Repository: tweag/nickel (rust)

Issue:
Cannot put string interpolation inside quotes in a multiline string if using multiple #
**Describe the bug**
When creating a multiline string with multiple `#`, if I want to put the result of an interpolation between quotes, I got this parsing error:
```
> let s = "Hello" in m##""##{s}" World"##m

error: Unexpected token
  ┌─ repl-input-1:1:34
  │
1 │let s = "Hello" in m##""##{s}" World"##m
  │                       ^^^
```

**Expected behavior**
I expected this to generate `""Hello" World"`, since this other example with only one `#` works:
```
> let s = "Hello" in m#""#{s}" World"#m
""Hello" World"
```

Also, this example with an extra space works too:
```
> let s = "Hello" in m##"" ##{s}" World"##m
"" Hello" World"
```

**Environment**
 - rust version: 1.58.1
 - Version of the code: 9f618e6ca6d6d7cee6826dbf44d462f4ceb28e8a

Pull request description:
[Fix]Interpolation in multiline strings
Fix #596.

The main issue was a `token` annotation instead of a `regex`one in the lexer. Since this bug was preventing the corresponding code path in the lexer from being triggered, the initial fix unveiled a few other small issues that we fixed along the way. 

This PR bumps the `logos` dependency and adds a regression test as well.

Patch:
diff --git a/Cargo.lock b/Cargo.lock
index b7306dd3..d73ac2f2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -128,9 +128,9 @@ dependencies = [
 
 [[package]]
 name = "beef"
-version = "0.4.4"
+version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "474a626a67200bd107d44179bb3d4fc61891172d11696609264589be6a0e6a43"
+checksum = "bed554bd50246729a1ec158d08aa3235d1b69d94ad120ebe187e28894787e736"
 
 [[package]]
 name = "bit-set"
@@ -788,18 +788,18 @@ dependencies = [
 
 [[package]]
 name = "logos"
-version = "0.11.4"
+version = "0.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b91c49573597a5d6c094f9031617bb1fed15c0db68c81e6546d313414ce107e4"
+checksum = "427e2abca5be13136da9afdbf874e6b34ad9001dd70f2b103b083a85daa7b345"
 dependencies = [
  "logos-derive",
 ]
 
 [[package]]
 name = "logos-derive"
-version = "0.11.5"
+version = "0.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "797b1f8a0571b331c1b47e7db245af3dc634838da7a92b3bef4e30376ae1c347"
+checksum = "56a7d287fd2ac3f75b11f19a1c8a874a7d55744bd91f7a1b3e7cf87d4343c36d"
 dependencies = [
  "beef",
  "fnv",
diff --git a/Cargo.toml b/Cargo.toml
index 8a3dc8b5..973fd10f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -30,7 +30,7 @@ regex = "0.2.1"
 simple-counter = "0.1.0"
 codespan = "0.11"
 codespan-reporting = "0.11"
-logos = "0.11.4"
+logos = "0.12.0"
 serde = { version = "1.0.117", features = ["derive"] }
 serde_json = "1.0.59"
 serde_yaml = "0.8.15"
diff --git a/src/parser/lexer.rs b/src/parser/lexer.rs
index 322843df..b98a97da 100644
--- a/src/parser/lexer.rs
+++ b/src/parser/lexer.rs
@@ -333,7 +333,7 @@ pub enum MultiStringToken<'input> {
     /// The other rules should be sufficient to match this as a double quote followed by a
     /// `CandidateInterpolation`, but if we omit this token, the lexer can fail unexpectedly on
     /// valid inputs because of #200.
-    #[token("\"#+\\{")]
+    #[regex("\"#+\\{")]
     QuotesCandidateInterpolation(&'input str),
     /// Token emitted by the modal lexer for the parser once it has decided that a `CandidateEnd` is
     /// an actual end token.
@@ -437,7 +437,7 @@ impl<'input> Lexer<'input> {
             //  `Normal`
             Some(ModalLexer::Normal(lexer)) => {
                 self.stack.push(ModeElt::Normal(self.count));
-                self.lexer.replace(morph(lexer));
+                self.lexer = Some(morph(lexer));
             }
             _ => panic!("lexer::enter_strlike"),
         }
@@ -457,11 +457,11 @@ impl<'input> Lexer<'input> {
         match self.lexer.take() {
             //count must be zero, and we do not push it on the stack
             Some(ModalLexer::Str(lexer)) => {
-                self.lexer.replace(ModalLexer::Normal(lexer.morph()));
+                self.lexer = Some(ModalLexer::Normal(lexer.morph()));
                 self.stack.push(ModeElt::Str);
             }
             Some(ModalLexer::MultiStr(lexer)) => {
-                self.lexer.replace(ModalLexer::Normal(lexer.morph()));
+                self.lexer = Some(ModalLexer::Normal(lexer.morph()));
                 self.stack.push(ModeElt::MultiStr(self.count));
             }
             _ => panic!("lexer::enter_normal"),
@@ -479,7 +479,7 @@ impl<'input> Lexer<'input> {
                     mode => panic!("lexer::leave_str (popped mode {:?})", mode),
                 };
 
-                self.lexer.replace(ModalLexer::Normal(lexer.morph()));
+                self.lexer = Some(ModalLexer::Normal(lexer.morph()));
             }
             _ => panic!("lexer::leave_str"),
         }
@@ -494,7 +494,7 @@ impl<'input> Lexer<'input> {
                     mode => panic!("lexer::leave_str (popped mode {:?})", mode),
                 };
 
-                self.lexer.replace(ModalLexer::Normal(lexer.morph()));
+                self.lexer = Some(ModalLexer::Normal(lexer.morph()));
             }
             _ => panic!("lexer::leave_str"),
         }
@@ -505,10 +505,10 @@ impl<'input> Lexer<'input> {
             Some(ModalLexer::Normal(lexer)) => {
                 // count must be 0
                 match self.stack.pop() {
-                    Some(ModeElt::Str) => self.lexer.replace(ModalLexer::Str(lexer.morph())),
+                    Some(ModeElt::Str) => self.lexer = Some(ModalLexer::Str(lexer.morph())),
                     Some(ModeElt::MultiStr(count)) => {
                         self.count = count;
-                        self.lexer.replace(ModalLexer::MultiStr(lexer.morph()))
+                        self.lexer = Some(ModalLexer::MultiStr(lexer.morph()))
                     }
                     mode => panic!("lexer::leave_normal (popped mode {:?})", mode),
                 };
@@ -566,28 +566,36 @@ impl<'input> Iterator for Lexer<'input> {
                 token = Some(MultiStr(MultiStringToken::Interpolation));
                 self.enter_normal();
             }
+            // We never lex something as a `MultiStringToken::Interpolation` directly, but rather
+            // generate it in this very function from other tokens. However, such a token could
+            // have still been buffered in the previous iteration, and can thus be matched here,
+            // which is why we need the case below.
+            Some(MultiStr(MultiStringToken::Interpolation)) => self.enter_normal(),
             // If we encouter a `QuotesCandidateInterpolation` token with the right number of
             // characters, we need to split it into two tokens:
-            // - a simple `"` literal
-            // - a interpolation token
+            // - a literal starting by a `"` followed by between 0 and k hashes `#`
+            // - an interpolation token
             // The interpolation token is put in the buffer such that it will be returned next
             // time.
+            //
+            // For example, in `m##""###{exp}"##m`, the `"###{` is a `QuotesCandidateInterpolation`
+            // which is split as a `"#` literal followed by an interpolation token.
             Some(MultiStr(MultiStringToken::QuotesCandidateInterpolation(s)))
-                if s.len() == self.count =>
+                if s.len() >= self.count =>
             {
+                let split_at = s.len() - self.count + 1;
                 let next_token = MultiStr(MultiStringToken::Interpolation);
                 let next_span = Range {
-                    start: span.start + 1,
+                    start: span.start + split_at,
                     end: span.end,
                 };
-                self.buffer.replace((next_token, next_span));
+                self.buffer = Some((next_token, next_span));
 
-                token = Some(MultiStr(MultiStringToken::Literal(&s[0..1])));
+                token = Some(MultiStr(MultiStringToken::Literal(&s[0..split_at])));
                 span = Range {
                     start: span.start,
-                    end: span.start + 1,
+                    end: span.start + split_at,
                 };
-                self.enter_normal();
             }
             // Otherwise, it is just part of the string, so we transform the token into a
             // `FalseInterpolation` one
@@ -637,7 +645,7 @@ impl<'input> Iterator for Lexer<'input> {
             // Ignore comment
             Some(Normal(NormalToken::LineComment)) => return self.next(),
             _ => (),
-        }
+        };
 
         token.map(|t| Ok((span.start, t, span.end)))
     }


Test patch:
diff --git a/src/parser/tests.rs b/src/parser/tests.rs
index 792d979c..83f9479a 100644
--- a/src/parser/tests.rs
+++ b/src/parser/tests.rs
@@ -279,8 +279,7 @@ fn string_lexing() {
         lex_without_pos(r##"m#""#"#m"##),
         Ok(vec![
             Token::Normal(NormalToken::MultiStringStart(3)),
-            Token::MultiStr(MultiStringToken::Literal("\"")),
-            Token::MultiStr(MultiStringToken::Literal("#")),
+            Token::MultiStr(MultiStringToken::Literal("\"#")),
             Token::MultiStr(MultiStringToken::End),
         ])
     );
diff --git a/tests/pass/strings.ncl b/tests/pass/strings.ncl
index 9648f296..6ed8c7b9 100644
--- a/tests/pass/strings.ncl
+++ b/tests/pass/strings.ncl
@@ -15,5 +15,10 @@ let Assert = fun l x => x || %blame% l in
   m#""#{"foo"}""#m == "\"foo\"",
   m#"""#m == "\"",
   m#""#"#"#"#m == "\"#\"#\"#",
+
+  // regression test for issue #596 (https://github.com/tweag/nickel/issues/596)
+  let s = "Hello" in m##""##{s}" World"##m == "\"Hello\" World",
+  let s = "Hello" in m##""###{s}" World"##m == "\"#Hello\" World",
+  m##""##s"##m == "\"##s",
 ]
 |> lists.foldl (fun x y => (x | #Assert) && y) true