Implement beginnings of generics

- add HIR for generic params
 - resolve generic params in type paths
 - add substitions for ADTs
 - insert type variables for substitutions

crates/ra_hir/src/generics.rs

@@ -0,0 +1,48 @@
+//! Many kinds of items or constructs can have generic parameters: functions,
+//! structs, impls, traits, etc. This module provides a common HIR for these
+//! generic parameters. See also the `Generics` type and the `generics_of` query
+//! in rustc.
+
+use std::sync::Arc;
+
+use ra_syntax::ast::{TypeParamList, AstNode, NameOwner};
+
+use crate::{db::HirDatabase, DefId, Name, AsName};
+
+/// Data about a generic parameter (to a function, struct, impl, ...).
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub struct GenericParam {
+    pub(crate) idx: u32,
+    pub(crate) name: Name,
+}
+
+/// Data about the generic parameters of a function, struct, impl, etc.
+#[derive(Clone, PartialEq, Eq, Debug, Default)]
+pub struct Generics {
+    pub(crate) params: Vec<GenericParam>,
+}
+
+impl Generics {
+    pub(crate) fn generics_query(db: &impl HirDatabase, def_id: DefId) -> Arc<Generics> {
+        let (_file_id, node) = def_id.source(db);
+        let mut generics = Generics::default();
+        if let Some(type_param_list) = node.children().find_map(TypeParamList::cast) {
+            for (idx, type_param) in type_param_list.type_params().enumerate() {
+                let name = type_param
+                    .name()
+                    .map(AsName::as_name)
+                    .unwrap_or_else(Name::missing);
+                let param = GenericParam {
+                    idx: idx as u32,
+                    name,
+                };
+                generics.params.push(param);
+            }
+        }
+        Arc::new(generics)
+    }
+
+    pub(crate) fn find_by_name(&self, name: &Name) -> Option<&GenericParam> {
+        self.params.iter().find(|p| &p.name == name)
+    }
+}