Merge #3108

3108: Magic Completion for `impl Trait for` Associated Items r=matklad a=kdelorey

# Summary
This PR adds a set of magic completions to auto complete associated trait items (functions/consts/types). 

![Associated Trait Impl](https://user-images.githubusercontent.com/2295721/74493144-d8f1af00-4e96-11ea-93a4-82725bf89646.gif)

## Notes
Since the assist and completion share the same logic when figuring out the associated items that are missing, a shared utility was created in the `ra_assists::utils` module.

Resolves #1046 

As this is my first PR to the rust-analyzer project, I'm new to the codebase, feedback welcomed!

Co-authored-by: Kevin DeLorey <[email protected]>

3 files changed, 448 insertions, 0 deletions

diff --git a/crates/ra_ide/src/completion.rs b/crates/ra_ide/src/completion.rs
index fedc02e14..4bdc6ba23 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_trait_impl;
 
 use ra_db::SourceDatabase;
 use ra_ide_db::RootDatabase;
@@ -74,5 +75,7 @@ pub(crate) fn completions(db: &RootDatabase, position: FilePosition) -> Option<C
     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_trait_impl::complete_trait_impl(&mut acc, &ctx);
+
     Some(acc)
 }
diff --git a/crates/ra_ide/src/completion/complete_trait_impl.rs b/crates/ra_ide/src/completion/complete_trait_impl.rs
new file mode 100644
index 000000000..6ff10c017
--- /dev/null
+++ b/crates/ra_ide/src/completion/complete_trait_impl.rs
@@ -0,0 +1,436 @@
+//! Completion for associated items in a trait implementation.
+//!
+//! This module adds the completion items related to implementing associated
+//! items within a `impl Trait for Struct` block. The current context node
+//! must be within either a `FN_DEF`, `TYPE_ALIAS_DEF`, or `CONST_DEF` node
+//! and an direct child of an `IMPL_BLOCK`.
+//!
+//! # Examples
+//!
+//! Considering the following trait `impl`:
+//!
+//! ```ignore
+//! trait SomeTrait {
+//!     fn foo();
+//! }
+//!
+//! impl SomeTrait for () {
+//!     fn f<|>
+//! }
+//! ```
+//!
+//! may result in the completion of the following method:
+//!
+//! ```ignore
+//! # trait SomeTrait {
+//! #    fn foo();
+//! # }
+//!
+//! impl SomeTrait for () {
+//!     fn foo() {}<|>
+//! }
+//! ```
+
+use hir::{self, Docs, HasSource};
+use ra_assists::utils::get_missing_impl_items;
+use ra_syntax::{
+    ast::{self, edit},
+    AstNode, SyntaxKind, SyntaxNode, TextRange,
+};
+use ra_text_edit::TextEdit;
+
+use crate::{
+    completion::{
+        CompletionContext, CompletionItem, CompletionItemKind, CompletionKind, Completions,
+    },
+    display::FunctionSignature,
+};
+
+pub(crate) fn complete_trait_impl(acc: &mut Completions, ctx: &CompletionContext) {
+    let trigger = ctx.token.ancestors().find(|p| match p.kind() {
+        SyntaxKind::FN_DEF
+        | SyntaxKind::TYPE_ALIAS_DEF
+        | SyntaxKind::CONST_DEF
+        | SyntaxKind::BLOCK_EXPR => true,
+        _ => false,
+    });
+
+    let impl_block = trigger
+        .as_ref()
+        .and_then(|node| node.parent())
+        .and_then(|node| node.parent())
+        .and_then(|node| ast::ImplBlock::cast(node));
+
+    if let (Some(trigger), Some(impl_block)) = (trigger, impl_block) {
+        match trigger.kind() {
+            SyntaxKind::FN_DEF => {
+                for missing_fn in get_missing_impl_items(ctx.db, &ctx.analyzer, &impl_block)
+                    .iter()
+                    .filter_map(|item| match item {
+                        hir::AssocItem::Function(fn_item) => Some(fn_item),
+                        _ => None,
+                    })
+                {
+                    add_function_impl(&trigger, acc, ctx, &missing_fn);
+                }
+            }
+
+            SyntaxKind::TYPE_ALIAS_DEF => {
+                for missing_fn in get_missing_impl_items(ctx.db, &ctx.analyzer, &impl_block)
+                    .iter()
+                    .filter_map(|item| match item {
+                        hir::AssocItem::TypeAlias(type_item) => Some(type_item),
+                        _ => None,
+                    })
+                {
+                    add_type_alias_impl(&trigger, acc, ctx, &missing_fn);
+                }
+            }
+
+            SyntaxKind::CONST_DEF => {
+                for missing_fn in get_missing_impl_items(ctx.db, &ctx.analyzer, &impl_block)
+                    .iter()
+                    .filter_map(|item| match item {
+                        hir::AssocItem::Const(const_item) => Some(const_item),
+                        _ => None,
+                    })
+                {
+                    add_const_impl(&trigger, acc, ctx, &missing_fn);
+                }
+            }
+
+            _ => {}
+        }
+    }
+}
+
+fn add_function_impl(
+    fn_def_node: &SyntaxNode,
+    acc: &mut Completions,
+    ctx: &CompletionContext,
+    func: &hir::Function,
+) {
+    let display = FunctionSignature::from_hir(ctx.db, func.clone());
+
+    let fn_name = func.name(ctx.db).to_string();
+
+    let label = if func.params(ctx.db).len() > 0 {
+        format!("fn {}(..)", fn_name)
+    } else {
+        format!("fn {}()", fn_name)
+    };
+
+    let builder = CompletionItem::new(CompletionKind::Magic, ctx.source_range(), label.clone())
+        .lookup_by(fn_name)
+        .set_documentation(func.docs(ctx.db));
+
+    let completion_kind = if func.has_self_param(ctx.db) {
+        CompletionItemKind::Method
+    } else {
+        CompletionItemKind::Function
+    };
+
+    let snippet = format!("{} {{}}", display);
+
+    let range = TextRange::from_to(fn_def_node.text_range().start(), ctx.source_range().end());
+
+    builder.text_edit(TextEdit::replace(range, snippet)).kind(completion_kind).add_to(acc);
+}
+
+fn add_type_alias_impl(
+    type_def_node: &SyntaxNode,
+    acc: &mut Completions,
+    ctx: &CompletionContext,
+    type_alias: &hir::TypeAlias,
+) {
+    let alias_name = type_alias.name(ctx.db).to_string();
+
+    let snippet = format!("type {} = ", alias_name);
+
+    let range = TextRange::from_to(type_def_node.text_range().start(), ctx.source_range().end());
+
+    CompletionItem::new(CompletionKind::Magic, ctx.source_range(), snippet.clone())
+        .text_edit(TextEdit::replace(range, snippet))
+        .lookup_by(alias_name)
+        .kind(CompletionItemKind::TypeAlias)
+        .set_documentation(type_alias.docs(ctx.db))
+        .add_to(acc);
+}
+
+fn add_const_impl(
+    const_def_node: &SyntaxNode,
+    acc: &mut Completions,
+    ctx: &CompletionContext,
+    const_: &hir::Const,
+) {
+    let const_name = const_.name(ctx.db).map(|n| n.to_string());
+
+    if let Some(const_name) = const_name {
+        let snippet = make_const_compl_syntax(&const_.source(ctx.db).value);
+
+        let range =
+            TextRange::from_to(const_def_node.text_range().start(), ctx.source_range().end());
+
+        CompletionItem::new(CompletionKind::Magic, ctx.source_range(), snippet.clone())
+            .text_edit(TextEdit::replace(range, snippet))
+            .lookup_by(const_name)
+            .kind(CompletionItemKind::Const)
+            .set_documentation(const_.docs(ctx.db))
+            .add_to(acc);
+    }
+}
+
+fn make_const_compl_syntax(const_: &ast::ConstDef) -> String {
+    let const_ = edit::strip_attrs_and_docs(const_);
+
+    let const_start = const_.syntax().text_range().start();
+    let const_end = const_.syntax().text_range().end();
+
+    let start =
+        const_.syntax().first_child_or_token().map_or(const_start, |f| f.text_range().start());
+
+    let end = const_
+        .syntax()
+        .children_with_tokens()
+        .find(|s| s.kind() == SyntaxKind::SEMI || s.kind() == SyntaxKind::EQ)
+        .map_or(const_end, |f| f.text_range().start());
+
+    let len = end - start;
+    let range = TextRange::from_to(0.into(), len);
+
+    let syntax = const_.syntax().text().slice(range).to_string();
+
+    format!("{} = ", syntax.trim_end())
+}
+
+#[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::Magic)
+    }
+
+    #[test]
+    fn single_function() {
+        let completions = complete(
+            r"
+            trait Test {
+                fn foo();
+            }
+
+            struct T1;
+
+            impl Test for T1 {
+                fn f<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "fn foo()",
+                source_range: [141; 142),
+                delete: [138; 142),
+                insert: "fn foo() {}",
+                kind: Function,
+                lookup: "foo",
+            },
+        ]
+        "###);
+    }
+
+    #[test]
+    fn hide_implemented_fn() {
+        let completions = complete(
+            r"
+            trait Test {
+                fn foo();
+                fn foo_bar();
+            }
+
+            struct T1;
+
+            impl Test for T1 {
+                fn foo() {}
+
+                fn f<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "fn foo_bar()",
+                source_range: [200; 201),
+                delete: [197; 201),
+                insert: "fn foo_bar() {}",
+                kind: Function,
+                lookup: "foo_bar",
+            },
+        ]
+        "###);
+    }
+
+    #[test]
+    fn completes_only_on_top_level() {
+        let completions = complete(
+            r"
+            trait Test {
+                fn foo();
+
+                fn foo_bar();
+            }
+
+            struct T1;
+
+            impl Test for T1 {
+                fn foo() {
+                    <|>
+                }
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"[]"###);
+    }
+
+    #[test]
+    fn generic_fn() {
+        let completions = complete(
+            r"
+            trait Test {
+                fn foo<T>();
+            }
+
+            struct T1;
+
+            impl Test for T1 {
+                fn f<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "fn foo()",
+                source_range: [144; 145),
+                delete: [141; 145),
+                insert: "fn foo<T>() {}",
+                kind: Function,
+                lookup: "foo",
+            },
+        ]
+        "###);
+    }
+
+    #[test]
+    fn generic_constrait_fn() {
+        let completions = complete(
+            r"
+            trait Test {
+                fn foo<T>() where T: Into<String>;
+            }
+
+            struct T1;
+
+            impl Test for T1 {
+                fn f<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "fn foo()",
+                source_range: [166; 167),
+                delete: [163; 167),
+                insert: "fn foo<T>()\nwhere T: Into<String> {}",
+                kind: Function,
+                lookup: "foo",
+            },
+        ]
+        "###);
+    }
+
+    #[test]
+    fn associated_type() {
+        let completions = complete(
+            r"
+            trait Test {
+                type SomeType;
+            }
+
+            impl Test for () {
+                type S<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "type SomeType = ",
+                source_range: [124; 125),
+                delete: [119; 125),
+                insert: "type SomeType = ",
+                kind: TypeAlias,
+                lookup: "SomeType",
+            },
+        ]
+        "###);
+    }
+
+    #[test]
+    fn associated_const() {
+        let completions = complete(
+            r"
+            trait Test {
+                const SOME_CONST: u16;
+            }
+
+            impl Test for () {
+                const S<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "const SOME_CONST: u16 = ",
+                source_range: [133; 134),
+                delete: [127; 134),
+                insert: "const SOME_CONST: u16 = ",
+                kind: Const,
+                lookup: "SOME_CONST",
+            },
+        ]
+        "###);
+    }
+
+    #[test]
+    fn associated_const_with_default() {
+        let completions = complete(
+            r"
+            trait Test {
+                const SOME_CONST: u16 = 42;
+            }
+
+            impl Test for () {
+                const S<|>
+            }
+            ",
+        );
+        assert_debug_snapshot!(completions, @r###"
+        [
+            CompletionItem {
+                label: "const SOME_CONST: u16 = ",
+                source_range: [138; 139),
+                delete: [132; 139),
+                insert: "const SOME_CONST: u16 = ",
+                kind: Const,
+                lookup: "SOME_CONST",
+            },
+        ]
+        "###);
+    }
+}
diff --git a/crates/ra_ide/src/completion/completion_context.rs b/crates/ra_ide/src/completion/completion_context.rs
index 5a0407fd7..8678a3234 100644
--- a/crates/ra_ide/src/completion/completion_context.rs
+++ b/crates/ra_ide/src/completion/completion_context.rs
@@ -25,6 +25,7 @@ pub(crate) struct CompletionContext<'a> {
     pub(super) use_item_syntax: Option<ast::UseItem>,
     pub(super) record_lit_syntax: Option<ast::RecordLit>,
     pub(super) record_lit_pat: Option<ast::RecordPat>,
+    pub(super) impl_block: Option<ast::ImplBlock>,
     pub(super) is_param: bool,
     /// If a name-binding or reference to a const in a pattern.
     /// Irrefutable patterns (like let) are excluded.
@@ -72,6 +73,7 @@ impl<'a> CompletionContext<'a> {
             use_item_syntax: None,
             record_lit_syntax: None,
             record_lit_pat: None,
+            impl_block: None,
             is_param: false,
             is_pat_binding: false,
             is_trivial_path: false,
@@ -148,6 +150,13 @@ impl<'a> CompletionContext<'a> {
             self.record_lit_syntax = find_node_at_offset(original_file.syntax(), self.offset);
         }
 
+        self.impl_block = self
+            .token
+            .parent()
+            .ancestors()
+            .take_while(|it| it.kind() != SOURCE_FILE && it.kind() != MODULE)
+            .find_map(ast::ImplBlock::cast);
+
         let top_node = name_ref
             .syntax()
             .ancestors()