From e3cda83f90a5e64ccf288438466e6c447df253a8 Mon Sep 17 00:00:00 2001
From: Kevin DeLorey <2295721+kdelorey@users.noreply.github.com>
Date: Wed, 22 Jan 2020 22:25:41 -0600
Subject: Added basic support for completing `fn` for trait impl.

---
 crates/ra_ide/src/completion.rs                  |  3 +
 crates/ra_ide/src/completion/complete_impl_fn.rs | 80 ++++++++++++++++++++++++
 crates/ra_ide/src/completion/presentation.rs     | 33 ++++++++++
 3 files changed, 116 insertions(+)
 create mode 100644 crates/ra_ide/src/completion/complete_impl_fn.rs

diff --git a/crates/ra_ide/src/completion.rs b/crates/ra_ide/src/completion.rs
index abe1f36ce..f31f2593c 100644
--- a/crates/ra_ide/src/completion.rs
+++ b/crates/ra_ide/src/completion.rs
@@ -15,6 +15,7 @@ mod complete_path;
 mod complete_scope;
 mod complete_postfix;
 mod complete_macro_in_item_position;
+mod complete_impl_fn;
 
 use ra_db::SourceDatabase;
 
@@ -73,5 +74,7 @@ pub(crate) fn completions(db: &db::RootDatabase, position: FilePosition) -> Opti
     complete_pattern::complete_pattern(&mut acc, &ctx);
     complete_postfix::complete_postfix(&mut acc, &ctx);
     complete_macro_in_item_position::complete_macro_in_item_position(&mut acc, &ctx);
+    complete_impl_fn::complete_impl_fn(&mut acc, &ctx);
+    
     Some(acc)
 }
diff --git a/crates/ra_ide/src/completion/complete_impl_fn.rs b/crates/ra_ide/src/completion/complete_impl_fn.rs
new file mode 100644
index 000000000..6d464cc1f
--- /dev/null
+++ b/crates/ra_ide/src/completion/complete_impl_fn.rs
@@ -0,0 +1,80 @@
+
+use crate::completion::{CompletionContext, Completions};
+
+use hir::{ self, db::HirDatabase, HasSource };
+
+use ra_syntax::{ ast, ast::AstNode };
+
+pub(crate) fn complete_impl_fn(acc: &mut Completions, ctx: &CompletionContext) {
+    let impl_trait = ast::ItemList::cast(ctx.token.parent())
+        .and_then(|item_list| item_list.syntax().parent())
+        .and_then(|item_list_parent| ast::ImplBlock::cast(item_list_parent))
+        .and_then(|impl_block| resolve_target_trait(ctx.db, &ctx.analyzer, &impl_block));
+
+    if let Some(x) = &impl_trait {
+        for trait_item in x.0.items(ctx.db) {
+            match trait_item {
+                hir::AssocItem::Function(f) => acc.add_function_impl(ctx, f),
+                _ => {}
+            }
+        }
+    }
+}
+
+fn resolve_target_trait(
+    db: &impl HirDatabase,
+    analyzer: &hir::SourceAnalyzer,
+    impl_block: &ast::ImplBlock
+) -> Option<(hir::Trait, ast::TraitDef)> {
+    let ast_path = impl_block
+        .target_trait()
+        .map(|it| it.syntax().clone())
+        .and_then(ast::PathType::cast)?
+        .path()?;
+
+    match analyzer.resolve_path(db, &ast_path) {
+        Some(hir::PathResolution::Def(hir::ModuleDef::Trait(def))) => {
+            Some((def, def.source(db).value))
+        }
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::completion::{do_completion, CompletionItem, CompletionKind};
+    use insta::assert_debug_snapshot;
+
+    fn complete(code: &str) -> Vec<CompletionItem> {
+        do_completion(code, CompletionKind::Reference)
+    }
+
+    #[test]
+    fn single_function() {
+        let completions = complete(
+            r"
+            trait Test {
+                fn foo();
+            }
+
+            struct T1;
+
+            impl Test for T1 {
+                <|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "fn foo()",
+                source_range: [138; 138),
+                delete: [138; 138),
+                insert: "fn foo() { $0 }",
+                kind: Function,
+                lookup: "foo",
+            },
+        ]
+        "###);
+    }
+}
\ No newline at end of file
diff --git a/crates/ra_ide/src/completion/presentation.rs b/crates/ra_ide/src/completion/presentation.rs
index 97475fc0b..d0a43261f 100644
--- a/crates/ra_ide/src/completion/presentation.rs
+++ b/crates/ra_ide/src/completion/presentation.rs
@@ -129,6 +129,39 @@ impl Completions {
         self.add_function_with_name(ctx, None, func)
     }
 
+    pub(crate) fn add_function_impl(&mut self, ctx: &CompletionContext, func: hir::Function) {
+        use crate::display::FunctionSignature;
+
+        let display = FunctionSignature::from_hir(ctx.db, func.clone());
+
+        let func_name = func.name(ctx.db);
+
+        let mut builder = CompletionItem::new(
+            CompletionKind::Reference, 
+            ctx.source_range(), 
+            format!("fn {}()", func_name.to_string()))
+            .set_documentation(func.docs(ctx.db));
+
+        let completion_kind = if func.has_self_param(ctx.db) {
+            CompletionItemKind::Method
+        } else {
+            CompletionItemKind::Function
+        };
+        
+        let snippet = {
+            let mut s = format!("{}", display);
+            s.push_str(" { $0 }");
+            s
+        };
+
+        builder = builder
+            .insert_text(snippet)
+            .kind(completion_kind)
+            .lookup_by(func_name.to_string());
+
+        self.add(builder.build());
+    }
+
     fn guess_macro_braces(&self, macro_name: &str, docs: &str) -> &'static str {
         let mut votes = [0, 0, 0];
         for (idx, s) in docs.match_indices(&macro_name) {
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