1 files changed, 86 insertions, 0 deletions

diff --git a/crates/ra_assists/src/assists/add_explicit_type.rs b/crates/ra_assists/src/assists/add_explicit_type.rs
new file mode 100644
index 000000000..78f0f7f28
--- /dev/null
+++ b/crates/ra_assists/src/assists/add_explicit_type.rs
@@ -0,0 +1,86 @@
+use hir::{db::HirDatabase, HirDisplay, Ty};
+use ra_syntax::{
+    ast::{self, AstNode, LetStmt, NameOwner},
+    T,
+};
+
+use crate::{Assist, AssistCtx, AssistId};
+
+/// Add explicit type assist.
+pub(crate) fn add_explicit_type(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let stmt = ctx.node_at_offset::<LetStmt>()?;
+    let expr = stmt.initializer()?;
+    let pat = stmt.pat()?;
+    // Must be a binding
+    let pat = match pat {
+        ast::Pat::BindPat(bind_pat) => bind_pat,
+        _ => return None,
+    };
+    let pat_range = pat.syntax().text_range();
+    // The binding must have a name
+    let name = pat.name()?;
+    let name_range = name.syntax().text_range();
+    // Assist not applicable if the type has already been specified
+    if stmt.syntax().children_with_tokens().any(|child| child.kind() == T![:]) {
+        return None;
+    }
+    // Infer type
+    let db = ctx.db;
+    let analyzer = hir::SourceAnalyzer::new(db, ctx.frange.file_id, stmt.syntax(), None);
+    let ty = analyzer.type_of(db, &expr)?;
+    // Assist not applicable if the type is unknown
+    if is_unknown(&ty) {
+        return None;
+    }
+
+    ctx.add_action(AssistId("add_explicit_type"), "add explicit type", |edit| {
+        edit.target(pat_range);
+        edit.insert(name_range.end(), format!(": {}", ty.display(db)));
+    });
+    ctx.build()
+}
+
+/// Returns true if any type parameter is unknown
+fn is_unknown(ty: &Ty) -> bool {
+    match ty {
+        Ty::Unknown => true,
+        Ty::Apply(a_ty) => a_ty.parameters.iter().any(is_unknown),
+        _ => false,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::helpers::{check_assist, check_assist_not_applicable, check_assist_target};
+
+    #[test]
+    fn add_explicit_type_target() {
+        check_assist_target(add_explicit_type, "fn f() { let a<|> = 1; }", "a");
+    }
+
+    #[test]
+    fn add_explicit_type_works_for_simple_expr() {
+        check_assist(
+            add_explicit_type,
+            "fn f() { let a<|> = 1; }",
+            "fn f() { let a<|>: i32 = 1; }",
+        );
+    }
+
+    #[test]
+    fn add_explicit_type_not_applicable_if_ty_not_inferred() {
+        check_assist_not_applicable(add_explicit_type, "fn f() { let a<|> = None; }");
+    }
+
+    #[test]
+    fn add_explicit_type_not_applicable_if_ty_already_specified() {
+        check_assist_not_applicable(add_explicit_type, "fn f() { let a<|>: i32 = 1; }");
+    }
+
+    #[test]
+    fn add_explicit_type_not_applicable_if_specified_ty_is_tuple() {
+        check_assist_not_applicable(add_explicit_type, "fn f() { let a<|>: (i32, i32) = (3, 4); }");
+    }
+}