From ed216e285d7c0770913cbfc88e2dc4f07f89810f Mon Sep 17 00:00:00 2001
From: Luiz Carvalho <luizcmpc@gmail.com>
Date: Sat, 16 Dec 2023 11:49:26 -0300
Subject: [PATCH] fix(completion): make the expected type a tad smarter with
 `Fn`s

This commit changes how the expected type is calculated when working
with Fn pointers, making the parenthesis stop vanishing when completing
the function name.

I've been bugged by the behaviour on parenthesis completion for a long
while now. R-a assumes that the `LetStmt` type is the same as the
function type I've just written. Worse is that all parenthesis vanish,
even from functions that have completely different signatures. It will
now verify if the signature is the same.

While working on this, I noticed that record fields behave the same, so
I also made it prioritize the field type instead of the current
expression when possible, but I'm unsure if this is OK, so input is
appreciated.

ImplTraits as return types will still behave weirdly because lowering is
disallowed at the time it resolves the function types.
---
 crates/hir/src/lib.rs                         |  3 +++
 crates/ide-completion/src/context/analysis.rs | 23 ++++++++++---------
 crates/ide-completion/src/render/function.rs  | 15 +++++++-----
 3 files changed, 24 insertions(+), 17 deletions(-)

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 85762603ed15..0d0901d89e98 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -4657,6 +4657,9 @@ impl Callable {
     pub fn return_type(&self) -> Type {
         self.ty.derived(self.sig.ret().clone())
     }
+    pub fn sig(&self) -> &CallableSig {
+        &self.sig
+    }
 }
 
 fn closure_source(db: &dyn HirDatabase, closure: ClosureId) -> Option<ast::ClosureExpr> {
diff --git a/crates/ide-completion/src/context/analysis.rs b/crates/ide-completion/src/context/analysis.rs
index 2d62c45174fd..7da664836574 100644
--- a/crates/ide-completion/src/context/analysis.rs
+++ b/crates/ide-completion/src/context/analysis.rs
@@ -361,7 +361,12 @@ fn expected_type_and_name(
                     let ty = it.pat()
                         .and_then(|pat| sema.type_of_pat(&pat))
                         .or_else(|| it.initializer().and_then(|it| sema.type_of_expr(&it)))
-                        .map(TypeInfo::original);
+                        .map(TypeInfo::original)
+                        .filter(|ty| {
+                            // don't infer the let type if the expr is a function,
+                            // preventing parenthesis from vanishing
+                            it.ty().is_some() || !ty.is_fn()
+                        });
                     let name = match it.pat() {
                         Some(ast::Pat::IdentPat(ident)) => ident.name().map(NameOrNameRef::Name),
                         Some(_) | None => None,
@@ -415,20 +420,16 @@ fn expected_type_and_name(
                     })().unwrap_or((None, None))
                 },
                 ast::RecordExprField(it) => {
+                    let field_ty = sema.resolve_record_field(&it).map(|(_, _, ty)| ty);
+                    let field_name = it.field_name().map(NameOrNameRef::NameRef);
                     if let Some(expr) = it.expr() {
                         cov_mark::hit!(expected_type_struct_field_with_leading_char);
-                        (
-                            sema.type_of_expr(&expr).map(TypeInfo::original),
-                            it.field_name().map(NameOrNameRef::NameRef),
-                        )
+                        let ty = field_ty
+                            .or_else(|| sema.type_of_expr(&expr).map(TypeInfo::original));
+                        (ty, field_name)
                     } else {
                         cov_mark::hit!(expected_type_struct_field_followed_by_comma);
-                        let ty = sema.resolve_record_field(&it)
-                            .map(|(_, _, ty)| ty);
-                        (
-                            ty,
-                            it.field_name().map(NameOrNameRef::NameRef),
-                        )
+                        (field_ty, field_name)
                     }
                 },
                 // match foo { $0 }
diff --git a/crates/ide-completion/src/render/function.rs b/crates/ide-completion/src/render/function.rs
index d23ed71fdcc6..b306bede653b 100644
--- a/crates/ide-completion/src/render/function.rs
+++ b/crates/ide-completion/src/render/function.rs
@@ -305,12 +305,15 @@ fn params(
         return None;
     }
 
-    // Don't add parentheses if the expected type is some function reference.
-    if let Some(ty) = &ctx.expected_type {
-        // FIXME: check signature matches?
-        if ty.is_fn() {
-            cov_mark::hit!(no_call_parens_if_fn_ptr_needed);
-            return None;
+    // Don't add parentheses if the expected type is a function reference with the same signature.
+    if let Some(expected) = ctx.expected_type.as_ref().filter(|e| e.is_fn()) {
+        if let Some(expected) = expected.as_callable(ctx.db) {
+            if let Some(completed) = func.ty(ctx.db).as_callable(ctx.db) {
+                if expected.sig() == completed.sig() {
+                    cov_mark::hit!(no_call_parens_if_fn_ptr_needed);
+                    return None;
+                }
+            }
         }
     }