13 files changed, 467 insertions, 145 deletions

diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
new file mode 100644
index 000000000..b497cc5d7
--- /dev/null
+++ b/ARCHITECTURE.md
@@ -0,0 +1,133 @@
+# Architecture
+
+This document describes high-level architecture of rust-analyzer.
+If you want to familiarize yourself with the code base, you are just
+in the right place!
+
+
+## Code generation
+
+Some of the components of this repository are generated through automatic
+processes. These are outlined below:
+
+- `gen-kinds`: The kinds of tokens are reused in several places, so a generator
+  is used. We use tera templates to generate the files listed below, based on
+  the grammar described in [grammar.ron]:
+  - [ast/generated.rs][ast generated] in `ra_syntax` based on
+    [ast/generated.tera.rs][ast source]
+  - [syntax_kinds/generated.rs][syntax_kinds generated] in `ra_syntax` based on
+    [syntax_kinds/generated.tera.rs][syntax_kinds source]
+
+[tera]: https://tera.netlify.com/
+[grammar.ron]: ./crates/ra_syntax/src/grammar.ron
+[ast generated]: ./crates/ra_syntax/src/ast/generated.rs
+[ast source]: ./crates/ra_syntax/src/ast/generated.rs.tera
+[syntax_kinds generated]: ./crates/ra_syntax/src/syntax_kinds/generated.rs
+[syntax_kinds source]: ./crates/ra_syntax/src/syntax_kinds/generated.rs.tera
+
+
+## Code Walk-Through
+
+### `crates/ra_syntax`
+
+Rust syntax tree structure and parser. See
+[RFC](https://github.com/rust-lang/rfcs/pull/2256) for some design
+notes.
+
+- [rowan](https://github.com/rust-analyzer/rowan) library is used for constructing syntax trees.
+- `grammar` module is the actual parser. It is a hand-written recursive descent parsers, which
+  produces a sequence of events like "start node X", "finish not Y". It works similarly to  [kotlin parser](https://github.com/JetBrains/kotlin/blob/4d951de616b20feca92f3e9cc9679b2de9e65195/compiler/frontend/src/org/jetbrains/kotlin/parsing/KotlinParsing.java),
+  which is a good source for inspiration for dealing with syntax errors and incomplete input. Original [libsyntax parser](https://github.com/rust-lang/rust/blob/6b99adeb11313197f409b4f7c4083c2ceca8a4fe/src/libsyntax/parse/parser.rs)
+  is what we use for the definition of the Rust language.
+- `parser_api/parser_impl` bridges the tree-agnostic parser from `grammar` with `rowan` trees.
+  This is the thing that turns a flat list of events into a tree (see `EventProcessor`)
+- `ast` a type safe API on top of the raw `rowan` tree.
+- `grammar.ron` RON description of the grammar, which is used to
+  generate `syntax_kinds` and `ast` modules, using `cargo gen-kinds` command.
+- `algo`: generic tree algorithms, including `walk` for O(1) stack
+  space tree traversal (this is cool) and `visit` for type-driven
+  visiting the nodes (this is double plus cool, if you understand how
+  `Visitor` works, you understand rust-analyzer).
+
+Test for ra_syntax are mostly data-driven: `tests/data/parser` contains a bunch of `.rs`
+(test vectors) and `.txt` files with corresponding syntax trees. During testing, we check
+`.rs` against `.txt`. If the `.txt` file is missing, it is created (this is how you update
+tests). Additionally, running `cargo gen-tests` will walk the grammar module and collect
+all `//test test_name` comments into files inside `tests/data` directory.
+
+See [#93](https://github.com/rust-analyzer/rust-analyzer/pull/93) for an example PR which
+fixes a bug in the grammar.
+
+
+### `crates/ra_editor`
+
+All IDE features which can be implemented if you only have access to a
+single file. `ra_editor` could be used to enhance editing of Rust code
+without the need to fiddle with build-systems, file
+synchronization and such.
+
+In a sense, `ra_editor` is just a bunch of pure functions which take a
+syntax tree as an input.
+
+The tests for `ra_editor` are `#[cfg(test)] mod tests` unit-tests spread
+throughout its modules.
+
+### `crates/salsa`
+
+An implementation of red-green incremental compilation algorithm from
+rust compiler. It makes all rust-analyzer features on-demand. To be replaced
+with `salsa-rs/salsa` soon.
+
+
+### `crates/ra_analysis`
+
+A stateful library for analyzing many Rust files as they change.
+`AnalysisHost` is a mutable entity (clojure's atom) which holds
+current state, incorporates changes and handles out `Analysis` --- an
+immutable consistent snapshot of world state at a point in time, which
+actually powers analysis.
+
+
+### `crates/ra_lsp_server`
+
+An LSP implementation which uses `ra_analysis` for managing state and
+`ra_editor` for actually doing useful stuff.
+
+See [#79](https://github.com/rust-analyzer/rust-analyzer/pull/79/) as an
+example of PR which adds a new feature to `ra_editor` and exposes it
+to `ra_lsp_server`.
+
+
+### `crates/cli`
+
+A CLI interface to rust-analyzer.
+
+### `crate/tools`
+
+Custom Cargo tasks used to develop rust-analyzer:
+
+- `cargo gen-kinds` -- generate `ast` and `syntax_kinds`
+- `cargo gen-tests` -- collect inline tests from grammar
+- `cargo install-code` -- build and install VS Code extension and server
+
+### `editors/code`
+
+VS Code plugin
+
+
+## Common workflows
+
+To try out VS Code extensions, run `cargo install-code`.  This installs both the
+`ra_lsp_server` binary and VS Code extension. To install only the binary, `use
+cargo install --path crates/ra_lsp_server --force`
+
+To see logs from the language server, set `RUST_LOG=info` env variable. To see
+all communication between the server and the client, use
+`RUST_LOG=gen_lsp_server=debug` (will print quite a bit of stuff).
+
+To run tests, just `cargo test`.
+
+To work on VS Code extension, launch code inside `editors/code` and use `F5` to
+launch/debug. To automatically apply formatter and linter suggestions, use `npm
+run fix`.
+
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index c952078cf..a2efc7afa 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,58 +1,18 @@
 The project is in its early stages: contributions are welcome and would be
 **very** helpful, but the project is not _yet_ optimized for contribution.
 Moreover, it is doubly experimental, so there's no guarantee that any work here
-would reach production. That said, here are some areas where contributions would
-be **especially** welcome:
+would reach production.
 
-- Designing internal data structures: RFC only outlines the constraints, it's an
-  open question how to satisfy them in the optimal way. See `ARCHITECTURE.md`
-  for current design questions.
+To get an idea of how rust-analyzer works, take a look at the [ARCHITECTURE.md](./ARCHITECTURE.md)
+document.
 
-- Porting libsyntax parser to rust-analyzer: currently rust-analyzer parses only
-  a tiny subset of Rust. This should be fixed by porting parsing functions from
-  libsyntax one by one. Take a look at the [libsyntax parser] for "what to port"
-  and at the [Kotlin parser] for "how to port".
+Useful labels on the issue tracker:
+  * [E-mentor](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-mentor)
+    issues have links to the code in question and tests,
+  * [E-easy](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-easy),
+    [E-medium](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-medium),
+    [E-hard](https://github.com/rust-analyzer/rust-analyzer/issues?q=is%3Aopen+is%3Aissue+label%3AE-hard),
+    labels are *estimates* for how hard would be to write a fix.
 
-- Writing validators: by design, rust-analyzer is very lax about the input. For
-  example, the lexer happily accepts unclosed strings. The idea is that there
-  should be a higher level visitor, which walks the syntax tree after parsing
-  and produces all the warnings. Alas, there's no such visitor yet :( Would you
-  like to write one? :)
-
-- Creating tests: it would be tremendously helpful to read each of libsyntax and
-  rust-analyzer parser functions and crate a small separate test cases to cover
-  each and every edge case.
-
-- Building stuff with rust-analyzer: it would be really cool to compile
-  rust-analyzer to WASM and add _client side_ syntax validation to rust
-  playground!
-
-Do take a look at the issue tracker.
-
-If you don't know where to start, or have _any_ questions or suggestions, don't
-hesitate to chat at [Gitter]!
-
-# Code generation
-
-Some of the components of this repository are generated through automatic
-processes. These are outlined below:
-
-- `gen-kinds`: The kinds of tokens are reused in several places, so a generator
-  is used. This process uses [tera] to generate, using data in [grammar.ron],
-  the files:
-  - [ast/generated.rs][ast generated] in `ra_syntax` based on
-    [ast/generated.tera.rs][ast source]
-  - [syntax_kinds/generated.rs][syntax_kinds generated] in `ra_syntax` based on
-    [syntax_kinds/generated.tera.rs][syntax_kinds source]
-
-[libsyntax parser]:
-  https://github.com/rust-lang/rust/blob/6b99adeb11313197f409b4f7c4083c2ceca8a4fe/src/libsyntax/parse/parser.rs
-[kotlin parser]:
-  https://github.com/JetBrains/kotlin/blob/4d951de616b20feca92f3e9cc9679b2de9e65195/compiler/frontend/src/org/jetbrains/kotlin/parsing/KotlinParsing.java
-[gitter]: https://gitter.im/libsyntax2/Lobby
-[tera]: https://tera.netlify.com/
-[grammar.ron]: ./crates/ra_syntax/src/grammar.ron
-[ast generated]: ./crates/ra_syntax/src/ast/generated.rs
-[ast source]: ./crates/ra_syntax/src/ast/generated.tera.rs
-[syntax_kinds generated]: ./crates/ra_syntax/src/syntax_kinds/generated.rs
-[syntax_kinds source]: ./crates/ra_syntax/src/syntax_kinds/generated.tera.rs
+There's no formal PR check list: everything that passes CI (we use [bors](https://bors.tech/)) is valid,
+but it's a good idea to write nice commit messages, test code thoroughly, maintain consistent style, etc.
diff --git a/README.md b/README.md
index 481d065b0..1c6facbbd 100644
--- a/README.md
+++ b/README.md
@@ -61,99 +61,20 @@ fold:
 * to quickly bootstrap usable and useful language server: solution
   that covers 80% of Rust code will be useful for IDEs, and will be
   vastly simpler than 100% solution.
-  
+
 * to understand how the consumer-side of compiler API should look like
   (especially it's on-demand aspects). If you have
   `get_expression_type` function, you can write a ton of purely-IDE
   features on top of it, even if the function is only partially
   correct. Plugin in the precise function afterwards should just make
   IDE features more reliable.
-  
+
 The long term plan is to merge with the mainline rustc compiler,
 probably around the HIR boundary? That is, use rust analyzer for
 parsing, macro expansion and related bits of name resolution, but
 leave the rest (including type inference and trait selection) to the
 existing rustc.
 
-## Code Walk-Through
-
-### `crates/ra_syntax`
-
-Rust syntax tree structure and parser. See
-[RFC](https://github.com/rust-lang/rfcs/pull/2256) for some design
-notes.
-
-- `yellow`, red/green syntax tree, heavily inspired [by this](https://github.com/apple/swift/tree/ab68f0d4cbf99cdfa672f8ffe18e433fddc8b371/lib/Syntax)
-- `grammar`, the actual parser
-- `parser_api/parser_impl` bridges the tree-agnostic parser from `grammar` with `yellow` trees
-- `grammar.ron` RON description of the grammar, which is used to
-  generate `syntax_kinds` and `ast` modules.
-- `algo`: generic tree algorithms, including `walk` for O(1) stack
-  space tree traversal (this is cool) and `visit` for type-driven
-  visiting the nodes (this is double plus cool, if you understand how
-  `Visitor` works, you understand rust-analyzer).
-
-
-### `crates/ra_editor`
-
-All IDE features which can be implemented if you only have access to a
-single file. `ra_editor` could be used to enhance editing of Rust code
-without the need to fiddle with build-systems, file
-synchronization and such.
-
-In a sense, `ra_editor` is just a bunch of pure functions which take a
-syntax tree as an input.
-
-### `crates/salsa`
-
-An implementation of red-green incremental compilation algorithm from
-rust compiler. It makes all rust-analyzer features on-demand.
-
-
-### `crates/ra_analysis`
-
-A stateful library for analyzing many Rust files as they change.
-`AnalysisHost` is a mutable entity (clojure's atom) which holds
-current state, incorporates changes and handles out `Analysis` --- an
-immutable consistent snapshot of world state at a point in time, which
-actually powers analysis.
-
-
-### `crates/ra_lsp_server`
-
-An LSP implementation which uses `ra_analysis` for managing state and
-`ra_editor` for actually doing useful stuff.
-
-
-### `crates/cli`
-
-A CLI interface to libsyntax
-
-### `crate/tools`
-
-Code-gen tasks, used to develop rust-analyzer:
-
-- `cargo gen-kinds` -- generate `ast` and `syntax_kinds`
-- `cargo gen-tests` -- collect inline tests from grammar
-- `cargo install-code` -- build and install VS Code extension and server
-
-### `editors/code`
-
-VS Code plugin
-
-
-## Performance
-
-Non-incremental, but seems pretty fast:
-
-```
-$ cargo build --release --package ra_cli
-$ wc -l ~/projects/rust/src/libsyntax/parse/parser.rs
-7546 /home/matklad/projects/rust/src/libsyntax/parse/parser.rs
-$ ./target/release/ra_cli parse < ~/projects/rust/src/libsyntax/parse/parser.rs --no-dump  > /dev/null
-parsing: 21.067065ms
-```
-
 ## Getting in touch
 
 @matklad can be found at Rust
@@ -161,6 +82,11 @@ parsing: 21.067065ms
 #ides-and-editors.
 
 
+## Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md) and [ARCHITECTURE.md](./ARCHITECTURE.md)
+
+
 ## License
 
 Rust analyzer is primarily distributed under the terms of both the MIT
diff --git a/crates/ra_analysis/src/descriptors.rs b/crates/ra_analysis/src/descriptors.rs
index 0731b5572..faf945a41 100644
--- a/crates/ra_analysis/src/descriptors.rs
+++ b/crates/ra_analysis/src/descriptors.rs
@@ -4,7 +4,8 @@ use std::{
 use relative_path::RelativePathBuf;
 use ra_syntax::{
     SmolStr,
-    ast::{self, NameOwner},
+    ast::{self, NameOwner, AstNode},
+    text_utils::is_subrange
 };
 use {
     FileId,
@@ -218,3 +219,56 @@ fn resolve_submodule(
     }
     (points_to, problem)
 }
+
+#[derive(Debug, Clone)]
+pub struct FnDescriptor {
+    pub name: String,
+    pub label : String,
+    pub ret_type: Option<String>,
+    pub params: Vec<String>,
+}
+
+impl FnDescriptor {
+    pub fn new(node: ast::FnDef) -> Option<Self> {
+        let name = node.name()?.text().to_string();
+
+        // Strip the body out for the label.
+        let label : String = if let Some(body) = node.body() {
+            let body_range = body.syntax().range();
+            let label : String = node.syntax().children()
+                .filter(|child| !is_subrange(body_range, child.range()))
+                .map(|node| node.text().to_string())
+                .collect();
+            label
+        } else {
+            node.syntax().text().to_string()
+        };
+
+        let params = FnDescriptor::param_list(node);
+        let ret_type = node.ret_type().map(|r| r.syntax().text().to_string());
+
+        Some(FnDescriptor {
+            name,
+            ret_type,
+            params,
+            label
+        })
+    }
+
+    fn param_list(node: ast::FnDef) -> Vec<String> {
+        let mut res = vec![];
+        if let Some(param_list) = node.param_list() {
+            if let Some(self_param) = param_list.self_param() {
+                res.push(self_param.syntax().text().to_string())
+            }
+
+            // Maybe use param.pat here? See if we can just extract the name?
+            //res.extend(param_list.params().map(|p| p.syntax().text().to_string()));
+            res.extend(param_list.params()
+                .filter_map(|p| p.pat())
+                .map(|pat| pat.syntax().text().to_string())
+            );
+        }
+        res
+    }
+}
\ No newline at end of file
diff --git a/crates/ra_analysis/src/imp.rs b/crates/ra_analysis/src/imp.rs
index 47bc0032b..aad54b977 100644
--- a/crates/ra_analysis/src/imp.rs
+++ b/crates/ra_analysis/src/imp.rs
@@ -12,19 +12,18 @@ use relative_path::RelativePath;
 use rustc_hash::FxHashSet;
 use ra_editor::{self, FileSymbol, LineIndex, find_node_at_offset, LocalEdit, resolve_local_name};
 use ra_syntax::{
-    TextUnit, TextRange, SmolStr, File, AstNode,
+    TextUnit, TextRange, SmolStr, File, AstNode, SyntaxNodeRef,
     SyntaxKind::*,
-    ast::{self, NameOwner},
+    ast::{self, NameOwner, ArgListOwner, Expr},
 };
 
 use {
     FileId, FileResolver, Query, Diagnostic, SourceChange, SourceFileEdit, Position, FileSystemEdit,
     JobToken, CrateGraph, CrateId,
     roots::{SourceRoot, ReadonlySourceRoot, WritableSourceRoot},
-    descriptors::{ModuleTreeDescriptor, Problem},
+    descriptors::{FnDescriptor, ModuleTreeDescriptor, Problem},
 };
 
-
 #[derive(Clone, Debug)]
 pub(crate) struct FileResolverImp {
     inner: Arc<FileResolver>
@@ -306,6 +305,68 @@ impl AnalysisImpl {
             .collect()
     }
 
+    pub fn resolve_callable(&self, file_id: FileId, offset: TextUnit, token: &JobToken)
+        -> Option<(FnDescriptor, Option<usize>)> {
+
+        let root = self.root(file_id);
+        let file = root.syntax(file_id);
+        let syntax = file.syntax();
+
+        // Find the calling expression and it's NameRef
+        let calling_node = FnCallNode::with_node(syntax, offset)?;
+        let name_ref = calling_node.name_ref()?;
+
+        // Resolve the function's NameRef (NOTE: this isn't entirely accurate).
+        let file_symbols = self.index_resolve(name_ref, token);
+        for (_, fs) in file_symbols {
+            if fs.kind == FN_DEF {
+                if let Some(fn_def) = find_node_at_offset(syntax, fs.node_range.start()) {
+                    if let Some(descriptor) = FnDescriptor::new(fn_def) {
+                        // If we have a calling expression let's find which argument we are on
+                        let mut current_parameter = None;
+
+                        let num_params = descriptor.params.len();
+                        let has_self = fn_def.param_list()
+                            .and_then(|l| l.self_param())
+                            .is_some();
+
+                        if num_params == 1 {
+                            if !has_self {
+                                current_parameter = Some(1);
+                            }
+                        } else if num_params > 1 {
+                            // Count how many parameters into the call we are.
+                            // TODO: This is best effort for now and should be fixed at some point.
+                            // It may be better to see where we are in the arg_list and then check
+                            // where offset is in that list (or beyond).
+                            // Revisit this after we get documentation comments in.
+                            if let Some(ref arg_list) = calling_node.arg_list() {
+                                let start = arg_list.syntax().range().start();
+
+                                let range_search = TextRange::from_to(start, offset);
+                                let mut commas: usize = arg_list.syntax().text()
+                                    .slice(range_search).to_string()
+                                    .matches(",")
+                                    .count();
+
+                                // If we have a method call eat the first param since it's just self.
+                                if has_self {
+                                    commas = commas + 1;
+                                }
+
+                                current_parameter = Some(commas);
+                            }
+                        }
+
+                        return Some((descriptor, current_parameter));
+                    }
+                }
+            }
+        }
+
+        None
+    }
+
     fn index_resolve(&self, name_ref: ast::NameRef, token: &JobToken) -> Vec<(FileId, FileSymbol)> {
         let name = name_ref.text();
         let mut query = Query::new(name.to_string());
@@ -355,3 +416,46 @@ impl CrateGraph {
         Some(crate_id)
     }
 }
+
+enum FnCallNode<'a> {
+    CallExpr(ast::CallExpr<'a>),
+    MethodCallExpr(ast::MethodCallExpr<'a>)
+}
+
+impl<'a> FnCallNode<'a> {
+    pub fn with_node(syntax: SyntaxNodeRef, offset: TextUnit) -> Option<FnCallNode> {
+        if let Some(expr) = find_node_at_offset::<ast::CallExpr>(syntax, offset) {
+            return Some(FnCallNode::CallExpr(expr));
+        }
+        if let Some(expr) = find_node_at_offset::<ast::MethodCallExpr>(syntax, offset) {
+            return Some(FnCallNode::MethodCallExpr(expr));
+        }
+        None
+    }
+
+    pub fn name_ref(&self) -> Option<ast::NameRef> {
+        match *self {
+            FnCallNode::CallExpr(call_expr) => {
+                Some(match call_expr.expr()? {
+                    Expr::PathExpr(path_expr) => {
+                        path_expr.path()?.segment()?.name_ref()?
+                    },
+                    _ => return None
+                })
+            },
+
+            FnCallNode::MethodCallExpr(call_expr) => {
+                call_expr.syntax().children()
+                    .filter_map(ast::NameRef::cast)
+                    .nth(0)
+            }
+        }
+    }
+
+    pub fn arg_list(&self) -> Option<ast::ArgList> {
+        match *self {
+            FnCallNode::CallExpr(expr) => expr.arg_list(),
+            FnCallNode::MethodCallExpr(expr) => expr.arg_list()
+        }
+    }
+}
\ No newline at end of file
diff --git a/crates/ra_analysis/src/lib.rs b/crates/ra_analysis/src/lib.rs
index 849fd93e4..1aca72ae0 100644
--- a/crates/ra_analysis/src/lib.rs
+++ b/crates/ra_analysis/src/lib.rs
@@ -38,6 +38,7 @@ pub use ra_editor::{
     Fold, FoldKind
 };
 pub use job::{JobToken, JobHandle};
+pub use descriptors::FnDescriptor;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct FileId(pub u32);
@@ -236,6 +237,11 @@ impl Analysis {
         let file = self.imp.file_syntax(file_id);
         ra_editor::folding_ranges(&file)
     }
+
+    pub fn resolve_callable(&self, file_id: FileId, offset: TextUnit, token: &JobToken)
+        -> Option<(FnDescriptor, Option<usize>)> {
+        self.imp.resolve_callable(file_id, offset, token)
+    }
 }
 
 #[derive(Debug)]
diff --git a/crates/ra_analysis/tests/tests.rs b/crates/ra_analysis/tests/tests.rs
index a886cd0ff..755640fb4 100644
--- a/crates/ra_analysis/tests/tests.rs
+++ b/crates/ra_analysis/tests/tests.rs
@@ -1,6 +1,8 @@
 extern crate relative_path;
 extern crate ra_analysis;
 extern crate rustc_hash;
+extern crate ra_editor;
+extern crate ra_syntax;
 extern crate test_utils;
 
 use std::{
@@ -9,8 +11,8 @@ use std::{
 
 use rustc_hash::FxHashMap;
 use relative_path::{RelativePath, RelativePathBuf};
-use ra_analysis::{Analysis, AnalysisHost, FileId, FileResolver, JobHandle, CrateGraph, CrateId};
-use test_utils::assert_eq_dbg;
+use ra_analysis::{Analysis, AnalysisHost, FileId, FileResolver, JobHandle, CrateGraph, CrateId, FnDescriptor};
+use test_utils::{assert_eq_dbg, extract_offset};
 
 #[derive(Debug)]
 struct FileMap(Vec<(FileId, RelativePathBuf)>);
@@ -39,7 +41,7 @@ impl FileResolver for FileMap {
     }
 }
 
-fn analysis_host(files: &'static [(&'static str, &'static str)]) -> AnalysisHost {
+fn analysis_host(files: &[(&str, &str)]) -> AnalysisHost {
     let mut host = AnalysisHost::new();
     let mut file_map = Vec::new();
     for (id, &(path, contents)) in files.iter().enumerate() {
@@ -53,10 +55,20 @@ fn analysis_host(files: &'static [(&'static str, &'static str)]) -> AnalysisHost
     host
 }
 
-fn analysis(files: &'static [(&'static str, &'static str)]) -> Analysis {
+fn analysis(files: &[(&str, &str)]) -> Analysis {
     analysis_host(files).analysis()
 }
 
+fn get_signature(text: &str) -> (FnDescriptor, Option<usize>) {
+    let (offset, code) = extract_offset(text);
+    let code = code.as_str();
+
+    let (_handle, token) = JobHandle::new();
+    let snap = analysis(&[("/lib.rs", code)]);
+
+    snap.resolve_callable(FileId(1), offset, &token).unwrap()
+}
+
 #[test]
 fn test_resolve_module() {
     let snap = analysis(&[
@@ -145,3 +157,85 @@ fn test_resolve_crate_root() {
         vec![CrateId(1)],
     );
 }
+
+#[test]
+fn test_fn_signature_two_args_first() {
+    let (desc, param) = get_signature(
+r#"fn foo(x: u32, y: u32) -> u32 {x + y}
+fn bar() { foo(<|>3, ); }"#);
+
+    assert_eq!(desc.name, "foo".to_string());
+    assert_eq!(desc.params, vec!("x".to_string(),"y".to_string()));
+    assert_eq!(desc.ret_type, Some("-> u32".into()));
+    assert_eq!(param, Some(0));
+}
+
+#[test]
+fn test_fn_signature_two_args_second() {
+    let (desc, param) = get_signature(
+        r#"fn foo(x: u32, y: u32) -> u32 {x + y}
+fn bar() { foo(3, <|>); }"#);
+
+    assert_eq!(desc.name, "foo".to_string());
+    assert_eq!(desc.params, vec!("x".to_string(),"y".to_string()));
+    assert_eq!(desc.ret_type, Some("-> u32".into()));
+    assert_eq!(param, Some(1));
+}
+
+#[test]
+fn test_fn_signature_for_impl() {
+    let (desc, param) = get_signature(
+r#"struct F; impl F { pub fn new() { F{}} }
+fn bar() {let _ : F = F::new(<|>);}"#);
+
+    assert_eq!(desc.name, "new".to_string());
+    assert_eq!(desc.params, Vec::<String>::new());
+    assert_eq!(desc.ret_type, None);
+    assert_eq!(param, None);
+}
+
+#[test]
+fn test_fn_signature_for_method_self() {
+    let (desc, param) = get_signature(
+r#"struct F;
+impl F {
+    pub fn new() -> F{
+        F{}
+    }
+
+    pub fn do_it(&self) {}
+}
+
+fn bar() {
+    let f : F = F::new();
+    f.do_it(<|>);
+}"#);
+
+    assert_eq!(desc.name, "do_it".to_string());
+    assert_eq!(desc.params, vec!["&self".to_string()]);
+    assert_eq!(desc.ret_type, None);
+    assert_eq!(param, None);
+}
+
+#[test]
+fn test_fn_signature_for_method_with_arg() {
+    let (desc, param) = get_signature(
+r#"struct F;
+impl F {
+    pub fn new() -> F{
+        F{}
+    }
+
+    pub fn do_it(&self, x: i32) {}
+}
+
+fn bar() {
+    let f : F = F::new();
+    f.do_it(<|>);
+}"#);
+
+    assert_eq!(desc.name, "do_it".to_string());
+    assert_eq!(desc.params, vec!["&self".to_string(), "x".to_string()]);
+    assert_eq!(desc.ret_type, None);
+    assert_eq!(param, Some(1));
+}
\ No newline at end of file
diff --git a/crates/ra_lsp_server/src/caps.rs b/crates/ra_lsp_server/src/caps.rs
index 3c628f29c..5598ec75f 100644
--- a/crates/ra_lsp_server/src/caps.rs
+++ b/crates/ra_lsp_server/src/caps.rs
@@ -7,6 +7,7 @@ use languageserver_types::{
     TextDocumentSyncKind,
     ExecuteCommandOptions,
     CompletionOptions,
+    SignatureHelpOptions,
     DocumentOnTypeFormattingOptions,
 };
 
@@ -26,7 +27,9 @@ pub fn server_capabilities() -> ServerCapabilities {
             resolve_provider: None,
             trigger_characters: None,
         }),
-        signature_help_provider: None,
+        signature_help_provider: Some(SignatureHelpOptions {
+            trigger_characters: Some(vec!["(".to_string(), ",".to_string()])
+        }),
         definition_provider: Some(true),
         type_definition_provider: None,
         implementation_provider: None,
diff --git a/crates/ra_lsp_server/src/main_loop/handlers.rs b/crates/ra_lsp_server/src/main_loop/handlers.rs
index ab8be15e9..f65e2a889 100644
--- a/crates/ra_lsp_server/src/main_loop/handlers.rs
+++ b/crates/ra_lsp_server/src/main_loop/handlers.rs
@@ -411,6 +411,42 @@ pub fn handle_folding_range(
     Ok(res)
 }
 
+pub fn handle_signature_help(
+    world: ServerWorld,
+    params: req::TextDocumentPositionParams,
+    token: JobToken,
+) -> Result<Option<req::SignatureHelp>> {
+    use languageserver_types::{ParameterInformation, SignatureInformation};
+
+    let file_id = params.text_document.try_conv_with(&world)?;
+    let line_index = world.analysis().file_line_index(file_id);
+    let offset = params.position.conv_with(&line_index);
+
+    if let Some((descriptor, active_param)) = world.analysis().resolve_callable(file_id, offset, &token) {
+        let parameters : Vec<ParameterInformation> =
+            descriptor.params.iter().map(|param|
+                ParameterInformation {
+                    label: param.clone(),
+                    documentation: None
+                }
+            ).collect();
+
+        let sig_info = SignatureInformation {
+            label: descriptor.label,
+            documentation: None,
+            parameters: Some(parameters)
+        };
+
+        Ok(Some(req::SignatureHelp {
+            signatures: vec![sig_info],
+            active_signature: Some(0),
+            active_parameter: active_param.map(|a| a as u64)
+        }))
+    } else {
+        Ok(None)
+    }
+}
+
 pub fn handle_code_action(
     world: ServerWorld,
     params: req::CodeActionParams,
diff --git a/crates/ra_lsp_server/src/main_loop/mod.rs b/crates/ra_lsp_server/src/main_loop/mod.rs
index 402615e42..f4e7cfc33 100644
--- a/crates/ra_lsp_server/src/main_loop/mod.rs
+++ b/crates/ra_lsp_server/src/main_loop/mod.rs
@@ -255,6 +255,7 @@ fn on_request(
         .on::<req::Completion>(handlers::handle_completion)?
         .on::<req::CodeActionRequest>(handlers::handle_code_action)?
         .on::<req::FoldingRangeRequest>(handlers::handle_folding_range)?
+        .on::<req::SignatureHelpRequest>(handlers::handle_signature_help)?
         .finish();
     match req {
         Ok((id, handle)) => {
diff --git a/crates/ra_lsp_server/src/req.rs b/crates/ra_lsp_server/src/req.rs
index f80957589..1630edf7f 100644
--- a/crates/ra_lsp_server/src/req.rs
+++ b/crates/ra_lsp_server/src/req.rs
@@ -14,6 +14,7 @@ pub use languageserver_types::{
     CompletionParams, CompletionResponse,
     DocumentOnTypeFormattingParams,
     TextDocumentEdit,
+    SignatureHelp, Hover
 };
 
 pub enum SyntaxTree {}
diff --git a/crates/ra_syntax/src/ast/generated.rs b/crates/ra_syntax/src/ast/generated.rs
index 85aa5e0dd..4db1bcbf9 100644
--- a/crates/ra_syntax/src/ast/generated.rs
+++ b/crates/ra_syntax/src/ast/generated.rs
@@ -1387,7 +1387,11 @@ impl<'a> AstNode<'a> for PathExpr<'a> {
     fn syntax(self) -> SyntaxNodeRef<'a> { self.syntax }
 }
 
-impl<'a> PathExpr<'a> {}
+impl<'a> PathExpr<'a> {
+    pub fn path(self) -> Option<Path<'a>> {
+        super::child_opt(self)
+    }
+}
 
 // PathPat
 #[derive(Debug, Clone, Copy)]
diff --git a/crates/ra_syntax/src/grammar.ron b/crates/ra_syntax/src/grammar.ron
index d538739de..ea8063d3b 100644
--- a/crates/ra_syntax/src/grammar.ron
+++ b/crates/ra_syntax/src/grammar.ron
@@ -342,7 +342,7 @@ Grammar(
         "TupleExpr": (),
         "ArrayExpr": (),
         "ParenExpr": (),
-        "PathExpr": (),
+        "PathExpr": (options: ["Path"]),
         "LambdaExpr": (
             options: [
                 "ParamList",