Add lsp signature help (#1755)

* Add lsp signature help

* Do not move signature help popup on multiple triggers

* Highlight current parameter in signature help

* Auto close signature help

* Position signature help above to not block completion

* Update signature help on backspace/insert mode delete

* Add lsp.auto-signature-help config option

* Add serde default annotation for LspConfig

* Show LSP inactive message only if signature help is invoked manually

* Do not assume valid signature help response from LSP

Malformed LSP responses are common, and these should not crash the
editor.

* Check signature help capability before sending request

* Reuse Open enum for PositionBias in popup

* Close signature popup and exit insert mode on escape

* Add config to control signature help docs display

* Use new Margin api in signature help

* Invoke signature help on changing to insert mode

6 files changed, 202 insertions, 13 deletions

diff --git a/helix-term/src/ui/completion.rs b/helix-term/src/ui/completion.rs
index a3637415..c1816db1 100644
--- a/helix-term/src/ui/completion.rs
+++ b/helix-term/src/ui/completion.rs
@@ -85,6 +85,8 @@ pub struct Completion {
 }
 
 impl Completion {
+    pub const ID: &'static str = "completion";
+
     pub fn new(
         editor: &Editor,
         items: Vec<CompletionItem>,
@@ -214,7 +216,7 @@ impl Completion {
                 }
             };
         });
-        let popup = Popup::new("completion", menu);
+        let popup = Popup::new(Self::ID, menu);
         let mut completion = Self {
             popup,
             start_offset,
diff --git a/helix-term/src/ui/editor.rs b/helix-term/src/ui/editor.rs
index 911ee0f0..849f0b0b 100644
--- a/helix-term/src/ui/editor.rs
+++ b/helix-term/src/ui/editor.rs
@@ -1,7 +1,7 @@
 use crate::{
     commands,
     compositor::{Component, Context, EventResult},
-    key,
+    job, key,
     keymap::{KeymapResult, Keymaps},
     ui::{Completion, ProgressSpinners},
 };
@@ -28,6 +28,7 @@ use std::borrow::Cow;
 use crossterm::event::{Event, MouseButton, MouseEvent, MouseEventKind};
 use tui::buffer::Buffer as Surface;
 
+use super::lsp::SignatureHelp;
 use super::statusline;
 
 pub struct EditorView {
@@ -1205,10 +1206,21 @@ impl Component for EditorView {
                             _ => unimplemented!(),
                         };
                         self.last_insert.1.clear();
+                        commands::signature_help_impl(
+                            &mut cx,
+                            commands::SignatureHelpInvoked::Automatic,
+                        );
                     }
                     (Mode::Insert, Mode::Normal) => {
                         // if exiting insert mode, remove completion
                         self.completion = None;
+                        // TODO: Use an on_mode_change hook to remove signature help
+                        context.jobs.callback(async {
+                            let call: job::Callback = Box::new(|_editor, compositor| {
+                                compositor.remove(SignatureHelp::ID);
+                            });
+                            Ok(call)
+                        });
                     }
                     _ => (),
                 }
diff --git a/helix-term/src/ui/lsp.rs b/helix-term/src/ui/lsp.rs
new file mode 100644
index 00000000..f2854551
--- /dev/null
+++ b/helix-term/src/ui/lsp.rs
@@ -0,0 +1,133 @@
+use std::sync::Arc;
+
+use helix_core::syntax;
+use helix_view::graphics::{Margin, Rect, Style};
+use tui::buffer::Buffer;
+use tui::widgets::{BorderType, Paragraph, Widget, Wrap};
+
+use crate::compositor::{Component, Compositor, Context};
+
+use crate::ui::Markdown;
+
+use super::Popup;
+
+pub struct SignatureHelp {
+    signature: String,
+    signature_doc: Option<String>,
+    /// Part of signature text
+    active_param_range: Option<(usize, usize)>,
+
+    language: String,
+    config_loader: Arc<syntax::Loader>,
+}
+
+impl SignatureHelp {
+    pub const ID: &'static str = "signature-help";
+
+    pub fn new(signature: String, language: String, config_loader: Arc<syntax::Loader>) -> Self {
+        Self {
+            signature,
+            signature_doc: None,
+            active_param_range: None,
+            language,
+            config_loader,
+        }
+    }
+
+    pub fn set_signature_doc(&mut self, signature_doc: Option<String>) {
+        self.signature_doc = signature_doc;
+    }
+
+    pub fn set_active_param_range(&mut self, offset: Option<(usize, usize)>) {
+        self.active_param_range = offset;
+    }
+
+    pub fn visible_popup(compositor: &mut Compositor) -> Option<&mut Popup<Self>> {
+        compositor.find_id::<Popup<Self>>(Self::ID)
+    }
+}
+
+impl Component for SignatureHelp {
+    fn render(&mut self, area: Rect, surface: &mut Buffer, cx: &mut Context) {
+        let margin = Margin::horizontal(1);
+
+        let active_param_span = self.active_param_range.map(|(start, end)| {
+            vec![(
+                cx.editor.theme.find_scope_index("ui.selection").unwrap(),
+                start..end,
+            )]
+        });
+
+        let sig_text = crate::ui::markdown::highlighted_code_block(
+            self.signature.clone(),
+            &self.language,
+            Some(&cx.editor.theme),
+            Arc::clone(&self.config_loader),
+            active_param_span,
+        );
+
+        let (_, sig_text_height) = crate::ui::text::required_size(&sig_text, area.width);
+        let sig_text_area = area.clip_top(1).with_height(sig_text_height);
+        let sig_text_para = Paragraph::new(sig_text).wrap(Wrap { trim: false });
+        sig_text_para.render(sig_text_area.inner(&margin), surface);
+
+        if self.signature_doc.is_none() {
+            return;
+        }
+
+        let sep_style = Style::default();
+        let borders = BorderType::line_symbols(BorderType::Plain);
+        for x in sig_text_area.left()..sig_text_area.right() {
+            if let Some(cell) = surface.get_mut(x, sig_text_area.bottom()) {
+                cell.set_symbol(borders.horizontal).set_style(sep_style);
+            }
+        }
+
+        let sig_doc = match &self.signature_doc {
+            None => return,
+            Some(doc) => Markdown::new(doc.clone(), Arc::clone(&self.config_loader)),
+        };
+        let sig_doc = sig_doc.parse(Some(&cx.editor.theme));
+        let sig_doc_area = area.clip_top(sig_text_area.height + 2);
+        let sig_doc_para = Paragraph::new(sig_doc)
+            .wrap(Wrap { trim: false })
+            .scroll((cx.scroll.unwrap_or_default() as u16, 0));
+        sig_doc_para.render(sig_doc_area.inner(&margin), surface);
+    }
+
+    fn required_size(&mut self, viewport: (u16, u16)) -> Option<(u16, u16)> {
+        const PADDING: u16 = 2;
+        const SEPARATOR_HEIGHT: u16 = 1;
+
+        if PADDING >= viewport.1 || PADDING >= viewport.0 {
+            return None;
+        }
+        let max_text_width = (viewport.0 - PADDING).min(120);
+
+        let signature_text = crate::ui::markdown::highlighted_code_block(
+            self.signature.clone(),
+            &self.language,
+            None,
+            Arc::clone(&self.config_loader),
+            None,
+        );
+        let (sig_width, sig_height) =
+            crate::ui::text::required_size(&signature_text, max_text_width);
+
+        let (width, height) = match self.signature_doc {
+            Some(ref doc) => {
+                let doc_md = Markdown::new(doc.clone(), Arc::clone(&self.config_loader));
+                let doc_text = doc_md.parse(None);
+                let (doc_width, doc_height) =
+                    crate::ui::text::required_size(&doc_text, max_text_width);
+                (
+                    sig_width.max(doc_width),
+                    sig_height + SEPARATOR_HEIGHT + doc_height,
+                )
+            }
+            None => (sig_width, sig_height),
+        };
+
+        Some((width + PADDING, height + PADDING))
+    }
+}
diff --git a/helix-term/src/ui/markdown.rs b/helix-term/src/ui/markdown.rs
index a5c78c41..c53b3b66 100644
--- a/helix-term/src/ui/markdown.rs
+++ b/helix-term/src/ui/markdown.rs
@@ -144,7 +144,7 @@ impl Markdown {
         }
     }
 
-    fn parse(&self, theme: Option<&Theme>) -> tui::text::Text<'_> {
+    pub fn parse(&self, theme: Option<&Theme>) -> tui::text::Text<'_> {
         fn push_line<'a>(spans: &mut Vec<Span<'a>>, lines: &mut Vec<Spans<'a>>) {
             let spans = std::mem::take(spans);
             if !spans.is_empty() {
diff --git a/helix-term/src/ui/mod.rs b/helix-term/src/ui/mod.rs
index 88a226e9..257608f0 100644
--- a/helix-term/src/ui/mod.rs
+++ b/helix-term/src/ui/mod.rs
@@ -1,11 +1,12 @@
 mod completion;
 pub(crate) mod editor;
 mod info;
+pub mod lsp;
 mod markdown;
 pub mod menu;
 pub mod overlay;
 mod picker;
-mod popup;
+pub mod popup;
 mod prompt;
 mod spinner;
 mod statusline;
diff --git a/helix-term/src/ui/popup.rs b/helix-term/src/ui/popup.rs
index f5b79526..77ab2462 100644
--- a/helix-term/src/ui/popup.rs
+++ b/helix-term/src/ui/popup.rs
@@ -1,4 +1,5 @@
 use crate::{
+    commands::Open,
     compositor::{Callback, Component, Context, EventResult},
     ctrl, key,
 };
@@ -17,8 +18,10 @@ pub struct Popup<T: Component> {
     margin: Margin,
     size: (u16, u16),
     child_size: (u16, u16),
+    position_bias: Open,
     scroll: usize,
     auto_close: bool,
+    ignore_escape_key: bool,
     id: &'static str,
 }
 
@@ -29,15 +32,27 @@ impl<T: Component> Popup<T> {
             position: None,
             margin: Margin::none(),
             size: (0, 0),
+            position_bias: Open::Below,
             child_size: (0, 0),
             scroll: 0,
             auto_close: false,
+            ignore_escape_key: false,
             id,
         }
     }
 
-    pub fn set_position(&mut self, pos: Option<Position>) {
+    pub fn position(mut self, pos: Option<Position>) -> Self {
         self.position = pos;
+        self
+    }
+
+    pub fn get_position(&self) -> Option<Position> {
+        self.position
+    }
+
+    pub fn position_bias(mut self, bias: Open) -> Self {
+        self.position_bias = bias;
+        self
     }
 
     pub fn margin(mut self, margin: Margin) -> Self {
@@ -50,6 +65,18 @@ impl<T: Component> Popup<T> {
         self
     }
 
+    /// Ignores an escape keypress event, letting the outer layer
+    /// (usually the editor) handle it. This is useful for popups
+    /// in insert mode like completion and signature help where
+    /// the popup is closed on the mode change from insert to normal
+    /// which is done with the escape key. Otherwise the popup consumes
+    /// the escape key event and closes it, and an additional escape
+    /// would be required to exit insert mode.
+    pub fn ignore_escape_key(mut self, ignore: bool) -> Self {
+        self.ignore_escape_key = ignore;
+        self
+    }
+
     pub fn get_rel_position(&mut self, viewport: Rect, cx: &Context) -> (u16, u16) {
         let position = self
             .position
@@ -68,13 +95,23 @@ impl<T: Component> Popup<T> {
             rel_x = rel_x.saturating_sub((rel_x + width).saturating_sub(viewport.width));
         }
 
-        // TODO: be able to specify orientation preference. We want above for most popups, below
-        // for menus/autocomplete.
-        if viewport.height > rel_y + height {
-            rel_y += 1 // position below point
-        } else {
-            rel_y = rel_y.saturating_sub(height) // position above point
-        }
+        let can_put_below = viewport.height > rel_y + height;
+        let can_put_above = rel_y.checked_sub(height).is_some();
+        let final_pos = match self.position_bias {
+            Open::Below => match can_put_below {
+                true => Open::Below,
+                false => Open::Above,
+            },
+            Open::Above => match can_put_above {
+                true => Open::Above,
+                false => Open::Below,
+            },
+        };
+
+        rel_y = match final_pos {
+            Open::Above => rel_y.saturating_sub(height),
+            Open::Below => rel_y + 1,
+        };
 
         (rel_x, rel_y)
     }
@@ -112,9 +149,13 @@ impl<T: Component> Component for Popup<T> {
             _ => return EventResult::Ignored(None),
         };
 
+        if key!(Esc) == key.into() && self.ignore_escape_key {
+            return EventResult::Ignored(None);
+        }
+
         let close_fn: Callback = Box::new(|compositor, _| {
             // remove the layer
-            compositor.pop();
+            compositor.remove(self.id.as_ref());
         });
 
         match key.into() {