Merge #9993

9993: feat: join lines merges assignments r=matklad a=unexge

Closes https://github.com/rust-analyzer/rust-analyzer/issues/9967.
![Peek 2021-08-22 21-46](https://user-images.githubusercontent.com/16212576/130366571-3ebb1753-f8d5-4884-be8f-222cda71a2a7.gif)


Co-authored-by: unexge <unexge@gmail.com>

crates/ide/src/join_lines.rs

@@ -15,6 +15,7 @@ pub struct JoinLinesConfig {
     pub join_else_if: bool,
     pub remove_trailing_comma: bool,
     pub unwrap_trivial_blocks: bool,
+    pub join_assignments: bool,
 }
 
 // Feature: Join Lines
@@ -162,6 +163,12 @@ fn remove_newline(
         }
     }
 
+    if config.join_assignments {
+        if join_assignments(edit, &prev, &next).is_some() {
+            return;
+        }
+    }
+
     if config.unwrap_trivial_blocks {
         // Special case that turns something like:
         //
@@ -232,6 +239,41 @@ fn join_single_use_tree(edit: &mut TextEditBuilder, token: &SyntaxToken) -> Opti
     Some(())
 }
 
+fn join_assignments(
+    edit: &mut TextEditBuilder,
+    prev: &SyntaxElement,
+    next: &SyntaxElement,
+) -> Option<()> {
+    let let_stmt = ast::LetStmt::cast(prev.as_node()?.clone())?;
+    if let_stmt.eq_token().is_some() {
+        cov_mark::hit!(join_assignments_already_initialized);
+        return None;
+    }
+    let let_ident_pat = match let_stmt.pat()? {
+        ast::Pat::IdentPat(it) => it,
+        _ => return None,
+    };
+
+    let expr_stmt = ast::ExprStmt::cast(next.as_node()?.clone())?;
+    let bin_expr = match expr_stmt.expr()? {
+        ast::Expr::BinExpr(it) => it,
+        _ => return None,
+    };
+    if !matches!(bin_expr.op_kind()?, ast::BinaryOp::Assignment { op: None }) {
+        return None;
+    }
+    let lhs = bin_expr.lhs()?;
+    let name_ref = lhs.name_ref()?;
+
+    if name_ref.to_string() != let_ident_pat.syntax().to_string() {
+        cov_mark::hit!(join_assignments_mismatch);
+        return None;
+    }
+
+    edit.delete(let_stmt.semicolon_token()?.text_range().cover(lhs.syntax().text_range()));
+    Some(())
+}
+
 fn as_if_expr(element: &SyntaxElement) -> Option<ast::IfExpr> {
     let mut node = element.as_node()?.clone();
     if let Some(stmt) = ast::ExprStmt::cast(node.clone()) {
@@ -275,6 +317,7 @@ mod tests {
             join_else_if: true,
             remove_trailing_comma: true,
             unwrap_trivial_blocks: true,
+            join_assignments: true,
         };
 
         let (before_cursor_pos, before) = extract_offset(ra_fixture_before);
@@ -300,6 +343,7 @@ mod tests {
             join_else_if: true,
             remove_trailing_comma: true,
             unwrap_trivial_blocks: true,
+            join_assignments: true,
         };
 
         let (sel, before) = extract_range(ra_fixture_before);
@@ -990,6 +1034,55 @@ fn main() {
 
     }
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn join_assignments() {
+        check_join_lines(
+            r#"
+fn foo() {
+    $0let foo;
+    foo = "bar";
+}
+"#,
+            r#"
+fn foo() {
+    $0let foo = "bar";
+}
+"#,
+        );
+
+        cov_mark::check!(join_assignments_mismatch);
+        check_join_lines(
+            r#"
+fn foo() {
+    let foo;
+    let qux;$0
+    foo = "bar";
+}
+"#,
+            r#"
+fn foo() {
+    let foo;
+    let qux;$0 foo = "bar";
+}
+"#,
+        );
+
+        cov_mark::check!(join_assignments_already_initialized);
+        check_join_lines(
+            r#"
+fn foo() {
+    let foo = "bar";$0
+    foo = "bar";
+}
+"#,
+            r#"
+fn foo() {
+    let foo = "bar";$0 foo = "bar";
+}
 "#,
         );
     }

crates/rust-analyzer/src/config.rs

@@ -206,6 +206,8 @@ config_data! {
         joinLines_removeTrailingComma: bool = "true",
         /// Join lines unwraps trivial blocks.
         joinLines_unwrapTrivialBlock: bool = "true",
+        /// Join lines merges consecutive declaration and initialization of an assignment.
+        joinLines_joinAssignments: bool = "true",
 
         /// Whether to show `Debug` lens. Only applies when
         /// `#rust-analyzer.lens.enable#` is set.
@@ -786,6 +788,7 @@ impl Config {
             join_else_if: self.data.joinLines_joinElseIf,
             remove_trailing_comma: self.data.joinLines_removeTrailingComma,
             unwrap_trivial_blocks: self.data.joinLines_unwrapTrivialBlock,
+            join_assignments: self.data.joinLines_joinAssignments,
         }
     }
     pub fn call_info_full(&self) -> bool {

docs/user/generated_config.adoc

@@ -318,6 +318,11 @@ Join lines removes trailing commas.
 --
 Join lines unwraps trivial blocks.
 --
+[[rust-analyzer.joinLines.joinAssignments]]rust-analyzer.joinLines.joinAssignments (default: `true`)::
++
+--
+Join lines merges consecutive declaration and initialization of an assignment.
+--
 [[rust-analyzer.lens.debug]]rust-analyzer.lens.debug (default: `true`)::
 +
 --

editors/code/package.json

@@ -761,6 +761,11 @@
                     "default": true,
                     "type": "boolean"
                 },
+                "rust-analyzer.joinLines.joinAssignments": {
+                    "markdownDescription": "Join lines merges consecutive declaration and initialization of an assignment.",
+                    "default": true,
+                    "type": "boolean"
+                },
                 "rust-analyzer.lens.debug": {
                     "markdownDescription": "Whether to show `Debug` lens. Only applies when\n`#rust-analyzer.lens.enable#` is set.",
                     "default": true,