Use narrow type for string parsing patterns (#7211)

This PR adds a new enum type `StringType` which is either a string
literal, byte literal or f-string. The motivation behind this is to have
a narrow type which is accepted in `concatenate_strings` as that
function is only applicable for the mentioned 3 types. This makes the
code more readable and easy to reason about.

A future improvement (which was prototyped here and removed) is to split
the current string literal pattern in LALRPOP definition into two parts:
1. A single string literal or a f-string: This means no checking for
bytes / non-bytes and other unnecessary compution
2. Two or more of string/byte/f-string: This will call the
`concatenate_strings` function.

The reason for removing the second change is because of how ranges work.
The range for an individual string/byte is the entire range which
includes the quotes as well but if the same string/byte is part of a
f-string, then it only includes the range for the content (without the
quotes / inner range). The current string parser returns with the former
range.

To give an example, for `"foo"`, the range of the string would be
`0..5`, but for `f"foo"` the range of the string would be `2..5` while
the range for the f-string expression would be `0..6`. The ranges are
correct but they differ in the context the string constant itself is
being used for. Is it part of a f-string or is it a standalone string?

`cargo test`

crates/ruff_python_ast/src/nodes.rs

@@ -2628,6 +2628,16 @@ impl Deref for StringConstant {
     }
 }
 
+impl From<String> for StringConstant {
+    fn from(value: String) -> StringConstant {
+        Self {
+            value,
+            unicode: false,
+            implicit_concatenated: false,
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct BytesConstant {
     /// The bytes value as resolved by the parser (i.e., without quotes, or escape sequences, or
@@ -2644,6 +2654,15 @@ impl Deref for BytesConstant {
     }
 }
 
+impl From<Vec<u8>> for BytesConstant {
+    fn from(value: Vec<u8>) -> BytesConstant {
+        Self {
+            value,
+            implicit_concatenated: false,
+        }
+    }
+}
+
 impl From<Vec<u8>> for Constant {
     fn from(value: Vec<u8>) -> Constant {
         Self::Bytes(BytesConstant {

crates/ruff_python_parser/src/python.lalrpop

@@ -11,7 +11,7 @@ use crate::{
     lexer::{LexicalError, LexicalErrorType},
     function::{ArgumentList, parse_arguments, validate_pos_params, validate_arguments},
     context::set_context,
-    string::{concatenate_strings, parse_fstring_middle, parse_string_literal},
+    string::{StringType, concatenate_strings, parse_fstring_middle, parse_string_literal},
     token::{self, StringKind},
 };
 use lalrpop_util::ParseError;
@@ -669,7 +669,7 @@ LiteralPattern: ast::Pattern = {
         range: (location..end_location).into()
     }.into(),
     <location:@L> <strings:StringLiteralOrFString+> <end_location:@R> =>? Ok(ast::PatternMatchValue {
-        value: Box::new(concatenate_strings(strings, (location..end_location).into())?.into()),
+        value: Box::new(concatenate_strings(strings, (location..end_location).into())?),
         range: (location..end_location).into()
     }.into()),
 }
@@ -726,7 +726,7 @@ MappingKey: ast::Expr = {
         value: false.into(),
         range: (location..end_location).into()
     }.into(),
-    <location:@L> <strings:StringLiteralOrFString+> <end_location:@R> =>? Ok(concatenate_strings(strings, (location..end_location).into())?.into()),
+    <location:@L> <strings:StringLiteralOrFString+> <end_location:@R> =>? Ok(concatenate_strings(strings, (location..end_location).into())?),
 }
 
 MatchMappingEntry: (ast::Expr, ast::Pattern) = {
@@ -1573,25 +1573,25 @@ SliceOp: Option<ast::ParenthesizedExpr> = {
     <location:@L> ":" <e:Test<"all">?> => e,
 }
 
-StringLiteralOrFString: ast::ParenthesizedExpr = {
+StringLiteralOrFString: StringType = {
     StringLiteral,
     FStringExpr,
 };
 
-StringLiteral: ast::ParenthesizedExpr = {
+StringLiteral: StringType = {
     <start_location:@L> <string:string> =>? {
         let (source, kind, triple_quoted) = string;
-        Ok(parse_string_literal(&source, kind, triple_quoted, start_location)?.into())
+        Ok(parse_string_literal(&source, kind, triple_quoted, start_location)?)
     }
 };
 
-FStringExpr: ast::ParenthesizedExpr = {
+FStringExpr: StringType = {
     <location:@L> FStringStart <values:FStringMiddlePattern*> FStringEnd <end_location:@R> => {
-        ast::ExprFString {
+        StringType::FString(ast::ExprFString {
             values: values.into_iter().flatten().collect(),
             implicit_concatenated: false,
             range: (location..end_location).into()
-        }.into()
+        })
     }
 };
 
@@ -1665,7 +1665,7 @@ FStringConversion: (TextSize, ast::ConversionFlag) = {
 };
 
 Atom<Goal>: ast::ParenthesizedExpr = {
-    <location:@L> <strings:StringLiteralOrFString+> <end_location:@L> =>? Ok(concatenate_strings(strings, (location..end_location).into())?),
+    <location:@L> <strings:StringLiteralOrFString+> <end_location:@R> =>? Ok(concatenate_strings(strings, (location..end_location).into())?.into()),
     <location:@L> <value:Constant> <end_location:@R> => ast::ExprConstant {
         value,
         range: (location..end_location).into(),