refactor: use generate_trait_impl_text_intransitive for From-like…

… traits

crates/ide-assists/src/handlers/generate_from_impl_for_enum.rs

@@ -1,7 +1,9 @@
 use ide_db::{famous_defs::FamousDefs, RootDatabase};
 use syntax::ast::{self, AstNode, HasName};
 
-use crate::{utils::generate_trait_impl_text, AssistContext, AssistId, AssistKind, Assists};
+use crate::{
+    utils::generate_trait_impl_text_intransitive, AssistContext, AssistId, AssistKind, Assists,
+};
 
 // Assist: generate_from_impl_for_enum
 //
@@ -70,7 +72,7 @@ pub(crate) fn generate_from_impl_for_enum(
     }}"#
                 )
             };
-            let from_impl = generate_trait_impl_text(&enum_, &from_trait, &impl_code);
+            let from_impl = generate_trait_impl_text_intransitive(&enum_, &from_trait, &impl_code);
             edit.insert(start_offset, from_impl);
         },
     )

crates/ide-assists/src/handlers/generate_impl.rs

@@ -1,7 +1,7 @@
 use syntax::ast::{self, AstNode, HasName};
 
 use crate::{
-    utils::{generate_impl_text, generate_trait_impl_text},
+    utils::{generate_impl_text, generate_trait_impl_text_intransitive},
     AssistContext, AssistId, AssistKind, Assists,
 };
 
@@ -89,11 +89,11 @@ pub(crate) fn generate_trait_impl(acc: &mut Assists, ctx: &AssistContext<'_>) ->
             let start_offset = nominal.syntax().text_range().end();
             match ctx.config.snippet_cap {
                 Some(cap) => {
-                    let snippet = generate_trait_impl_text(&nominal, "$0", "");
+                    let snippet = generate_trait_impl_text_intransitive(&nominal, "$0", "");
                     edit.insert_snippet(cap, start_offset, snippet);
                 }
                 None => {
-                    let text = generate_trait_impl_text(&nominal, "", "");
+                    let text = generate_trait_impl_text_intransitive(&nominal, "", "");
                     edit.insert(start_offset, text);
                 }
             }

crates/ide-assists/src/utils.rs

@@ -5,7 +5,6 @@ use std::ops;
 pub(crate) use gen_trait_fn_body::gen_trait_fn_body;
 use hir::{db::HirDatabase, HirDisplay, Semantics};
 use ide_db::{famous_defs::FamousDefs, path_transform::PathTransform, RootDatabase, SnippetCap};
-use itertools::Itertools;
 use stdx::format_to;
 use syntax::{
     ast::{
@@ -435,52 +434,67 @@ pub(crate) fn find_impl_block_end(impl_def: ast::Impl, buf: &mut String) -> Opti
     Some(end)
 }
 
-// Generates the surrounding `impl Type { <code> }` including type and lifetime
-// parameters
+/// Generates the surrounding `impl Type { <code> }` including type and lifetime
+/// parameters.
 pub(crate) fn generate_impl_text(adt: &ast::Adt, code: &str) -> String {
-    generate_impl_text_inner(adt, None, code)
+    generate_impl_text_inner(adt, None, true, code)
 }
 
-// Generates the surrounding `impl <trait> for Type { <code> }` including type
-// and lifetime parameters
+/// Generates the surrounding `impl <trait> for Type { <code> }` including type
+/// and lifetime parameters, with `<trait>` appended to `impl`'s generic parameters' bounds.
+///
+/// This is useful for traits like `PartialEq`, since `impl<T> PartialEq for U<T>` often requires `T: PartialEq`.
 pub(crate) fn generate_trait_impl_text(adt: &ast::Adt, trait_text: &str, code: &str) -> String {
-    generate_impl_text_inner(adt, Some(trait_text), code)
+    generate_impl_text_inner(adt, Some(trait_text), true, code)
+}
+
+/// Generates the surrounding `impl <trait> for Type { <code> }` including type
+/// and lifetime parameters, with `impl`'s generic parameters' bounds kept as-is.
+///
+/// This is useful for traits like `From<T>`, since `impl<T> From<T> for U<T>` doesn't require `T: From<T>`.
+pub(crate) fn generate_trait_impl_text_intransitive(
+    adt: &ast::Adt,
+    trait_text: &str,
+    code: &str,
+) -> String {
+    generate_impl_text_inner(adt, Some(trait_text), false, code)
 }
 
-fn generate_impl_text_inner(adt: &ast::Adt, trait_text: Option<&str>, code: &str) -> String {
+fn generate_impl_text_inner(
+    adt: &ast::Adt,
+    trait_text: Option<&str>,
+    trait_is_transitive: bool,
+    code: &str,
+) -> String {
     // Ensure lifetime params are before type & const params
     let generic_params = adt.generic_param_list().map(|generic_params| {
         let lifetime_params =
             generic_params.lifetime_params().map(ast::GenericParam::LifetimeParam);
-        let ty_or_const_params = generic_params.type_or_const_params().filter_map(|param| {
+        let ty_or_const_params = generic_params.type_or_const_params().map(|param| {
             match param {
                 ast::TypeOrConstParam::Type(param) => {
                     let param = param.clone_for_update();
                     // remove defaults since they can't be specified in impls
                     param.remove_default();
-                    let mut bounds = param
-                        .type_bound_list()
-                        .map_or_else(Vec::new, |it| it.bounds().collect_vec());
-                    // `{ty_param}: {trait_text}`
+                    let mut bounds =
+                        param.type_bound_list().map_or_else(Vec::new, |it| it.bounds().collect());
                     if let Some(trait_) = trait_text {
-                        // Defense against the following cases:
-                        // - The trait is undetermined, e.g. `$0`.
-                        // - The trait is a `From`, e.g. `From<T>`.
-                        if !trait_.starts_with('$')
-                            && !matches!(trait_.split_once('<'), Some((left, _right)) if left.trim() == "From")
-                        {
+                        // Add the current trait to `bounds` if the trait is transitive,
+                        // meaning `impl<T> Trait for U<T>` requires `T: Trait`.
+                        if trait_is_transitive {
                             bounds.push(make::type_bound(trait_));
                         }
                     };
+                    // `{ty_param}: {bounds}`
                     let param =
                         make::type_param(param.name().unwrap(), make::type_bound_list(bounds));
-                    Some(ast::GenericParam::TypeParam(param))
+                    ast::GenericParam::TypeParam(param)
                 }
                 ast::TypeOrConstParam::Const(param) => {
                     let param = param.clone_for_update();
                     // remove defaults since they can't be specified in impls
                     param.remove_default();
-                    Some(ast::GenericParam::ConstParam(param))
+                    ast::GenericParam::ConstParam(param)
                 }
             }
         });

crates/syntax/src/ast/make.rs

@@ -734,8 +734,8 @@ pub fn type_bound_list(
 }
 
 pub fn type_param(name: ast::Name, bounds: Option<ast::TypeBoundList>) -> ast::TypeParam {
-    let bound = bounds.map_or_else(String::new, |it| format!(": {it}"));
-    ast_from_text(&format!("fn f<{name}{bound}>() {{ }}"))
+    let bounds = bounds.map_or_else(String::new, |it| format!(": {it}"));
+    ast_from_text(&format!("fn f<{name}{bounds}>() {{ }}"))
 }
 
 pub fn lifetime_param(lifetime: ast::Lifetime) -> ast::LifetimeParam {