Language Server: textDocument/signatureHelp

Implements a pretty barebones function signature help mechanism in
the language server.

Users can use `Analysis::resolve_callback()` to get basic information
about a call site.

Fixes #102

3 files changed, 171 insertions, 5 deletions

diff --git a/crates/ra_analysis/src/descriptors.rs b/crates/ra_analysis/src/descriptors.rs
index 0731b5572..4dcac1aa2 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, TypeParamsOwner},
+    text_utils::is_subrange
 };
 use {
     FileId,
@@ -218,3 +219,56 @@ fn resolve_submodule(
     }
     (points_to, problem)
 }
+
+#[derive(Debug, Clone)]
+pub struct FnDescriptor {
+    pub name: Option<String>,
+    pub label : String,
+    pub ret_type: Option<String>,
+    pub params: Vec<String>,
+}
+
+impl FnDescriptor {
+    pub fn new(node: ast::FnDef) -> Self {
+        let name = node.name().map(|name| 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());
+
+        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..9e3ae2b03 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 +305,70 @@ 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()) {
+                    let 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 +418,46 @@ 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)]