Merge #2641

2641: Parse const generics r=matklad a=roblabla

Adds very primitive support for parsing const generics (`const IDENT: TY`) so that rust-analyzer stops complaining about the syntax being invalid.

Fixes #1574
Fixes #2281 

Co-authored-by: roblabla <unfiltered@roblab.la>

crates/ra_syntax/src/ast/generated.rs

@@ -551,6 +551,36 @@ impl ConstDef {
     }
 }
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct ConstParam {
+    pub(crate) syntax: SyntaxNode,
+}
+impl AstNode for ConstParam {
+    fn can_cast(kind: SyntaxKind) -> bool {
+        match kind {
+            CONST_PARAM => true,
+            _ => false,
+        }
+    }
+    fn cast(syntax: SyntaxNode) -> Option<Self> {
+        if Self::can_cast(syntax.kind()) {
+            Some(Self { syntax })
+        } else {
+            None
+        }
+    }
+    fn syntax(&self) -> &SyntaxNode {
+        &self.syntax
+    }
+}
+impl ast::NameOwner for ConstParam {}
+impl ast::AttrsOwner for ConstParam {}
+impl ast::TypeAscriptionOwner for ConstParam {}
+impl ConstParam {
+    pub fn default_val(&self) -> Option<Expr> {
+        AstChildren::new(&self.syntax).next()
+    }
+}
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct ContinueExpr {
     pub(crate) syntax: SyntaxNode,
 }