Rollup merge of rust-lang#66913 - VirrageS:help-self, r=varkor

Suggest calling method when first argument is `self`

Closes: rust-lang#66782

I've explored different approaches for this MR but I think the most straightforward is the best one.

I've tried to find out if the methods for given type exist (to maybe have a better suggestion), but we don't collect them anywhere and collecting them is quite problematic. Moreover, collecting all the methods would require rewriting big part of the code and also could potentially include performance degradation, which I don't think is necessary for this simple case.

src/librustc_resolve/late.rs

@@ -345,7 +345,7 @@ struct DiagnosticMetadata {
     /// The current self item if inside an ADT (used for better errors).
     current_self_item: Option<NodeId>,
 
-    /// The current enclosing funciton (used for better errors).
+    /// The current enclosing function (used for better errors).
     current_function: Option<Span>,
 
     /// A list of labels as of yet unused. Labels will be removed from this map when

src/librustc_resolve/late/diagnostics.rs

@@ -253,6 +253,56 @@ impl<'a> LateResolutionVisitor<'a, '_> {
                 }
                 return (err, candidates);
             }
+
+            // Check if the first argument is `self` and suggest calling a method.
+            let mut has_self_arg = None;
+            if let PathSource::Expr(parent) = source {
+                match &parent.map(|p| &p.kind) {
+                    Some(ExprKind::Call(_, args)) if args.len() > 0 => {
+                        let mut expr_kind = &args[0].kind;
+                        loop {
+                            match expr_kind {
+                                ExprKind::Path(_, arg_name) if arg_name.segments.len() == 1 => {
+                                    if arg_name.segments[0].ident.name == kw::SelfLower {
+                                        let call_span = parent.unwrap().span;
+                                        let args_span = if args.len() > 1 {
+                                            Some(Span::new(
+                                                args[1].span.lo(),
+                                                args.last().unwrap().span.hi(),
+                                                call_span.ctxt(),
+                                            ))
+                                        } else {
+                                            None
+                                        };
+                                        has_self_arg = Some((call_span, args_span));
+                                    }
+                                    break;
+                                },
+                                ExprKind::AddrOf(_, _, expr) => expr_kind = &expr.kind,
+                                _ => break,
+                            }
+                        }
+                    }
+                    _ => (),
+                }
+            };
+
+            if let Some((call_span, args_span)) = has_self_arg {
+                let mut args_snippet: String = String::from("");
+                if let Some(args_span) = args_span {
+                    if let Ok(snippet) = self.r.session.source_map().span_to_snippet(args_span) {
+                        args_snippet = snippet;
+                    }
+                }
+
+                err.span_suggestion(
+                    call_span,
+                    &format!("try calling `{}` as a method", ident),
+                    format!("self.{}({})", path_str, args_snippet),
+                    Applicability::MachineApplicable,
+                );
+                return (err, candidates);
+            }
         }
 
         // Try Levenshtein algorithm.
@@ -553,13 +603,13 @@ impl<'a> LateResolutionVisitor<'a, '_> {
         // Look for associated items in the current trait.
         if let Some((module, _)) = self.current_trait_ref {
             if let Ok(binding) = self.r.resolve_ident_in_module(
-                    ModuleOrUniformRoot::Module(module),
-                    ident,
-                    ns,
-                    &self.parent_scope,
-                    false,
-                    module.span,
-                ) {
+                ModuleOrUniformRoot::Module(module),
+                ident,
+                ns,
+                &self.parent_scope,
+                false,
+                module.span,
+            ) {
                 let res = binding.res();
                 if filter_fn(res) {
                     return Some(if self.r.has_self.contains(&res.def_id()) {

src/test/ui/self/suggest-self-2.rs

@@ -0,0 +1,25 @@
+struct Foo {}
+
+impl Foo {
+    fn foo(&self) {
+        bar(self);
+        //~^ ERROR cannot find function `bar` in this scope
+        //~| HELP try calling `bar` as a method
+
+        bar(&&self, 102);
+        //~^ ERROR cannot find function `bar` in this scope
+        //~| HELP try calling `bar` as a method
+
+        bar(&mut self, 102, &"str");
+        //~^ ERROR cannot find function `bar` in this scope
+        //~| HELP try calling `bar` as a method
+
+        bar();
+        //~^ ERROR cannot find function `bar` in this scope
+
+        self.bar();
+        //~^ ERROR no method named `bar` found for type
+    }
+}
+
+fn main() {}

src/test/ui/self/suggest-self-2.stderr

@@ -0,0 +1,40 @@
+error[E0425]: cannot find function `bar` in this scope
+  --> $DIR/suggest-self-2.rs:5:9
+   |
+LL |         bar(self);
+   |         ^^^------
+   |         |
+   |         help: try calling `bar` as a method: `self.bar()`
+
+error[E0425]: cannot find function `bar` in this scope
+  --> $DIR/suggest-self-2.rs:9:9
+   |
+LL |         bar(&&self, 102);
+   |         ^^^-------------
+   |         |
+   |         help: try calling `bar` as a method: `self.bar(102)`
+
+error[E0425]: cannot find function `bar` in this scope
+  --> $DIR/suggest-self-2.rs:13:9
+   |
+LL |         bar(&mut self, 102, &"str");
+   |         ^^^------------------------
+   |         |
+   |         help: try calling `bar` as a method: `self.bar(102, &"str")`
+
+error[E0425]: cannot find function `bar` in this scope
+  --> $DIR/suggest-self-2.rs:17:9
+   |
+LL |         bar();
+   |         ^^^ not found in this scope
+
+error[E0599]: no method named `bar` found for type `&Foo` in the current scope
+  --> $DIR/suggest-self-2.rs:20:14
+   |
+LL |         self.bar();
+   |              ^^^ method not found in `&Foo`
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0425, E0599.
+For more information about an error, try `rustc --explain E0425`.