Auto merge of #14816 - justahero:gh-14626, r=Veykril

feat: Assist to replace generic with impl trait

This adds a new assist named "Replace named generic with impl". It is the inverse operation to the existing "Replace impl trait with generic" assist.

It allows to refactor the following statement:

```rust
//      👇 cursor
fn new<T$0: ToString>(input: T) -> Self {}
```

to be transformed into:

```rust
fn new(input: impl ToString) -> Self {}
```

* adds new helper function `impl_trait_type` to create AST node
* add method to remove an existing generic param type from param list

Closes #14626

5 files changed, 247 insertions, 0 deletions

diff --git a/crates/ide-assists/src/handlers/replace_named_generic_with_impl.rs b/crates/ide-assists/src/handlers/replace_named_generic_with_impl.rs
new file mode 100644
index 00000000000..f32ceb42654
--- /dev/null
+++ b/crates/ide-assists/src/handlers/replace_named_generic_with_impl.rs
@@ -0,0 +1,213 @@
+use hir::Semantics;
+use ide_db::{
+    base_db::{FileId, FileRange},
+    defs::Definition,
+    search::SearchScope,
+    RootDatabase,
+};
+use syntax::{
+    ast::{self, make::impl_trait_type, HasGenericParams, HasName, HasTypeBounds},
+    ted, AstNode,
+};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: replace_named_generic_with_impl
+//
+// Replaces named generic with an `impl Trait` in function argument.
+//
+// ```
+// fn new<P$0: AsRef<Path>>(location: P) -> Self {}
+// ```
+// ->
+// ```
+// fn new(location: impl AsRef<Path>) -> Self {}
+// ```
+pub(crate) fn replace_named_generic_with_impl(
+    acc: &mut Assists,
+    ctx: &AssistContext<'_>,
+) -> Option<()> {
+    // finds `<P: AsRef<Path>>`
+    let type_param = ctx.find_node_at_offset::<ast::TypeParam>()?;
+    // returns `P`
+    let type_param_name = type_param.name()?;
+
+    // The list of type bounds / traits: `AsRef<Path>`
+    let type_bound_list = type_param.type_bound_list()?;
+
+    let fn_ = type_param.syntax().ancestors().find_map(ast::Fn::cast)?;
+    let params = fn_
+        .param_list()?
+        .params()
+        .filter_map(|param| {
+            // function parameter type needs to match generic type name
+            if let ast::Type::PathType(path_type) = param.ty()? {
+                let left = path_type.path()?.segment()?.name_ref()?.ident_token()?.to_string();
+                let right = type_param_name.to_string();
+                if left == right {
+                    Some(param)
+                } else {
+                    None
+                }
+            } else {
+                None
+            }
+        })
+        .collect::<Vec<_>>();
+
+    if params.is_empty() {
+        return None;
+    }
+
+    let type_param_hir_def = ctx.sema.to_def(&type_param)?;
+    let type_param_def = Definition::GenericParam(hir::GenericParam::TypeParam(type_param_hir_def));
+
+    if is_referenced_outside(&ctx.sema, type_param_def, &fn_, ctx.file_id()) {
+        return None;
+    }
+
+    let target = type_param.syntax().text_range();
+
+    acc.add(
+        AssistId("replace_named_generic_with_impl", AssistKind::RefactorRewrite),
+        "Replace named generic with impl",
+        target,
+        |edit| {
+            let type_param = edit.make_mut(type_param);
+            let fn_ = edit.make_mut(fn_);
+
+            // get all params
+            let param_types = params
+                .iter()
+                .filter_map(|param| match param.ty() {
+                    Some(ast::Type::PathType(param_type)) => Some(edit.make_mut(param_type)),
+                    _ => None,
+                })
+                .collect::<Vec<_>>();
+
+            if let Some(generic_params) = fn_.generic_param_list() {
+                generic_params.remove_generic_param(ast::GenericParam::TypeParam(type_param));
+                if generic_params.generic_params().count() == 0 {
+                    ted::remove(generic_params.syntax());
+                }
+            }
+
+            // get type bounds in signature type: `P` -> `impl AsRef<Path>`
+            let new_bounds = impl_trait_type(type_bound_list);
+            for param_type in param_types.iter().rev() {
+                ted::replace(param_type.syntax(), new_bounds.clone_for_update().syntax());
+            }
+        },
+    )
+}
+
+fn is_referenced_outside(
+    sema: &Semantics<'_, RootDatabase>,
+    type_param_def: Definition,
+    fn_: &ast::Fn,
+    file_id: FileId,
+) -> bool {
+    // limit search scope to function body & return type
+    let search_ranges = vec![
+        fn_.body().map(|body| body.syntax().text_range()),
+        fn_.ret_type().map(|ret_type| ret_type.syntax().text_range()),
+    ];
+
+    search_ranges.into_iter().flatten().any(|search_range| {
+        let file_range = FileRange { file_id, range: search_range };
+        !type_param_def.usages(sema).in_scope(SearchScope::file_range(file_range)).all().is_empty()
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn replace_generic_moves_into_function() {
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<T$0: ToString>(input: T) -> Self {}"#,
+            r#"fn new(input: impl ToString) -> Self {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_with_inner_associated_type() {
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<P$0: AsRef<Path>>(input: P) -> Self {}"#,
+            r#"fn new(input: impl AsRef<Path>) -> Self {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_trait_applies_to_all_matching_params() {
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<T$0: ToString>(a: T, b: T) -> Self {}"#,
+            r#"fn new(a: impl ToString, b: impl ToString) -> Self {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_with_multiple_generic_params() {
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<P: AsRef<Path>, T$0: ToString>(t: T, p: P) -> Self {}"#,
+            r#"fn new<P: AsRef<Path>>(t: impl ToString, p: P) -> Self {}"#,
+        );
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<T$0: ToString, P: AsRef<Path>>(t: T, p: P) -> Self {}"#,
+            r#"fn new<P: AsRef<Path>>(t: impl ToString, p: P) -> Self {}"#,
+        );
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<A: Send, B$0: ToString, C: Debug>(a: A, b: B, c: C) -> Self {}"#,
+            r#"fn new<A: Send, C: Debug>(a: A, b: impl ToString, c: C) -> Self {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_with_multiple_trait_bounds() {
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"fn new<P$0: Send + Sync>(p: P) -> Self {}"#,
+            r#"fn new(p: impl Send + Sync) -> Self {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_not_applicable_if_param_used_as_return_type() {
+        check_assist_not_applicable(
+            replace_named_generic_with_impl,
+            r#"fn new<P$0: Send + Sync>(p: P) -> P {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_not_applicable_if_param_used_in_fn_body() {
+        check_assist_not_applicable(
+            replace_named_generic_with_impl,
+            r#"fn new<P$0: ToString>(p: P) { let x: &dyn P = &O; }"#,
+        );
+    }
+
+    #[test]
+    fn replace_generic_ignores_another_function_with_same_param_type() {
+        check_assist(
+            replace_named_generic_with_impl,
+            r#"
+            fn new<P$0: Send + Sync>(p: P) {}
+            fn hello<P: Debug>(p: P) { println!("{:?}", p); }
+            "#,
+            r#"
+            fn new(p: impl Send + Sync) {}
+            fn hello<P: Debug>(p: P) { println!("{:?}", p); }
+            "#,
+        );
+    }
+}
diff --git a/crates/ide-assists/src/lib.rs b/crates/ide-assists/src/lib.rs
index fc03903e593..bd282e53434 100644
--- a/crates/ide-assists/src/lib.rs
+++ b/crates/ide-assists/src/lib.rs
@@ -193,6 +193,7 @@ mod handlers {
     mod replace_arith_op;
     mod introduce_named_generic;
     mod replace_let_with_if_let;
+    mod replace_named_generic_with_impl;
     mod replace_qualified_name_with_use;
     mod replace_string_with_char;
     mod replace_turbofish_with_explicit_type;
@@ -299,6 +300,7 @@ mod handlers {
             replace_let_with_if_let::replace_let_with_if_let,
             replace_method_eager_lazy::replace_with_eager_method,
             replace_method_eager_lazy::replace_with_lazy_method,
+            replace_named_generic_with_impl::replace_named_generic_with_impl,
             replace_turbofish_with_explicit_type::replace_turbofish_with_explicit_type,
             replace_qualified_name_with_use::replace_qualified_name_with_use,
             replace_arith_op::replace_arith_with_wrapping,
diff --git a/crates/ide-assists/src/tests/generated.rs b/crates/ide-assists/src/tests/generated.rs
index 0096254ecb7..8a35fd290e6 100644
--- a/crates/ide-assists/src/tests/generated.rs
+++ b/crates/ide-assists/src/tests/generated.rs
@@ -2339,6 +2339,19 @@ fn handle(action: Action) {
 }
 
 #[test]
+fn doctest_replace_named_generic_with_impl() {
+    check_doc_test(
+        "replace_named_generic_with_impl",
+        r#####"
+fn new<P$0: AsRef<Path>>(location: P) -> Self {}
+"#####,
+        r#####"
+fn new(location: impl AsRef<Path>) -> Self {}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_replace_qualified_name_with_use() {
     check_doc_test(
         "replace_qualified_name_with_use",
diff --git a/crates/syntax/src/ast/edit_in_place.rs b/crates/syntax/src/ast/edit_in_place.rs
index 995e8d8d152..b3ea6ca8d46 100644
--- a/crates/syntax/src/ast/edit_in_place.rs
+++ b/crates/syntax/src/ast/edit_in_place.rs
@@ -236,6 +236,21 @@ impl ast::GenericParamList {
         }
     }
 
+    /// Removes the existing generic param
+    pub fn remove_generic_param(&self, generic_param: ast::GenericParam) {
+        if let Some(previous) = generic_param.syntax().prev_sibling() {
+            if let Some(next_token) = previous.next_sibling_or_token() {
+                ted::remove_all(next_token..=generic_param.syntax().clone().into());
+            }
+        } else if let Some(next) = generic_param.syntax().next_sibling() {
+            if let Some(next_token) = next.prev_sibling_or_token() {
+                ted::remove_all(generic_param.syntax().clone().into()..=next_token);
+            }
+        } else {
+            ted::remove(generic_param.syntax());
+        }
+    }
+
     /// Constructs a matching [`ast::GenericArgList`]
     pub fn to_generic_args(&self) -> ast::GenericArgList {
         let args = self.generic_params().filter_map(|param| match param {
diff --git a/crates/syntax/src/ast/make.rs b/crates/syntax/src/ast/make.rs
index 8d2dc8709ba..3a61fb0a52c 100644
--- a/crates/syntax/src/ast/make.rs
+++ b/crates/syntax/src/ast/make.rs
@@ -232,6 +232,10 @@ pub fn impl_trait(
     ast_from_text(&format!("impl{ty_params_str} {trait_} for {ty}{ty_genargs_str} {{}}"))
 }
 
+pub fn impl_trait_type(bounds: ast::TypeBoundList) -> ast::ImplTraitType {
+    ast_from_text(&format!("fn f(x: impl {bounds}) {{}}"))
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("type __ = {name_ref};"))
 }