true,
+                    _ => false
+                };
                 match parent {
-                    (parent, Some(path)) if !self.privmod => {
+                    (parent, Some(path)) if !self.privmod && !hidden_field => {
                         self.search_index.push(IndexItem {
                             ty: shortty(&item),
                             name: s.to_string(),