1 files changed, 154 insertions, 0 deletions

diff --git a/crates/ra_assists/src/assist_ctx.rs b/crates/ra_assists/src/assist_ctx.rs
new file mode 100644
index 000000000..6d09bde52
--- /dev/null
+++ b/crates/ra_assists/src/assist_ctx.rs
@@ -0,0 +1,154 @@
+use hir::db::HirDatabase;
+use ra_text_edit::TextEditBuilder;
+use ra_db::FileRange;
+use ra_syntax::{
+    SourceFile, TextRange, AstNode, TextUnit, SyntaxNode,
+    algo::{find_leaf_at_offset, find_node_at_offset, find_covering_node, LeafAtOffset},
+};
+use ra_ide_api_light::formatting::{leading_indent, reindent};
+
+use crate::{AssistLabel, AssistAction};
+
+pub(crate) enum Assist {
+    Unresolved(AssistLabel),
+    Resolved(AssistLabel, AssistAction),
+}
+
+/// `AssistCtx` allows to apply an assist or check if it could be applied.
+///
+/// Assists use a somewhat overengineered approach, given the current needs. The
+/// assists workflow consists of two phases. In the first phase, a user asks for
+/// the list of available assists. In the second phase, the user picks a
+/// particular assist and it gets applied.
+///
+/// There are two peculiarities here:
+///
+/// * first, we ideally avoid computing more things then necessary to answer
+///   "is assist applicable" in the first phase.
+/// * second, when we are applying assist, we don't have a guarantee that there
+///   weren't any changes between the point when user asked for assists and when
+///   they applied a particular assist. So, when applying assist, we need to do
+///   all the checks from scratch.
+///
+/// To avoid repeating the same code twice for both "check" and "apply"
+/// functions, we use an approach reminiscent of that of Django's function based
+/// views dealing with forms. Each assist receives a runtime parameter,
+/// `should_compute_edit`. It first check if an edit is applicable (potentially
+/// computing info required to compute the actual edit). If it is applicable,
+/// and `should_compute_edit` is `true`, it then computes the actual edit.
+///
+/// So, to implement the original assists workflow, we can first apply each edit
+/// with `should_compute_edit = false`, and then applying the selected edit
+/// again, with `should_compute_edit = true` this time.
+///
+/// Note, however, that we don't actually use such two-phase logic at the
+/// moment, because the LSP API is pretty awkward in this place, and it's much
+/// easier to just compute the edit eagerly :-)#[derive(Debug, Clone)]
+#[derive(Debug)]
+pub(crate) struct AssistCtx<'a, DB> {
+    pub(crate) db: &'a DB,
+    pub(crate) frange: FileRange,
+    source_file: &'a SourceFile,
+    should_compute_edit: bool,
+}
+
+impl<'a, DB> Clone for AssistCtx<'a, DB> {
+    fn clone(&self) -> Self {
+        AssistCtx {
+            db: self.db,
+            frange: self.frange,
+            source_file: self.source_file,
+            should_compute_edit: self.should_compute_edit,
+        }
+    }
+}
+
+impl<'a, DB: HirDatabase> AssistCtx<'a, DB> {
+    pub(crate) fn with_ctx<F, T>(db: &DB, frange: FileRange, should_compute_edit: bool, f: F) -> T
+    where
+        F: FnOnce(AssistCtx<DB>) -> T,
+    {
+        let source_file = &db.parse(frange.file_id);
+        let ctx = AssistCtx {
+            db,
+            frange,
+            source_file,
+            should_compute_edit,
+        };
+        f(ctx)
+    }
+
+    pub(crate) fn build(
+        self,
+        label: impl Into<String>,
+        f: impl FnOnce(&mut AssistBuilder),
+    ) -> Option<Assist> {
+        let label = AssistLabel {
+            label: label.into(),
+        };
+        if !self.should_compute_edit {
+            return Some(Assist::Unresolved(label));
+        }
+        let action = {
+            let mut edit = AssistBuilder::default();
+            f(&mut edit);
+            edit.build()
+        };
+        Some(Assist::Resolved(label, action))
+    }
+
+    pub(crate) fn leaf_at_offset(&self) -> LeafAtOffset<&'a SyntaxNode> {
+        find_leaf_at_offset(self.source_file.syntax(), self.frange.range.start())
+    }
+
+    pub(crate) fn node_at_offset<N: AstNode>(&self) -> Option<&'a N> {
+        find_node_at_offset(self.source_file.syntax(), self.frange.range.start())
+    }
+    pub(crate) fn covering_node(&self) -> &'a SyntaxNode {
+        find_covering_node(self.source_file.syntax(), self.frange.range)
+    }
+}
+
+#[derive(Default)]
+pub(crate) struct AssistBuilder {
+    edit: TextEditBuilder,
+    cursor_position: Option<TextUnit>,
+}
+
+impl AssistBuilder {
+    pub(crate) fn replace(&mut self, range: TextRange, replace_with: impl Into<String>) {
+        self.edit.replace(range, replace_with.into())
+    }
+
+    pub(crate) fn replace_node_and_indent(
+        &mut self,
+        node: &SyntaxNode,
+        replace_with: impl Into<String>,
+    ) {
+        let mut replace_with = replace_with.into();
+        if let Some(indent) = leading_indent(node) {
+            replace_with = reindent(&replace_with, indent)
+        }
+        self.replace(node.range(), replace_with)
+    }
+
+    #[allow(unused)]
+    pub(crate) fn delete(&mut self, range: TextRange) {
+        self.edit.delete(range)
+    }
+
+    pub(crate) fn insert(&mut self, offset: TextUnit, text: impl Into<String>) {
+        self.edit.insert(offset, text.into())
+    }
+
+    pub(crate) fn set_cursor(&mut self, offset: TextUnit) {
+        self.cursor_position = Some(offset)
+    }
+
+    fn build(self) -> AssistAction {
+        AssistAction {
+            edit: self.edit.finish(),
+            cursor_position: self.cursor_position,
+        }
+    }
+}