[JSONSelection] Fix precedence of parsing NamedPathSelection within NamedSelection rule

apollo-federation/src/sources/connect/json_selection/README.md

@@ -75,10 +75,10 @@ below.
 JSONSelection        ::= NakedSubSelection | PathSelection
 NakedSubSelection    ::= NamedSelection* StarSelection?
 SubSelection         ::= "{" NakedSubSelection "}"
-NamedSelection       ::= NamedFieldSelection | NamedQuotedSelection | NamedPathSelection | NamedGroupSelection
+NamedSelection       ::= NamedPathSelection | NamedFieldSelection | NamedQuotedSelection | NamedGroupSelection
+NamedPathSelection   ::= Alias PathSelection
 NamedFieldSelection  ::= Alias? Identifier SubSelection?
 NamedQuotedSelection ::= Alias StringLiteral SubSelection?
-NamedPathSelection   ::= Alias PathSelection
 NamedGroupSelection  ::= Alias SubSelection
 Alias                ::= Identifier ":"
 PathSelection        ::= (VarPath | KeyPath) SubSelection?
@@ -112,17 +112,21 @@ operator. Every `CamelCase` identifier on the left side of the `::=` operator
 can be recursively expanded into one of its right-side alternatives.
 
 Methodically trying out all these alternatives is the fundamental job of the
-parser. While this grammar is not believed to have any ambiguities, ambiguities
-can be resolved by applying the alternatives left to right, accepting the first
-set of expansions that fully matches the input tokens. Parsing succeeds when
-only terminal tokens remain (quoted text or regular expression character
-classes).
-
-Much like regular expression syntax, the `*` and `+` operators denote repetition
-(_zero or more_ and _one or more_, respectively), `?` denotes optionality (_zero
-or one_), parentheses allow grouping, `"quoted"` or `'quoted'` text represents
-raw characters that cannot be expanded further, and `[...]` specifies character
-ranges.
+parser. Parsing succeeds when only terminal tokens remain (quoted text or
+regular expression character classes).
+
+Ambiguities can be resolved by applying the alternatives left to right,
+accepting the first set of expansions that fully matches the input tokens. An
+example where this kind of ordering matters is the `NamedSelection` rule, which
+specifies parsing `NamedPathSelection` before `NamedFieldSelection` and
+`NamedQuotedSelection`, so the entire path will be consumed, rather than
+mistakenly consuming only the first key in the path as a field name.
+
+As in many regular expression syntaxes, the `*` and `+` operators denote
+repetition (_zero or more_ and _one or more_, respectively), `?` denotes
+optionality (_zero or one_), parentheses allow grouping, `"quoted"` or
+`'quoted'` text represents raw characters that cannot be expanded further, and
+`[...]` specifies character ranges.
 
 ### Whitespace, comments, and `NO_SPACE`
 
@@ -253,6 +257,34 @@ Every possible production of the `NamedSelection` non-terminal corresponds to a
 named property in the output object, though each one obtains its value from the
 input object in a slightly different way.
 
+### `NamedPathSelection ::=`
+
+![NamedPathSelection](./grammar/NamedPathSelection.svg)
+
+Since `PathSelection` returns an anonymous value extracted from the given path,
+if you want to use a `PathSelection` alongside other `NamedSelection` items, you
+have to prefix it with an `Alias`, turning it into a `NamedPathSelection`.
+
+For example, you cannot omit the `pathName:` alias in the following
+`NakedSubSelection`, because `some.nested.path` has no output name by itself:
+
+```graphql
+position { x y }
+pathName: some.nested.path { a b c }
+scalarField
+```
+
+The ordering of alternatives in the `NamedSelection` rule is important, so the
+`NamedPathSelection` alternative can be considered before `NamedFieldSelection`
+and `NamedQuotedSelection`, because a `NamedPathSelection` such as `pathName:
+some.nested.path` has a prefix that looks like a `NamedFieldSelection`:
+`pathName: some`, causing an error when the parser encounters the remaining
+`.nested.path` text. Some parsers would resolve this ambiguity by forbidding `.`
+in the lookahead for `Named{Field,Quoted}Selection`, but negative lookahead is
+tricky for this parser (see similar discussion regarding `NO_SPACE`), so instead
+we greedily parse `NamedPathSelection` first, when possible, since that ensures
+the whole path will be consumed.
+
 ### `NamedFieldSelection ::=`
 
 ![NamedFieldSelection](./grammar/NamedFieldSelection.svg)
@@ -290,23 +322,6 @@ this selection extracts the `second property` field as `second`, subselecting
 `x`, `y`, and `z` from the extracted object. The final object will have the
 properties `first`, `second`, and `third`.
 
-### `NamedPathSelection ::=`
-
-![NamedPathSelection](./grammar/NamedPathSelection.svg)
-
-Since `PathSelection` returns an anonymous value extracted from the given path,
-if you want to use a `PathSelection` alongside other `NamedSelection` items, you
-have to prefix it with an `Alias`, turning it into a `NamedPathSelection`.
-
-For example, you cannot omit the `pathName:` alias in the following
-`NakedSubSelection`, because `some.nested.path` has no output name by itself:
-
-```graphql
-position { x y }
-pathName: some.nested.path { a b c }
-scalarField
-```
-
 ### `NamedGroupSelection ::=`
 
 ![NamedGroupSelection](./grammar/NamedGroupSelection.svg)

apollo-federation/src/sources/connect/json_selection/parser.rs

@@ -50,10 +50,10 @@ impl JSONSelection {
     }
 }
 
-// NamedSelection       ::= NamedFieldSelection | NamedQuotedSelection | NamedPathSelection | NamedGroupSelection
+// NamedSelection       ::= NamedPathSelection | NamedFieldSelection | NamedQuotedSelection | NamedGroupSelection
+// NamedPathSelection   ::= Alias PathSelection
 // NamedFieldSelection  ::= Alias? Identifier SubSelection?
 // NamedQuotedSelection ::= Alias StringLiteral SubSelection?
-// NamedPathSelection   ::= Alias PathSelection
 // NamedGroupSelection  ::= Alias SubSelection
 
 #[derive(Debug, PartialEq, Clone, Serialize)]
@@ -67,9 +67,17 @@ pub enum NamedSelection {
 impl NamedSelection {
     fn parse(input: &str) -> IResult<&str, Self> {
         alt((
+            // We must try parsing NamedPathSelection before NamedFieldSelection
+            // and NamedQuotedSelection because a NamedPathSelection without a
+            // leading `.`, such as `alias: some.nested.path` has a prefix that
+            // can be parsed as a NamedFieldSelection: `alias: some`. Parsing
+            // then fails when it finds the remaining `.nested.path` text. Some
+            // parsers would solve this by forbidding `.` in the "lookahead" for
+            // Named{Field,Quoted}Selection, but negative lookahead is tricky in
+            // nom, so instead we greedily parse NamedPathSelection first.
+            Self::parse_path,
             Self::parse_field,
             Self::parse_quoted,
-            Self::parse_path,
             Self::parse_group,
         ))(input)
     }