Assist: replace anonymous lifetime with a named one

(fixes #4523)

3 files changed, 149 insertions, 0 deletions

diff --git a/crates/ra_assists/src/handlers/change_lifetime_anon_to_named.rs b/crates/ra_assists/src/handlers/change_lifetime_anon_to_named.rs
new file mode 100644
index 000000000..63f0a7dab
--- /dev/null
+++ b/crates/ra_assists/src/handlers/change_lifetime_anon_to_named.rs
@@ -0,0 +1,123 @@
+use crate::{AssistContext, AssistId, Assists};
+use ra_syntax::{ast, ast::{TypeParamsOwner}, AstNode, SyntaxKind};
+
+/// Assist: change_lifetime_anon_to_named
+///
+/// Change an anonymous lifetime to a named lifetime.
+///
+/// ```
+/// impl Cursor<'_<|>> {
+///     fn node(self) -> &SyntaxNode {
+///         match self {
+///             Cursor::Replace(node) | Cursor::Before(node) => node,
+///         }
+///     }
+/// }
+/// ```
+/// ->
+/// ```
+/// impl<'a> Cursor<'a> {
+///     fn node(self) -> &SyntaxNode {
+///         match self {
+///             Cursor::Replace(node) | Cursor::Before(node) => node,
+///         }
+///     }
+/// }
+/// ```
+// TODO : How can we handle renaming any one of multiple anonymous lifetimes?
+pub(crate) fn change_lifetime_anon_to_named(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let lifetime_token = ctx.find_token_at_offset(SyntaxKind::LIFETIME)?;
+    let lifetime_arg = ast::LifetimeArg::cast(lifetime_token.parent())?;
+    if lifetime_arg.syntax().text() != "'_" {
+        return None;
+    }
+    let next_token = lifetime_token.next_token()?;
+    if next_token.kind() != SyntaxKind::R_ANGLE {
+        // only allow naming the last anonymous lifetime
+        return None;
+    }
+    match lifetime_arg.syntax().ancestors().find_map(ast::ImplDef::cast) {
+        Some(impl_def) => {
+            // get the `impl` keyword so we know where to add the lifetime argument
+            let impl_kw = impl_def.syntax().first_child_or_token()?.into_token()?;
+            if impl_kw.kind() != SyntaxKind::IMPL_KW {
+                return None;
+            }
+            acc.add(
+                AssistId("change_lifetime_anon_to_named"),
+                "Give anonymous lifetime a name",
+                lifetime_arg.syntax().text_range(),
+                |builder| {
+                    match impl_def.type_param_list() {
+                        Some(type_params) => {
+                            builder.insert(
+                                (u32::from(type_params.syntax().text_range().end()) - 1).into(),
+                                ", 'a",
+                            );
+                        },
+                        None => {
+                            builder.insert(
+                                impl_kw.text_range().end(),
+                                "<'a>",
+                            );
+                        },
+                    }
+                    builder.replace(lifetime_arg.syntax().text_range(), "'a");
+                },
+            )
+        }
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn test_example_case() {
+        check_assist(
+            change_lifetime_anon_to_named,
+            r#"impl Cursor<'_<|>> {
+                fn node(self) -> &SyntaxNode {
+                    match self {
+                        Cursor::Replace(node) | Cursor::Before(node) => node,
+                    }
+                }
+            }"#,
+            r#"impl<'a> Cursor<'a> {
+                fn node(self) -> &SyntaxNode {
+                    match self {
+                        Cursor::Replace(node) | Cursor::Before(node) => node,
+                    }
+                }
+            }"#,
+        );
+    }
+
+    #[test]
+    fn test_example_case_simplified() {
+        check_assist(
+            change_lifetime_anon_to_named,
+            r#"impl Cursor<'_<|>> {"#,
+            r#"impl<'a> Cursor<'a> {"#,
+        );
+    }
+
+    #[test]
+    fn test_not_applicable() {
+        check_assist_not_applicable(change_lifetime_anon_to_named, r#"impl Cursor<'_><|> {"#);
+        check_assist_not_applicable(change_lifetime_anon_to_named, r#"impl Cursor<|><'_> {"#);
+        check_assist_not_applicable(change_lifetime_anon_to_named, r#"impl Cursor<'a<|>> {"#);
+    }
+
+    #[test]
+    fn test_with_type_parameter() {
+        check_assist(
+            change_lifetime_anon_to_named,
+            r#"impl<T> Cursor<T, '_<|>>"#,
+            r#"impl<T, 'a> Cursor<T, 'a>"#,
+        );
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index 464bc03dd..3f8f7ffbf 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -112,6 +112,7 @@ mod handlers {
     mod add_turbo_fish;
     mod apply_demorgan;
     mod auto_import;
+    mod change_lifetime_anon_to_named;
     mod change_return_type_to_result;
     mod change_visibility;
     mod early_return;
@@ -151,6 +152,7 @@ mod handlers {
             add_turbo_fish::add_turbo_fish,
             apply_demorgan::apply_demorgan,
             auto_import::auto_import,
+            change_lifetime_anon_to_named::change_lifetime_anon_to_named,
             change_return_type_to_result::change_return_type_to_result,
             change_visibility::change_visibility,
             early_return::convert_to_guarded_return,
diff --git a/docs/user/assists.md b/docs/user/assists.md
index 4ad7ea59d..29b64330e 100644
--- a/docs/user/assists.md
+++ b/docs/user/assists.md
@@ -259,6 +259,30 @@ fn main() {
 }
 ```
 
+## `change_lifetime_anon_to_named`
+
+Change an anonymous lifetime to a named lifetime.
+
+```rust
+// BEFORE
+impl Cursor<'_<|>> {
+    fn node(self) -> &SyntaxNode {
+        match self {
+            Cursor::Replace(node) | Cursor::Before(node) => node,
+        }
+    }
+}
+
+// AFTER
+impl<'a> Cursor<'a> {
+    fn node(self) -> &SyntaxNode {
+        match self {
+            Cursor::Replace(node) | Cursor::Before(node) => node,
+        }
+    }
+}
+```
+
 ## `change_return_type_to_result`
 
 Change the function's return type to Result.