Merge #7335 #7691

7335: added region folding r=matklad a=LucianoBestia

Regions of code that you'd like to be folded can be wrapped with  `// #region` and `// #endregion` line comments.
This is called "Region Folding". It is originally available for many languages in VSCode. But Rust-analyzer has its own folding function and this is missing.
With this Pull Request I am suggesting a simple solution. 
The regions are a special kind of comments, so I added a bit of code in the comment folding function.
The regex to match are: `^\s*//\s*#?region\b` and `^\s*//\s*#?endregion\b`.
The number of space characters is not important. There is an optional # character. The line can end with a name of the region.
Example:
```rust
// 1. some normal comment
// region: test
// 2. some normal comment
calling_function(x,y);
// endregion: test
```
I added a test for this new functionality in `folding_ranges.rs`.
Please, take a look and comment. 
I found that these exact regexes are already present in the file `language-configuration.json`, but I don't find a way to read this configuration. So my regex is hardcoded in the code.

7691: Suggest name in extract variable r=matklad a=cpud36

Generate better default name in extract variable assist as was mentioned in issue #1587

# Currently supported
(in order of declining precedence)
1. Expr is argument to a function; use corresponding parameter name
2. Expr is result of a function or method call; use this function/method's name
3. Use expr type name (if possible)
4. Fallback to `var_name` otherwise

# Showcase

![generate_derive_variable_name_from_method](https://user-images.githubusercontent.com/4218373/108013304-72105400-701c-11eb-9f13-eec52e74d0cc.gif)
![generate_derive_variable_name_from_param](https://user-images.githubusercontent.com/4218373/108013305-72a8ea80-701c-11eb-957e-2214f7f005de.gif)

# Questions

* Should we more aggressively strip known types? E.g. we already strip `&T -> T`; should we strip `Option<T> -> T`, `Result<T, E> -> T`, and others?
* Integers and floats use `var_name` by default. Should we introduce a name, like `i`, `f` etc?
* Can we return a list and suggest a name when renaming(like IntelliJ does)?
* Should we add counters to remove duplicate variables? E.g. `type`, `type1`, type2`, etc.


Co-authored-by: Luciano Bestia <LucianoBestia@gmail.com>
Co-authored-by: Luciano <31509965+LucianoBestia@users.noreply.github.com>
Co-authored-by: Vladyslav Katasonov <cpud47@gmail.com>

crates/ide/src/folding_ranges.rs

@@ -6,7 +6,7 @@ use syntax::{
     ast::{self, AstNode, AstToken, VisibilityOwner},
     Direction, NodeOrToken, SourceFile,
     SyntaxKind::{self, *},
-    SyntaxNode, TextRange,
+    SyntaxNode, TextRange, TextSize,
 };
 
 #[derive(Debug, PartialEq, Eq)]
@@ -16,6 +16,7 @@ pub enum FoldKind {
     Mods,
     Block,
     ArgList,
+    Region,
 }
 
 #[derive(Debug)]
@@ -29,6 +30,8 @@ pub(crate) fn folding_ranges(file: &SourceFile) -> Vec<Fold> {
     let mut visited_comments = FxHashSet::default();
     let mut visited_imports = FxHashSet::default();
     let mut visited_mods = FxHashSet::default();
+    // regions can be nested, here is a LIFO buffer
+    let mut regions_starts: Vec<TextSize> = vec![];
 
     for element in file.syntax().descendants_with_tokens() {
         // Fold items that span multiple lines
@@ -48,10 +51,25 @@ pub(crate) fn folding_ranges(file: &SourceFile) -> Vec<Fold> {
                 // Fold groups of comments
                 if let Some(comment) = ast::Comment::cast(token) {
                     if !visited_comments.contains(&comment) {
-                        if let Some(range) =
-                            contiguous_range_for_comment(comment, &mut visited_comments)
-                        {
-                            res.push(Fold { range, kind: FoldKind::Comment })
+                        // regions are not real comments
+                        if comment.text().trim().starts_with("// region:") {
+                            regions_starts.push(comment.syntax().text_range().start());
+                        } else if comment.text().trim().starts_with("// endregion") {
+                            if let Some(region) = regions_starts.pop() {
+                                res.push(Fold {
+                                    range: TextRange::new(
+                                        region,
+                                        comment.syntax().text_range().end(),
+                                    ),
+                                    kind: FoldKind::Region,
+                                })
+                            }
+                        } else {
+                            if let Some(range) =
+                                contiguous_range_for_comment(comment, &mut visited_comments)
+                            {
+                                res.push(Fold { range, kind: FoldKind::Comment })
+                            }
                         }
                     }
                 }
@@ -175,9 +193,16 @@ fn contiguous_range_for_comment(
                 }
                 if let Some(c) = ast::Comment::cast(token) {
                     if c.kind() == group_kind {
-                        visited.insert(c.clone());
-                        last = c;
-                        continue;
+                        // regions are not real comments
+                        if c.text().trim().starts_with("// region:")
+                            || c.text().trim().starts_with("// endregion")
+                        {
+                            break;
+                        } else {
+                            visited.insert(c.clone());
+                            last = c;
+                            continue;
+                        }
                     }
                 }
                 // The comment group ends because either:
@@ -224,6 +249,7 @@ mod tests {
                 FoldKind::Mods => "mods",
                 FoldKind::Block => "block",
                 FoldKind::ArgList => "arglist",
+                FoldKind::Region => "region",
             };
             assert_eq!(kind, &attr.unwrap());
         }
@@ -415,6 +441,19 @@ fn foo<fold arglist>(
     x: i32,
     y: String,
 )</fold> {}
+"#,
+        )
+    }
+
+    #[test]
+    fn fold_region() {
+        check(
+            r#"
+// 1. some normal comment
+<fold region>// region: test
+// 2. some normal comment
+calling_function(x,y);
+// endregion: test</fold>
 "#,
         )
     }