1 files changed, 318 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/introduce_named_lifetime.rs b/crates/assists/src/handlers/introduce_named_lifetime.rs
new file mode 100644
index 000000000..5f623e5f7
--- /dev/null
+++ b/crates/assists/src/handlers/introduce_named_lifetime.rs
@@ -0,0 +1,318 @@
+use rustc_hash::FxHashSet;
+use syntax::{
+    ast::{self, GenericParamsOwner, NameOwner},
+    AstNode, SyntaxKind, TextRange, TextSize,
+};
+
+use crate::{assist_context::AssistBuilder, AssistContext, AssistId, AssistKind, Assists};
+
+static ASSIST_NAME: &str = "introduce_named_lifetime";
+static ASSIST_LABEL: &str = "Introduce named lifetime";
+
+// Assist: introduce_named_lifetime
+//
+// 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,
+//         }
+//     }
+// }
+// ```
+// FIXME: How can we handle renaming any one of multiple anonymous lifetimes?
+// FIXME: should also add support for the case fun(f: &Foo) -> &<|>Foo
+pub(crate) fn introduce_named_lifetime(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let lifetime_token = ctx
+        .find_token_at_offset(SyntaxKind::LIFETIME)
+        .filter(|lifetime| lifetime.text() == "'_")?;
+    if let Some(fn_def) = lifetime_token.ancestors().find_map(ast::Fn::cast) {
+        generate_fn_def_assist(acc, &fn_def, lifetime_token.text_range())
+    } else if let Some(impl_def) = lifetime_token.ancestors().find_map(ast::Impl::cast) {
+        generate_impl_def_assist(acc, &impl_def, lifetime_token.text_range())
+    } else {
+        None
+    }
+}
+
+/// Generate the assist for the fn def case
+fn generate_fn_def_assist(
+    acc: &mut Assists,
+    fn_def: &ast::Fn,
+    lifetime_loc: TextRange,
+) -> Option<()> {
+    let param_list: ast::ParamList = fn_def.param_list()?;
+    let new_lifetime_param = generate_unique_lifetime_param_name(&fn_def.generic_param_list())?;
+    let end_of_fn_ident = fn_def.name()?.ident_token()?.text_range().end();
+    let self_param =
+        // use the self if it's a reference and has no explicit lifetime
+        param_list.self_param().filter(|p| p.lifetime_token().is_none() && p.amp_token().is_some());
+    // compute the location which implicitly has the same lifetime as the anonymous lifetime
+    let loc_needing_lifetime = if let Some(self_param) = self_param {
+        // if we have a self reference, use that
+        Some(self_param.self_token()?.text_range().start())
+    } else {
+        // otherwise, if there's a single reference parameter without a named liftime, use that
+        let fn_params_without_lifetime: Vec<_> = param_list
+            .params()
+            .filter_map(|param| match param.ty() {
+                Some(ast::Type::RefType(ascribed_type))
+                    if ascribed_type.lifetime_token() == None =>
+                {
+                    Some(ascribed_type.amp_token()?.text_range().end())
+                }
+                _ => None,
+            })
+            .collect();
+        match fn_params_without_lifetime.len() {
+            1 => Some(fn_params_without_lifetime.into_iter().nth(0)?),
+            0 => None,
+            // multiple unnnamed is invalid. assist is not applicable
+            _ => return None,
+        }
+    };
+    acc.add(AssistId(ASSIST_NAME, AssistKind::Refactor), ASSIST_LABEL, lifetime_loc, |builder| {
+        add_lifetime_param(fn_def, builder, end_of_fn_ident, new_lifetime_param);
+        builder.replace(lifetime_loc, format!("'{}", new_lifetime_param));
+        loc_needing_lifetime.map(|loc| builder.insert(loc, format!("'{} ", new_lifetime_param)));
+    })
+}
+
+/// Generate the assist for the impl def case
+fn generate_impl_def_assist(
+    acc: &mut Assists,
+    impl_def: &ast::Impl,
+    lifetime_loc: TextRange,
+) -> Option<()> {
+    let new_lifetime_param = generate_unique_lifetime_param_name(&impl_def.generic_param_list())?;
+    let end_of_impl_kw = impl_def.impl_token()?.text_range().end();
+    acc.add(AssistId(ASSIST_NAME, AssistKind::Refactor), ASSIST_LABEL, lifetime_loc, |builder| {
+        add_lifetime_param(impl_def, builder, end_of_impl_kw, new_lifetime_param);
+        builder.replace(lifetime_loc, format!("'{}", new_lifetime_param));
+    })
+}
+
+/// Given a type parameter list, generate a unique lifetime parameter name
+/// which is not in the list
+fn generate_unique_lifetime_param_name(
+    existing_type_param_list: &Option<ast::GenericParamList>,
+) -> Option<char> {
+    match existing_type_param_list {
+        Some(type_params) => {
+            let used_lifetime_params: FxHashSet<_> = type_params
+                .lifetime_params()
+                .map(|p| p.syntax().text().to_string()[1..].to_owned())
+                .collect();
+            (b'a'..=b'z').map(char::from).find(|c| !used_lifetime_params.contains(&c.to_string()))
+        }
+        None => Some('a'),
+    }
+}
+
+/// Add the lifetime param to `builder`. If there are type parameters in `type_params_owner`, add it to the end. Otherwise
+/// add new type params brackets with the lifetime parameter at `new_type_params_loc`.
+fn add_lifetime_param<TypeParamsOwner: ast::GenericParamsOwner>(
+    type_params_owner: &TypeParamsOwner,
+    builder: &mut AssistBuilder,
+    new_type_params_loc: TextSize,
+    new_lifetime_param: char,
+) {
+    match type_params_owner.generic_param_list() {
+        // add the new lifetime parameter to an existing type param list
+        Some(type_params) => {
+            builder.insert(
+                (u32::from(type_params.syntax().text_range().end()) - 1).into(),
+                format!(", '{}", new_lifetime_param),
+            );
+        }
+        // create a new type param list containing only the new lifetime parameter
+        None => {
+            builder.insert(new_type_params_loc, format!("<'{}>", new_lifetime_param));
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn test_example_case() {
+        check_assist(
+            introduce_named_lifetime,
+            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(
+            introduce_named_lifetime,
+            r#"impl Cursor<'_<|>> {"#,
+            r#"impl<'a> Cursor<'a> {"#,
+        );
+    }
+
+    #[test]
+    fn test_example_case_cursor_after_tick() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"impl Cursor<'<|>_> {"#,
+            r#"impl<'a> Cursor<'a> {"#,
+        );
+    }
+
+    #[test]
+    fn test_impl_with_other_type_param() {
+        check_assist(
+            introduce_named_lifetime,
+            "impl<I> fmt::Display for SepByBuilder<'_<|>, I>
+        where
+            I: Iterator,
+            I::Item: fmt::Display,
+        {",
+            "impl<I, 'a> fmt::Display for SepByBuilder<'a, I>
+        where
+            I: Iterator,
+            I::Item: fmt::Display,
+        {",
+        )
+    }
+
+    #[test]
+    fn test_example_case_cursor_before_tick() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"impl Cursor<<|>'_> {"#,
+            r#"impl<'a> Cursor<'a> {"#,
+        );
+    }
+
+    #[test]
+    fn test_not_applicable_cursor_position() {
+        check_assist_not_applicable(introduce_named_lifetime, r#"impl Cursor<'_><|> {"#);
+        check_assist_not_applicable(introduce_named_lifetime, r#"impl Cursor<|><'_> {"#);
+    }
+
+    #[test]
+    fn test_not_applicable_lifetime_already_name() {
+        check_assist_not_applicable(introduce_named_lifetime, r#"impl Cursor<'a<|>> {"#);
+        check_assist_not_applicable(introduce_named_lifetime, r#"fn my_fun<'a>() -> X<'a<|>>"#);
+    }
+
+    #[test]
+    fn test_with_type_parameter() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"impl<T> Cursor<T, '_<|>>"#,
+            r#"impl<T, 'a> Cursor<T, 'a>"#,
+        );
+    }
+
+    #[test]
+    fn test_with_existing_lifetime_name_conflict() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"impl<'a, 'b> Cursor<'a, 'b, '_<|>>"#,
+            r#"impl<'a, 'b, 'c> Cursor<'a, 'b, 'c>"#,
+        );
+    }
+
+    #[test]
+    fn test_function_return_value_anon_lifetime_param() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun() -> X<'_<|>>"#,
+            r#"fn my_fun<'a>() -> X<'a>"#,
+        );
+    }
+
+    #[test]
+    fn test_function_return_value_anon_reference_lifetime() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun() -> &'_<|> X"#,
+            r#"fn my_fun<'a>() -> &'a X"#,
+        );
+    }
+
+    #[test]
+    fn test_function_param_anon_lifetime() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun(x: X<'_<|>>)"#,
+            r#"fn my_fun<'a>(x: X<'a>)"#,
+        );
+    }
+
+    #[test]
+    fn test_function_add_lifetime_to_params() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun(f: &Foo) -> X<'_<|>>"#,
+            r#"fn my_fun<'a>(f: &'a Foo) -> X<'a>"#,
+        );
+    }
+
+    #[test]
+    fn test_function_add_lifetime_to_params_in_presence_of_other_lifetime() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun<'other>(f: &Foo, b: &'other Bar) -> X<'_<|>>"#,
+            r#"fn my_fun<'other, 'a>(f: &'a Foo, b: &'other Bar) -> X<'a>"#,
+        );
+    }
+
+    #[test]
+    fn test_function_not_applicable_without_self_and_multiple_unnamed_param_lifetimes() {
+        // this is not permitted under lifetime elision rules
+        check_assist_not_applicable(
+            introduce_named_lifetime,
+            r#"fn my_fun(f: &Foo, b: &Bar) -> X<'_<|>>"#,
+        );
+    }
+
+    #[test]
+    fn test_function_add_lifetime_to_self_ref_param() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun<'other>(&self, f: &Foo, b: &'other Bar) -> X<'_<|>>"#,
+            r#"fn my_fun<'other, 'a>(&'a self, f: &Foo, b: &'other Bar) -> X<'a>"#,
+        );
+    }
+
+    #[test]
+    fn test_function_add_lifetime_to_param_with_non_ref_self() {
+        check_assist(
+            introduce_named_lifetime,
+            r#"fn my_fun<'other>(self, f: &Foo, b: &'other Bar) -> X<'_<|>>"#,
+            r#"fn my_fun<'other, 'a>(self, f: &'a Foo, b: &'other Bar) -> X<'a>"#,
+        );
+    }
+}