[red-knot] use declared types in inference/checking (#13335)

Use declared types in inference and checking. This means several things:

* Imports prefer declarations over inference, when declarations are
available.
* When we encounter a binding, we check that the bound value's inferred
type is assignable to the live declarations of the bound symbol, if any.
* When we encounter a declaration, we check that the declared type is
assignable from the inferred type of the symbol from previous bindings,
if any.
* When we encounter a binding+declaration, we check that the inferred
type of the bound value is assignable to the declared type.

crates/ruff_db/src/display.rs

@@ -0,0 +1,52 @@
+use std::fmt::{self, Display, Formatter};
+
+pub trait FormatterJoinExtension<'b> {
+    fn join<'a>(&'a mut self, separator: &'static str) -> Join<'a, 'b>;
+}
+
+impl<'b> FormatterJoinExtension<'b> for Formatter<'b> {
+    fn join<'a>(&'a mut self, separator: &'static str) -> Join<'a, 'b> {
+        Join {
+            fmt: self,
+            separator,
+            result: fmt::Result::Ok(()),
+            seen_first: false,
+        }
+    }
+}
+
+pub struct Join<'a, 'b> {
+    fmt: &'a mut Formatter<'b>,
+    separator: &'static str,
+    result: fmt::Result,
+    seen_first: bool,
+}
+
+impl<'a, 'b> Join<'a, 'b> {
+    pub fn entry(&mut self, item: &dyn Display) -> &mut Self {
+        if self.seen_first {
+            self.result = self
+                .result
+                .and_then(|()| self.fmt.write_str(self.separator));
+        } else {
+            self.seen_first = true;
+        }
+        self.result = self.result.and_then(|()| item.fmt(self.fmt));
+        self
+    }
+
+    pub fn entries<I, F>(&mut self, items: I) -> &mut Self
+    where
+        I: IntoIterator<Item = F>,
+        F: Display,
+    {
+        for item in items {
+            self.entry(&item);
+        }
+        self
+    }
+
+    pub fn finish(&mut self) -> fmt::Result {
+        self.result
+    }
+}

crates/ruff_db/src/lib.rs

@@ -6,6 +6,7 @@ use crate::files::Files;
 use crate::system::System;
 use crate::vendored::VendoredFileSystem;
 
+pub mod display;
 pub mod file_revision;
 pub mod files;
 pub mod parsed;