Merge #4596

4596: Strip leading underscores of argument names in function/method r=matklad a=kuy

Closes #4510 

### Goal

When I select a function/method from completions, I get a snippet that doesn't contain leading underscores of argument names.

### Solution

- Option 1: All signatures don't contain underscores
- Option 2: Keep same signature, but inserted snippet doesn't contain underscores

I choose Option 2 because I think that leading underscores is a part of "signature". Users should get correct signatures. On the other hand, trimming underscores is an assist by IDE.

### Other impls.

rls: Complete argument names with underscores (same as actual ra)
IntelliJ Rust: Doesn't complete argument names
VSCode (TypeScript): Doesn't complete argument names

### Working example

![Screen Shot 2020-05-25 at 0 03 21](https://user-images.githubusercontent.com/151614/82757771-a05e5b80-9e1d-11ea-9dbc-1263c960e2ae.png)


Co-authored-by: Yuki Kodama <endflow.net@gmail.com>

crates/ra_ide/src/completion/presentation.rs

@@ -211,7 +211,7 @@ impl Completions {
             .parameter_names
             .iter()
             .skip(if function_signature.has_self_param { 1 } else { 0 })
-            .cloned()
+            .map(|name| name.trim_start_matches('_').into())
             .collect();
 
         builder = builder.add_call_parens(ctx, name, Params::Named(params));
@@ -669,6 +669,37 @@ mod tests {
         ]
         "###
         );
+        assert_debug_snapshot!(
+            do_reference_completion(
+                r"
+                fn with_ignored_args(_foo: i32, ___bar: bool, ho_ge_: String) {}
+                fn main() { with_<|> }
+                "
+            ),
+            @r###"
+        [
+            CompletionItem {
+                label: "main()",
+                source_range: 110..115,
+                delete: 110..115,
+                insert: "main()$0",
+                kind: Function,
+                lookup: "main",
+                detail: "fn main()",
+            },
+            CompletionItem {
+                label: "with_ignored_args(…)",
+                source_range: 110..115,
+                delete: 110..115,
+                insert: "with_ignored_args(${1:foo}, ${2:bar}, ${3:ho_ge_})$0",
+                kind: Function,
+                lookup: "with_ignored_args",
+                detail: "fn with_ignored_args(_foo: i32, ___bar: bool, ho_ge_: String)",
+                trigger_call_info: true,
+            },
+        ]
+        "###
+        );
         assert_debug_snapshot!(
             do_reference_completion(
                 r"
@@ -695,6 +726,33 @@ mod tests {
         ]
         "###
         );
+        assert_debug_snapshot!(
+            do_reference_completion(
+                r"
+                struct S {}
+                impl S {
+                    fn foo_ignored_args(&self, _a: bool, b: i32) {}
+                }
+                fn bar(s: &S) {
+                    s.f<|>
+                }
+                "
+            ),
+            @r###"
+        [
+            CompletionItem {
+                label: "foo_ignored_args(…)",
+                source_range: 194..195,
+                delete: 194..195,
+                insert: "foo_ignored_args(${1:a}, ${2:b})$0",
+                kind: Method,
+                lookup: "foo_ignored_args",
+                detail: "fn foo_ignored_args(&self, _a: bool, b: i32)",
+                trigger_call_info: true,
+            },
+        ]
+        "###
+        );
     }
 
     #[test]