From e9b410d29ce996abc31bd85f8168936f6eeda2aa Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Mon, 20 Jan 2025 16:42:34 +0100
Subject: [PATCH] Goto `Display::fmt` when invoked on `to_string`

---
 .../rust-analyzer/crates/hir/src/semantics.rs |  2 +
 .../crates/ide-db/src/famous_defs.rs          |  7 ++++
 .../crates/ide/src/goto_definition.rs         | 39 +++++++++++++++++++
 .../crates/intern/src/symbol/symbols.rs       |  1 +
 4 files changed, 49 insertions(+)

diff --git a/src/tools/rust-analyzer/crates/hir/src/semantics.rs b/src/tools/rust-analyzer/crates/hir/src/semantics.rs
index 708db2c08dda..09470bed9cfb 100644
--- a/src/tools/rust-analyzer/crates/hir/src/semantics.rs
+++ b/src/tools/rust-analyzer/crates/hir/src/semantics.rs
@@ -1483,6 +1483,8 @@ impl<'db> SemanticsImpl<'db> {
         self.analyze(try_expr.syntax())?.resolve_try_expr(self.db, try_expr)
     }
 
+    // This does not resolve the method call to the correct trait impl!
+    // We should probably fix that.
     pub fn resolve_method_call_as_callable(&self, call: &ast::MethodCallExpr) -> Option<Callable> {
         self.analyze(call.syntax())?.resolve_method_call_as_callable(self.db, call)
     }
diff --git a/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs b/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs
index 1dc61e3f0a8d..7039d83a5d18 100644
--- a/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs
+++ b/src/tools/rust-analyzer/crates/ide-db/src/famous_defs.rs
@@ -142,6 +142,13 @@ impl FamousDefs<'_, '_> {
         self.find_macro("core:unimplemented")
     }
 
+    pub fn core_fmt_Display(&self) -> Option<Trait> {
+        self.find_trait("core:fmt:Display")
+    }
+
+    pub fn alloc_string_ToString(&self) -> Option<Trait> {
+        self.find_trait("alloc:string:ToString")
+    }
     pub fn builtin_crates(&self) -> impl Iterator<Item = Crate> {
         IntoIterator::into_iter([
             self.std(),
diff --git a/src/tools/rust-analyzer/crates/ide/src/goto_definition.rs b/src/tools/rust-analyzer/crates/ide/src/goto_definition.rs
index 99d0d6af716d..d18732a6b846 100644
--- a/src/tools/rust-analyzer/crates/ide/src/goto_definition.rs
+++ b/src/tools/rust-analyzer/crates/ide/src/goto_definition.rs
@@ -185,6 +185,15 @@ fn find_definition_for_known_blanket_dual_impls(
             // Extract the `T` from `Result<T, ..>`
             [return_type.type_arguments().next()?, callable.receiver_param(sema.db)?.1],
         )?
+    } else if fn_name == sym::to_string && fd.alloc_string_ToString() == Some(t) {
+        let dual = fd.core_fmt_Display()?;
+        let dual_f = dual.function(sema.db, &sym::fmt)?;
+        sema.resolve_trait_impl_method(
+            return_type.clone(),
+            dual,
+            dual_f,
+            [callable.receiver_param(sema.db)?.1.strip_reference()],
+        )?
     } else {
         return None;
     };
@@ -3231,6 +3240,36 @@ impl FromStr for A {
 }
 fn f() {
     let a: Result<A, _> = "aaaaaa".parse$0();
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn to_string_call_to_display_definition() {
+        check(
+            r#"
+//- minicore:fmt
+//- /alloc.rs crate:alloc
+pub mod string {
+    pub struct String;
+    pub trait ToString {
+        fn to_string(&self) -> String;
+    }
+
+    impl<T: core::fmt::Display> ToString for T {
+        fn to_string(&self) -> String { String }
+    }
+}
+//- /lib.rs crate:lib deps:alloc
+use alloc::string::ToString;
+struct A;
+impl core::fmt::Display for A {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {}
+    // ^^^
+}
+fn f() {
+    A.to_string$0();
 }
         "#,
         );
diff --git a/src/tools/rust-analyzer/crates/intern/src/symbol/symbols.rs b/src/tools/rust-analyzer/crates/intern/src/symbol/symbols.rs
index 59c0c9dca1c4..a29ebad31f03 100644
--- a/src/tools/rust-analyzer/crates/intern/src/symbol/symbols.rs
+++ b/src/tools/rust-analyzer/crates/intern/src/symbol/symbols.rs
@@ -460,6 +460,7 @@ define_symbols! {
     test_case,
     test,
     thiscall,
+    to_string,
     trace_macros,
     transmute_opts,
     transmute_trait,