Auto merge of rust-lang#17094 - Lindronics:convert-from-to-tryfrom, r=Veykril

feat: Add convert From to TryFrom assist

Adds a new code assist to convert a `From` impl into a `TryFrom` impl.

This is useful in situations where it turns out after or halfway through writing a `From` implementation that the conversion is actually fallible.

## Example

https://github.com/rust-lang/rust-analyzer/assets/26360861/872ec7c4-c9ff-451c-9453-4baaaad47326

Author: bors

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

@@ -0,0 +1,250 @@
+use ide_db::{famous_defs::FamousDefs, traits::resolve_target_trait};
+use itertools::Itertools;
+use syntax::{
+    ast::{self, make, AstNode, HasName},
+    ted,
+};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: convert_from_to_tryfrom
+//
+// Converts a From impl to a TryFrom impl, wrapping returns in `Ok`.
+//
+// ```
+// # //- minicore: from
+// impl $0From<usize> for Thing {
+//     fn from(val: usize) -> Self {
+//         Thing {
+//             b: val.to_string(),
+//             a: val
+//         }
+//     }
+// }
+// ```
+// ->
+// ```
+// impl TryFrom<usize> for Thing {
+//     type Error = ${0:()};
+//
+//     fn try_from(val: usize) -> Result<Self, Self::Error> {
+//         Ok(Thing {
+//             b: val.to_string(),
+//             a: val
+//         })
+//     }
+// }
+// ```
+pub(crate) fn convert_from_to_tryfrom(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let impl_ = ctx.find_node_at_offset::<ast::Impl>()?;
+    let trait_ty = impl_.trait_()?;
+
+    let module = ctx.sema.scope(impl_.syntax())?.module();
+
+    let from_type = match &trait_ty {
+        ast::Type::PathType(path) => {
+            path.path()?.segment()?.generic_arg_list()?.generic_args().next()?
+        }
+        _ => return None,
+    };
+
+    let associated_items = impl_.assoc_item_list()?;
+    let from_fn = associated_items.assoc_items().find_map(|item| {
+        if let ast::AssocItem::Fn(f) = item {
+            if f.name()?.text() == "from" {
+                return Some(f);
+            }
+        };
+        None
+    })?;
+
+    let from_fn_name = from_fn.name()?;
+    let from_fn_return_type = from_fn.ret_type()?.ty()?;
+
+    let return_exprs = from_fn.body()?.syntax().descendants().filter_map(ast::ReturnExpr::cast);
+    let tail_expr = from_fn.body()?.tail_expr()?;
+
+    if resolve_target_trait(&ctx.sema, &impl_)?
+        != FamousDefs(&ctx.sema, module.krate()).core_convert_From()?
+    {
+        return None;
+    }
+
+    acc.add(
+        AssistId("convert_from_to_tryfrom", AssistKind::RefactorRewrite),
+        "Convert From to TryFrom",
+        impl_.syntax().text_range(),
+        |builder| {
+            let trait_ty = builder.make_mut(trait_ty);
+            let from_fn_return_type = builder.make_mut(from_fn_return_type);
+            let from_fn_name = builder.make_mut(from_fn_name);
+            let tail_expr = builder.make_mut(tail_expr);
+            let return_exprs = return_exprs.map(|r| builder.make_mut(r)).collect_vec();
+            let associated_items = builder.make_mut(associated_items).clone();
+
+            ted::replace(
+                trait_ty.syntax(),
+                make::ty(&format!("TryFrom<{from_type}>")).syntax().clone_for_update(),
+            );
+            ted::replace(
+                from_fn_return_type.syntax(),
+                make::ty("Result<Self, Self::Error>").syntax().clone_for_update(),
+            );
+            ted::replace(from_fn_name.syntax(), make::name("try_from").syntax().clone_for_update());
+            ted::replace(
+                tail_expr.syntax(),
+                wrap_ok(tail_expr.clone()).syntax().clone_for_update(),
+            );
+
+            for r in return_exprs {
+                let t = r.expr().unwrap_or_else(make::expr_unit);
+                ted::replace(t.syntax(), wrap_ok(t.clone()).syntax().clone_for_update());
+            }
+
+            let error_type = ast::AssocItem::TypeAlias(make::ty_alias(
+                "Error",
+                None,
+                None,
+                None,
+                Some((make::ty_unit(), None)),
+            ))
+            .clone_for_update();
+
+            if let Some(cap) = ctx.config.snippet_cap {
+                if let ast::AssocItem::TypeAlias(type_alias) = &error_type {
+                    if let Some(ty) = type_alias.ty() {
+                        builder.add_placeholder_snippet(cap, ty);
+                    }
+                }
+            }
+
+            associated_items.add_item_at_start(error_type);
+        },
+    )
+}
+
+fn wrap_ok(expr: ast::Expr) -> ast::Expr {
+    make::expr_call(
+        make::expr_path(make::ext::ident_path("Ok")),
+        make::arg_list(std::iter::once(expr)),
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn converts_from_to_tryfrom() {
+        check_assist(
+            convert_from_to_tryfrom,
+            r#"
+//- minicore: from
+struct Foo(String);
+
+impl $0From<String> for Foo {
+    fn from(val: String) -> Self {
+        if val == "bar" {
+            return Foo(val);
+        }
+        Self(val)
+    }
+}
+            "#,
+            r#"
+struct Foo(String);
+
+impl TryFrom<String> for Foo {
+    type Error = ${0:()};
+
+    fn try_from(val: String) -> Result<Self, Self::Error> {
+        if val == "bar" {
+            return Ok(Foo(val));
+        }
+        Ok(Self(val))
+    }
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn converts_from_to_tryfrom_nested_type() {
+        check_assist(
+            convert_from_to_tryfrom,
+            r#"
+//- minicore: from
+struct Foo(String);
+
+impl $0From<Option<String>> for Foo {
+    fn from(val: Option<String>) -> Self {
+        match val {
+            Some(val) => Foo(val),
+            None => Foo("".to_string())
+        }
+    }
+}
+            "#,
+            r#"
+struct Foo(String);
+
+impl TryFrom<Option<String>> for Foo {
+    type Error = ${0:()};
+
+    fn try_from(val: Option<String>) -> Result<Self, Self::Error> {
+        Ok(match val {
+            Some(val) => Foo(val),
+            None => Foo("".to_string())
+        })
+    }
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn converts_from_to_tryfrom_preserves_lifetimes() {
+        check_assist(
+            convert_from_to_tryfrom,
+            r#"
+//- minicore: from
+struct Foo<'a>(&'a str);
+
+impl<'a> $0From<&'a str> for Foo<'a> {
+    fn from(val: &'a str) -> Self {
+        Self(val)
+    }
+}
+            "#,
+            r#"
+struct Foo<'a>(&'a str);
+
+impl<'a> TryFrom<&'a str> for Foo<'a> {
+    type Error = ${0:()};
+
+    fn try_from(val: &'a str) -> Result<Self, Self::Error> {
+        Ok(Self(val))
+    }
+}
+            "#,
+        );
+    }
+
+    #[test]
+    fn other_trait_not_applicable() {
+        check_assist_not_applicable(
+            convert_from_to_tryfrom,
+            r#"
+struct Foo(String);
+
+impl $0TryFrom<String> for Foo {
+    fn try_from(val: String) -> Result<Self, Self::Error> {
+        Ok(Self(val))
+    }
+}
+            "#,
+        );
+    }
+}

crates/ide-assists/src/lib.rs

@@ -116,6 +116,7 @@ mod handlers {
     mod change_visibility;
     mod convert_bool_then;
     mod convert_comment_block;
+    mod convert_from_to_tryfrom;
     mod convert_integer_literal;
     mod convert_into_to_from;
     mod convert_iter_for_each_to_for;
@@ -238,6 +239,7 @@ mod handlers {
             convert_bool_then::convert_bool_then_to_if,
             convert_bool_then::convert_if_to_bool_then,
             convert_comment_block::convert_comment_block,
+            convert_from_to_tryfrom::convert_from_to_tryfrom,
             convert_integer_literal::convert_integer_literal,
             convert_into_to_from::convert_into_to_from,
             convert_iter_for_each_to_for::convert_iter_for_each_to_for,

crates/ide-assists/src/tests/generated.rs

@@ -390,6 +390,36 @@ fn main() {
     )
 }
 
+#[test]
+fn doctest_convert_from_to_tryfrom() {
+    check_doc_test(
+        "convert_from_to_tryfrom",
+        r#####"
+//- minicore: from
+impl $0From<usize> for Thing {
+    fn from(val: usize) -> Self {
+        Thing {
+            b: val.to_string(),
+            a: val
+        }
+    }
+}
+"#####,
+        r#####"
+impl TryFrom<usize> for Thing {
+    type Error = ${0:()};
+
+    fn try_from(val: usize) -> Result<Self, Self::Error> {
+        Ok(Thing {
+            b: val.to_string(),
+            a: val
+        })
+    }
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_convert_if_to_bool_then() {
     check_doc_test(