feat: goto definition on an impl fn goes to that fn in the trait

e.g. if you have a trait T and `impl T for S` for some struct, if you
goto definition on some function name inside the impl, it will go to the
definition of that function inside the `trait T` block, rather than the
current behaviour of not going anywhere at all.

crates/ide/src/goto_definition.rs

@@ -1,10 +1,10 @@
 use std::convert::TryInto;
 
 use either::Either;
-use hir::{InFile, Semantics};
+use hir::{AsAssocItem, InFile, ModuleDef, Semantics};
 use ide_db::{
     base_db::{AnchoredPath, FileId, FileLoader},
-    defs::{NameClass, NameRefClass},
+    defs::{Definition, NameClass, NameRefClass},
     RootDatabase,
 };
 use syntax::{
@@ -57,7 +57,8 @@ pub(crate) fn goto_definition(
             },
             ast::Name(name) => {
                 let def = NameClass::classify(&sema, &name)?.referenced_or_defined(sema.db);
-                def.try_to_nav(sema.db)
+                try_find_trait_fn_definition(&sema.db, &def)
+                    .or_else(|| def.try_to_nav(sema.db))
             },
             ast::Lifetime(lt) => if let Some(name_class) = NameClass::classify_lifetime(&sema, &lt) {
                 let def = name_class.referenced_or_defined(sema.db);
@@ -99,6 +100,32 @@ fn try_lookup_include_path(
     })
 }
 
+/// finds the trait definition of an impl'd function
+/// e.g.
+/// ```rust
+/// trait A { fn a(); }
+/// struct S;
+/// impl A for S { fn a(); } // <-- on this function, will get the location of a() in the trait
+/// ```
+fn try_find_trait_fn_definition(db: &RootDatabase, def: &Definition) -> Option<NavigationTarget> {
+    match def {
+        Definition::ModuleDef(ModuleDef::Function(f)) => {
+            let name = def.name(db)?;
+            let assoc = f.as_assoc_item(db)?;
+            let imp = match assoc.container(db) {
+                hir::AssocItemContainer::Impl(imp) => imp,
+                _ => return None,
+            };
+            let trait_ = imp.trait_(db)?;
+            trait_
+                .items(db)
+                .iter()
+                .find_map(|itm| (itm.name(db)? == name).then(|| itm.try_to_nav(db)).flatten())
+        }
+        _ => None,
+    }
+}
+
 fn pick_best(tokens: TokenAtOffset<SyntaxToken>) -> Option<SyntaxToken> {
     return tokens.max_by_key(priority);
     fn priority(n: &SyntaxToken) -> usize {
@@ -1259,6 +1286,24 @@ fn main() {
 //- /foo.txt
 // empty
 //^ file
+"#,
+        );
+    }
+
+    #[test]
+    fn goto_def_of_trait_impl_fn() {
+        check(
+            r#"
+trait Twait {
+    fn a();
+    // ^
+}
+
+struct Stwuct;
+
+impl Twait for Stwuct {
+    fn a$0();
+}
 "#,
         );
     }