1 files changed, 178 insertions, 0 deletions

diff --git a/crates/ra_assists/src/handlers/add_explicit_type.rs b/crates/ra_assists/src/handlers/add_explicit_type.rs
new file mode 100644
index 000000000..2cb9d2f48
--- /dev/null
+++ b/crates/ra_assists/src/handlers/add_explicit_type.rs
@@ -0,0 +1,178 @@
+use hir::HirDisplay;
+use ra_syntax::{
+    ast::{self, AstNode, LetStmt, NameOwner, TypeAscriptionOwner},
+    TextRange,
+};
+
+use crate::{Assist, AssistCtx, AssistId};
+
+// Assist: add_explicit_type
+//
+// Specify type for a let binding.
+//
+// ```
+// fn main() {
+//     let x<|> = 92;
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     let x: i32 = 92;
+// }
+// ```
+pub(crate) fn add_explicit_type(ctx: AssistCtx) -> Option<Assist> {
+    let stmt = ctx.find_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();
+    let stmt_range = stmt.syntax().text_range();
+    let eq_range = stmt.eq_token()?.text_range();
+    // Assist should only be applicable if cursor is between 'let' and '='
+    let let_range = TextRange::from_to(stmt_range.start(), eq_range.start());
+    let cursor_in_range = ctx.frange.range.is_subrange(&let_range);
+    if !cursor_in_range {
+        return None;
+    }
+    // Assist not applicable if the type has already been specified
+    // and it has no placeholders
+    let ascribed_ty = stmt.ascribed_type();
+    if let Some(ref ty) = ascribed_ty {
+        if ty.syntax().descendants().find_map(ast::PlaceholderType::cast).is_none() {
+            return None;
+        }
+    }
+    // Infer type
+    let db = ctx.db;
+    let analyzer = ctx.source_analyzer(stmt.syntax(), None);
+    let ty = analyzer.type_of(db, &expr)?;
+    // Assist not applicable if the type is unknown
+    if ty.contains_unknown() {
+        return None;
+    }
+
+    ctx.add_assist(
+        AssistId("add_explicit_type"),
+        format!("Insert explicit type '{}'", ty.display(db)),
+        |edit| {
+            edit.target(pat_range);
+            if let Some(ascribed_ty) = ascribed_ty {
+                edit.replace(ascribed_ty.syntax().text_range(), format!("{}", ty.display(db)));
+            } else {
+                edit.insert(name_range.end(), format!(": {}", ty.display(db)));
+            }
+        },
+    )
+}
+
+#[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_works_for_underscore() {
+        check_assist(
+            add_explicit_type,
+            "fn f() { let a<|>: _ = 1; }",
+            "fn f() { let a<|>: i32 = 1; }",
+        );
+    }
+
+    #[test]
+    fn add_explicit_type_works_for_nested_underscore() {
+        check_assist(
+            add_explicit_type,
+            r#"
+            enum Option<T> {
+                Some(T),
+                None
+            }
+
+            fn f() {
+                let a<|>: Option<_> = Option::Some(1);
+            }"#,
+            r#"
+            enum Option<T> {
+                Some(T),
+                None
+            }
+
+            fn f() {
+                let a<|>: Option<i32> = Option::Some(1);
+            }"#,
+        );
+    }
+
+    #[test]
+    fn add_explicit_type_works_for_macro_call() {
+        check_assist(
+            add_explicit_type,
+            "macro_rules! v { () => {0u64} } fn f() { let a<|> = v!(); }",
+            "macro_rules! v { () => {0u64} } fn f() { let a<|>: u64 = v!(); }",
+        );
+    }
+
+    #[test]
+    fn add_explicit_type_works_for_macro_call_recursive() {
+        check_assist(
+            add_explicit_type,
+            "macro_rules! u { () => {0u64} } macro_rules! v { () => {u!()} } fn f() { let a<|> = v!(); }",
+            "macro_rules! u { () => {0u64} } macro_rules! v { () => {u!()} } fn f() { let a<|>: u64 = v!(); }",
+        );
+    }
+
+    #[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); }");
+    }
+
+    #[test]
+    fn add_explicit_type_not_applicable_if_cursor_after_equals() {
+        check_assist_not_applicable(
+            add_explicit_type,
+            "fn f() {let a =<|> match 1 {2 => 3, 3 => 5};}",
+        )
+    }
+
+    #[test]
+    fn add_explicit_type_not_applicable_if_cursor_before_let() {
+        check_assist_not_applicable(
+            add_explicit_type,
+            "fn f() <|>{let a = match 1 {2 => 3, 3 => 5};}",
+        )
+    }
+}