[red-knot] support fstring expressions (#13511)

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## Summary

Implement inference for `f-string`, contributes to #12701.

### First Implementation

When looking at the way `mypy` handles things, I noticed the following:
- No variables (e.g. `f"hello"`) ⇒ `LiteralString`
- Any variable (e.g. `f"number {1}"`) ⇒ `str`

My first commit (1ba5d0f) implements
exactly this logic, except that we deal with string literals just like
`infer_string_literal_expression` (if below `MAX_STRING_LITERAL_SIZE`,
show `Literal["exact string"]`)

### Second Implementation

My second commit (90326ce) pushes
things a bit further to handle cases where the expression within the
`f-string` are all literal values (string representation known at static
time).

Here's an example of when this could happen in code:
```python
BASE_URL = "https://httpbin.org"
VERSION = "v1"
endpoint = f"{BASE_URL}/{VERSION}/post"  # Literal["https://httpbin.org/v1/post"]
```
As this can be sightly more costly (additional allocations), I don't
know if we want this feature.

## Test Plan

- Added a test `fstring_expression` covering all cases I can think of

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/src/types.rs

@@ -380,6 +380,10 @@ impl<'db> Type<'db> {
         }
     }
 
+    pub fn builtin_str(db: &'db dyn Db) -> Self {
+        builtins_symbol_ty(db, "str")
+    }
+
     pub fn is_stdlib_symbol(&self, db: &'db dyn Db, module_name: &str, name: &str) -> bool {
         match self {
             Type::Class(class) => class.is_stdlib_symbol(db, module_name, name),
@@ -721,6 +725,44 @@ impl<'db> Type<'db> {
             Type::Tuple(_) => builtins_symbol_ty(db, "tuple"),
         }
     }
+
+    /// Return the string representation of this type when converted to string as it would be
+    /// provided by the `__str__` method.
+    ///
+    /// When not available, this should fall back to the value of `[Type::repr]`.
+    /// Note: this method is used in the builtins `format`, `print`, `str.format` and `f-strings`.
+    #[must_use]
+    pub fn str(&self, db: &'db dyn Db) -> Type<'db> {
+        match self {
+            Type::IntLiteral(_) | Type::BooleanLiteral(_) => self.repr(db),
+            Type::StringLiteral(_) | Type::LiteralString => *self,
+            // TODO: handle more complex types
+            _ => Type::builtin_str(db).to_instance(db),
+        }
+    }
+
+    /// Return the string representation of this type as it would be provided by the  `__repr__`
+    /// method at runtime.
+    #[must_use]
+    pub fn repr(&self, db: &'db dyn Db) -> Type<'db> {
+        match self {
+            Type::IntLiteral(number) => Type::StringLiteral(StringLiteralType::new(db, {
+                number.to_string().into_boxed_str()
+            })),
+            Type::BooleanLiteral(true) => {
+                Type::StringLiteral(StringLiteralType::new(db, "True".into()))
+            }
+            Type::BooleanLiteral(false) => {
+                Type::StringLiteral(StringLiteralType::new(db, "False".into()))
+            }
+            Type::StringLiteral(literal) => Type::StringLiteral(StringLiteralType::new(db, {
+                format!("'{}'", literal.value(db).escape_default()).into()
+            })),
+            Type::LiteralString => Type::LiteralString,
+            // TODO: handle more complex types
+            _ => Type::builtin_str(db).to_instance(db),
+        }
+    }
 }
 
 impl<'db> From<&Type<'db>> for Type<'db> {
@@ -1198,12 +1240,13 @@ mod tests {
 
     /// A test representation of a type that can be transformed unambiguously into a real Type,
     /// given a db.
-    #[derive(Debug)]
+    #[derive(Debug, Clone)]
     enum Ty {
         Never,
         Unknown,
         Any,
         IntLiteral(i64),
+        BoolLiteral(bool),
         StringLiteral(&'static str),
         LiteralString,
         BytesLiteral(&'static str),
@@ -1222,6 +1265,7 @@ mod tests {
                 Ty::StringLiteral(s) => {
                     Type::StringLiteral(StringLiteralType::new(db, (*s).into()))
                 }
+                Ty::BoolLiteral(b) => Type::BooleanLiteral(b),
                 Ty::LiteralString => Type::LiteralString,
                 Ty::BytesLiteral(s) => {
                     Type::BytesLiteral(BytesLiteralType::new(db, s.as_bytes().into()))
@@ -1331,4 +1375,28 @@ mod tests {
         let db = setup_db();
         assert_eq!(ty.into_type(&db).bool(&db), Truthiness::Ambiguous);
     }
+
+    #[test_case(Ty::IntLiteral(1), Ty::StringLiteral("1"))]
+    #[test_case(Ty::BoolLiteral(true), Ty::StringLiteral("True"))]
+    #[test_case(Ty::BoolLiteral(false), Ty::StringLiteral("False"))]
+    #[test_case(Ty::StringLiteral("ab'cd"), Ty::StringLiteral("ab'cd"))] // no quotes
+    #[test_case(Ty::LiteralString, Ty::LiteralString)]
+    #[test_case(Ty::BuiltinInstance("int"), Ty::BuiltinInstance("str"))]
+    fn has_correct_str(ty: Ty, expected: Ty) {
+        let db = setup_db();
+
+        assert_eq!(ty.into_type(&db).str(&db), expected.into_type(&db));
+    }
+
+    #[test_case(Ty::IntLiteral(1), Ty::StringLiteral("1"))]
+    #[test_case(Ty::BoolLiteral(true), Ty::StringLiteral("True"))]
+    #[test_case(Ty::BoolLiteral(false), Ty::StringLiteral("False"))]
+    #[test_case(Ty::StringLiteral("ab'cd"), Ty::StringLiteral("'ab\\'cd'"))] // single quotes
+    #[test_case(Ty::LiteralString, Ty::LiteralString)]
+    #[test_case(Ty::BuiltinInstance("int"), Ty::BuiltinInstance("str"))]
+    fn has_correct_repr(ty: Ty, expected: Ty) {
+        let db = setup_db();
+
+        assert_eq!(ty.into_type(&db).repr(&db), expected.into_type(&db));
+    }
 }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1653,50 +1653,50 @@ impl<'db> TypeInferenceBuilder<'db> {
     fn infer_fstring_expression(&mut self, fstring: &ast::ExprFString) -> Type<'db> {
         let ast::ExprFString { range: _, value } = fstring;
 
+        let mut collector = StringPartsCollector::new();
         for part in value {
+            // Make sure we iter through every parts to infer all sub-expressions. The `collector`
+            // struct ensures we don't allocate unnecessary strings.
             match part {
-                ast::FStringPart::Literal(_) => {
-                    // TODO string literal type
+                ast::FStringPart::Literal(literal) => {
+                    collector.push_str(&literal.value);
                 }
                 ast::FStringPart::FString(fstring) => {
-                    let ast::FString {
-                        range: _,
-                        elements,
-                        flags: _,
-                    } = fstring;
-                    for element in elements {
-                        self.infer_fstring_element(element);
-                    }
-                }
-            }
-        }
-
-        // TODO str type
-        Type::Unknown
-    }
-
-    fn infer_fstring_element(&mut self, element: &ast::FStringElement) {
-        match element {
-            ast::FStringElement::Literal(_) => {
-                // TODO string literal type
-            }
-            ast::FStringElement::Expression(expr_element) => {
-                let ast::FStringExpressionElement {
-                    range: _,
-                    expression,
-                    debug_text: _,
-                    conversion: _,
-                    format_spec,
-                } = expr_element;
-                self.infer_expression(expression);
-
-                if let Some(format_spec) = format_spec {
-                    for spec_element in &format_spec.elements {
-                        self.infer_fstring_element(spec_element);
+                    for element in &fstring.elements {
+                        match element {
+                            ast::FStringElement::Expression(expression) => {
+                                let ast::FStringExpressionElement {
+                                    range: _,
+                                    expression,
+                                    debug_text: _,
+                                    conversion,
+                                    format_spec,
+                                } = expression;
+                                let ty = self.infer_expression(expression);
+
+                                // TODO: handle format specifiers by calling a method
+                                // (`Type::format`?) that handles the `__format__` method.
+                                // Conversion flags should be handled before calling `__format__`.
+                                // https://docs.python.org/3/library/string.html#format-string-syntax
+                                if !conversion.is_none() || format_spec.is_some() {
+                                    collector.add_expression();
+                                } else {
+                                    if let Type::StringLiteral(literal) = ty.str(self.db) {
+                                        collector.push_str(literal.value(self.db));
+                                    } else {
+                                        collector.add_expression();
+                                    }
+                                }
+                            }
+                            ast::FStringElement::Literal(literal) => {
+                                collector.push_str(&literal.value);
+                            }
+                        }
                     }
                 }
             }
         }
+        collector.ty(self.db)
     }
 
     fn infer_ellipsis_literal_expression(
@@ -2659,6 +2659,53 @@ enum ModuleNameResolutionError {
     TooManyDots,
 }
 
+/// Struct collecting string parts when inferring a formatted string. Infers a string literal if the
+/// concatenated string is small enough, otherwise infers a literal string.
+///
+/// If the formatted string contains an expression (with a representation unknown at compile time),
+/// infers an instance of `builtins.str`.
+struct StringPartsCollector {
+    concatenated: Option<String>,
+    expression: bool,
+}
+
+impl StringPartsCollector {
+    fn new() -> Self {
+        Self {
+            concatenated: Some(String::new()),
+            expression: false,
+        }
+    }
+
+    fn push_str(&mut self, literal: &str) {
+        if let Some(mut concatenated) = self.concatenated.take() {
+            if concatenated.len().saturating_add(literal.len())
+                <= TypeInferenceBuilder::MAX_STRING_LITERAL_SIZE
+            {
+                concatenated.push_str(literal);
+                self.concatenated = Some(concatenated);
+            } else {
+                self.concatenated = None;
+            }
+        }
+    }
+
+    fn add_expression(&mut self) {
+        self.concatenated = None;
+        self.expression = true;
+    }
+
+    fn ty(self, db: &dyn Db) -> Type {
+        if self.expression {
+            Type::builtin_str(db).to_instance(db)
+        } else if let Some(concatenated) = self.concatenated {
+            Type::StringLiteral(StringLiteralType::new(db, concatenated.into_boxed_str()))
+        } else {
+            Type::LiteralString
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
 
@@ -3593,6 +3640,71 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn fstring_expression() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            x = 0
+            y = str()
+            z = False
+
+            a = f'hello'
+            b = f'h {x}'
+            c = 'one ' f'single ' f'literal'
+            d = 'first ' f'second({b})' f' third'
+            e = f'-{y}-'
+            f = f'-{y}-' f'--' '--'
+            g = f'{z} == {False} is {True}'
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "a", "Literal[\"hello\"]");
+        assert_public_ty(&db, "src/a.py", "b", "Literal[\"h 0\"]");
+        assert_public_ty(&db, "src/a.py", "c", "Literal[\"one single literal\"]");
+        assert_public_ty(&db, "src/a.py", "d", "Literal[\"first second(h 0) third\"]");
+        assert_public_ty(&db, "src/a.py", "e", "str");
+        assert_public_ty(&db, "src/a.py", "f", "str");
+        assert_public_ty(&db, "src/a.py", "g", "Literal[\"False == False is True\"]");
+
+        Ok(())
+    }
+
+    #[test]
+    fn fstring_expression_with_conversion_flags() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            string = 'hello'
+            a = f'{string!r}'
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "a", "str"); // Should be `Literal["'hello'"]`
+
+        Ok(())
+    }
+
+    #[test]
+    fn fstring_expression_with_format_specifier() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            a = f'{1:02}'
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "a", "str"); // Should be `Literal["01"]`
+
+        Ok(())
+    }
+
     #[test]
     fn basic_call_expression() -> anyhow::Result<()> {
         let mut db = setup_db();