3 files changed, 252 insertions, 0 deletions

diff --git a/crates/ide_assists/src/handlers/generate_is_empty_from_len.rs b/crates/ide_assists/src/handlers/generate_is_empty_from_len.rs
new file mode 100644
index 000000000..bd29dddb3
--- /dev/null
+++ b/crates/ide_assists/src/handlers/generate_is_empty_from_len.rs
@@ -0,0 +1,225 @@
+use hir::{AssocItem, HasSource, Impl};
+use syntax::{
+    ast::{self, NameOwner},
+    AstNode, TextRange,
+};
+
+use crate::{
+    assist_context::{AssistContext, Assists},
+    AssistId, AssistKind,
+};
+
+// Assist: generate_is_empty_from_len
+//
+// Generates is_empty implementation from the len method.
+//
+// ```
+// impl MyStruct {
+//     p$0ub fn len(&self) -> usize {
+//         self.data.len()
+//     }
+// }
+// ```
+// ->
+// ```
+// impl MyStruct {
+//     pub fn len(&self) -> usize {
+//         self.data.len()
+//     }
+//
+//     pub fn is_empty(&self) -> bool {
+//         self.len() == 0
+//     }
+// }
+// ```
+pub(crate) fn generate_is_empty_from_len(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let fn_node = ctx.find_node_at_offset::<ast::Fn>()?;
+    let fn_name = fn_node.name()?;
+
+    if fn_name.text() != "len" {
+        cov_mark::hit!(len_function_not_present);
+        return None;
+    }
+
+    if fn_node.param_list()?.params().next().is_some() {
+        cov_mark::hit!(len_function_with_parameters);
+        return None;
+    }
+
+    let impl_ = fn_node.syntax().ancestors().into_iter().find_map(ast::Impl::cast)?;
+    let impl_def = ctx.sema.to_def(&impl_)?;
+    if is_empty_implemented(ctx, &impl_def) {
+        cov_mark::hit!(is_empty_already_implemented);
+        return None;
+    }
+
+    let range = get_text_range_of_len_function(ctx, &impl_def)?;
+
+    acc.add(
+        AssistId("generate_is_empty_from_len", AssistKind::Generate),
+        "Generate a is_empty impl from a len function",
+        range,
+        |builder| {
+            let code = get_is_empty_code();
+            builder.insert(range.end(), code)
+        },
+    )
+}
+
+fn get_function_from_impl(ctx: &AssistContext, impl_def: &Impl, name: &str) -> Option<AssocItem> {
+    let db = ctx.sema.db;
+    impl_def.items(db).into_iter().filter(|item| matches!(item, AssocItem::Function(_value))).find(
+        |func| match func.name(db) {
+            Some(fn_name) => fn_name.to_string() == name,
+            None => false,
+        },
+    )
+}
+
+fn is_empty_implemented(ctx: &AssistContext, impl_def: &Impl) -> bool {
+    get_function_from_impl(ctx, impl_def, "is_empty").is_some()
+}
+
+fn get_text_range_of_len_function(ctx: &AssistContext, impl_def: &Impl) -> Option<TextRange> {
+    let db = ctx.sema.db;
+    let len_fn = get_function_from_impl(ctx, impl_def, "len")?;
+
+    let mut range = None;
+    if let AssocItem::Function(node) = len_fn {
+        let node = node.source(db)?;
+        range = Some(node.syntax().value.text_range());
+    }
+
+    range
+}
+
+fn get_is_empty_code() -> String {
+    r#"
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }"#
+    .to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn len_function_not_present() {
+        cov_mark::check!(len_function_not_present);
+        check_assist_not_applicable(
+            generate_is_empty_from_len,
+            r#"
+impl MyStruct {
+    p$0ub fn test(&self) -> usize {
+            self.data.len()
+        }
+    }
+"#,
+        );
+    }
+
+    #[test]
+    fn len_function_with_parameters() {
+        cov_mark::check!(len_function_with_parameters);
+        check_assist_not_applicable(
+            generate_is_empty_from_len,
+            r#"
+impl MyStruct {
+    p$0ub fn len(&self, _i: bool) -> usize {
+        self.data.len()
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn is_empty_already_implemented() {
+        cov_mark::check!(is_empty_already_implemented);
+        check_assist_not_applicable(
+            generate_is_empty_from_len,
+            r#"
+impl MyStruct {
+    p$0ub fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn generate_is_empty() {
+        check_assist(
+            generate_is_empty_from_len,
+            r#"
+impl MyStruct {
+    p$0ub fn len(&self) -> usize {
+        self.data.len()
+    }
+}
+"#,
+            r#"
+impl MyStruct {
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn multiple_functions_in_impl() {
+        check_assist(
+            generate_is_empty_from_len,
+            r#"
+impl MyStruct {
+    pub fn new() -> Self {
+        Self { data: 0 }
+    }
+
+    p$0ub fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    pub fn work(&self) -> Option<usize> {
+        // do some work
+    }
+}
+"#,
+            r#"
+impl MyStruct {
+    pub fn new() -> Self {
+        Self { data: 0 }
+    }
+
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    pub fn work(&self) -> Option<usize> {
+        // do some work
+    }
+}
+"#,
+        );
+    }
+}
diff --git a/crates/ide_assists/src/lib.rs b/crates/ide_assists/src/lib.rs
index f1aab74d4..8c068a6c0 100644
--- a/crates/ide_assists/src/lib.rs
+++ b/crates/ide_assists/src/lib.rs
@@ -129,6 +129,7 @@ mod handlers {
     mod flip_trait_bound;
     mod generate_default_from_enum_variant;
     mod generate_default_from_new;
+    mod generate_is_empty_from_len;
     mod generate_derive;
     mod generate_enum_is_method;
     mod generate_enum_projection_method;
@@ -193,6 +194,7 @@ mod handlers {
             flip_trait_bound::flip_trait_bound,
             generate_default_from_enum_variant::generate_default_from_enum_variant,
             generate_default_from_new::generate_default_from_new,
+            generate_is_empty_from_len::generate_is_empty_from_len,
             generate_derive::generate_derive,
             generate_enum_is_method::generate_enum_is_method,
             generate_enum_projection_method::generate_enum_as_method,
diff --git a/crates/ide_assists/src/tests/generated.rs b/crates/ide_assists/src/tests/generated.rs
index 3f77edd8d..66fbcc968 100644
--- a/crates/ide_assists/src/tests/generated.rs
+++ b/crates/ide_assists/src/tests/generated.rs
@@ -722,6 +722,31 @@ impl<T: Clone> Ctx<T> {
 }
 
 #[test]
+fn doctest_generate_is_empty_from_len() {
+    check_doc_test(
+        "generate_is_empty_from_len",
+        r#####"
+impl MyStruct {
+    p$0ub fn len(&self) -> usize {
+        self.data.len()
+    }
+}
+"#####,
+        r#####"
+impl MyStruct {
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_generate_new() {
     check_doc_test(
         "generate_new",