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.

3 files changed, 24 insertions, 17 deletions

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 85762603ed1..0d0901d89e9 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 2d62c45174f..7da66483657 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 d23ed71fdcc..b306bede653 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;
+                }
+            }
         }
     }