Properly handle different defaults for severity of lints

Previously all lints were assumed to be `#[warn]`, and we had a hand-coded list of `#[allow]` exceptions. Now the severity is autogenerated from rustdoc output.

Also support lints that change status between editions, and the `warnings` lint group.

crates/edition/src/lib.rs

@@ -0,0 +1,76 @@
+//! The edition of the Rust language used in a crate.
+// This should live in a separate crate because we use it in both actual code and codegen.
+use std::fmt;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(u8)]
+pub enum Edition {
+    Edition2015,
+    Edition2018,
+    Edition2021,
+    Edition2024,
+}
+
+impl Edition {
+    pub const DEFAULT: Edition = Edition::Edition2015;
+    pub const LATEST: Edition = Edition::Edition2024;
+    pub const CURRENT: Edition = Edition::Edition2021;
+    /// The current latest stable edition, note this is usually not the right choice in code.
+    pub const CURRENT_FIXME: Edition = Edition::Edition2021;
+
+    pub fn at_least_2024(self) -> bool {
+        self >= Edition::Edition2024
+    }
+
+    pub fn at_least_2021(self) -> bool {
+        self >= Edition::Edition2021
+    }
+
+    pub fn at_least_2018(self) -> bool {
+        self >= Edition::Edition2018
+    }
+
+    pub fn iter() -> impl Iterator<Item = Edition> {
+        [Edition::Edition2015, Edition::Edition2018, Edition::Edition2021, Edition::Edition2024]
+            .iter()
+            .copied()
+    }
+}
+
+#[derive(Debug)]
+pub struct ParseEditionError {
+    invalid_input: String,
+}
+
+impl std::error::Error for ParseEditionError {}
+impl fmt::Display for ParseEditionError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "invalid edition: {:?}", self.invalid_input)
+    }
+}
+
+impl std::str::FromStr for Edition {
+    type Err = ParseEditionError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let res = match s {
+            "2015" => Edition::Edition2015,
+            "2018" => Edition::Edition2018,
+            "2021" => Edition::Edition2021,
+            "2024" => Edition::Edition2024,
+            _ => return Err(ParseEditionError { invalid_input: s.to_owned() }),
+        };
+        Ok(res)
+    }
+}
+
+impl fmt::Display for Edition {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            Edition::Edition2015 => "2015",
+            Edition::Edition2018 => "2018",
+            Edition::Edition2021 => "2021",
+            Edition::Edition2024 => "2024",
+        })
+    }
+}