Auto merge of #16136 - saiintbrisson:fix/completion/a-tad-smarter-with-fns, r=Veykril

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 behavior of 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.

![image](https://github.com/rust-lang/rust-analyzer/assets/29989290/c06d6c93-5cac-4ebe-a93b-923017a6ae8c)
![image](https://github.com/rust-lang/rust-analyzer/assets/29989290/31594d82-fa4d-446c-a77e-47e9de1a9a67)
![image](https://github.com/rust-lang/rust-analyzer/assets/29989290/cf33856e-a485-411b-91af-11090d78a44e)

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> {

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 }

crates/ide-completion/src/render.rs

@@ -837,11 +837,11 @@ fn main() {
 }
 "#,
             expect![[r#"
-                fn main []
-                fn test []
+                fn main() []
+                fn test(…) []
                 md dep []
                 fn function (use dep::test_mod_a::function) [requires_import]
-                fn function (use dep::test_mod_b::function) [requires_import]
+                fn function(…) (use dep::test_mod_b::function) [requires_import]
             "#]],
         );
     }

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;
+                }
+            }
         }
     }