Format match expressions properly when they appear on an overflowing line.

Author: nrc

src/expr.rs

@@ -250,7 +250,7 @@ impl Rewrite for ast::Block {
         }
 
         let mut visitor = FmtVisitor::from_codemap(context.codemap, context.config);
-        visitor.block_indent = context.block_indent;
+        visitor.block_indent = context.block_indent + context.overflow_indent;
 
         let prefix = match self.rules {
             ast::BlockCheckMode::PushUnsafeBlock(..) |
@@ -541,9 +541,9 @@ fn rewrite_match(context: &RewriteContext,
     let cond_str = try_opt!(cond.rewrite(context, cond_budget, offset + 6));
     let mut result = format!("match {} {{", cond_str);
 
-    let block_indent = context.block_indent;
     let nested_context = context.nested_context();
-    let arm_indent_str = make_indent(nested_context.block_indent);
+    let arm_indent = nested_context.block_indent + context.overflow_indent;
+    let arm_indent_str = make_indent(arm_indent);
 
     let open_brace_pos = span_after(mk_sp(cond.span.hi, arm_start_pos(&arms[0])),
                                     "{",
@@ -579,8 +579,8 @@ fn rewrite_match(context: &RewriteContext,
 
         let arm_str = arm.rewrite(&nested_context,
                                   context.config.max_width -
-                                      nested_context.block_indent,
-                                  nested_context.block_indent);
+                                      arm_indent,
+                                  arm_indent);
         if let Some(ref arm_str) = arm_str {
             result.push_str(arm_str);
         } else {
@@ -594,7 +594,7 @@ fn rewrite_match(context: &RewriteContext,
     // match expression, but meh.
 
     result.push('\n');
-    result.push_str(&make_indent(block_indent));
+    result.push_str(&make_indent(context.block_indent + context.overflow_indent));
     result.push('}');
     Some(result)
 }
@@ -1192,7 +1192,7 @@ pub fn rewrite_assign_rhs<S: Into<String>>(context: &RewriteContext,
             result.push_str(&format!("\n{}", make_indent(new_offset)));
 
             let max_width = try_opt!(context.config.max_width.checked_sub(new_offset + 1));
-            let rhs = try_opt!(ex.rewrite(&context, max_width, new_offset));
+            let rhs = try_opt!(ex.rewrite(&context.overflow_context(), max_width, new_offset));
 
             result.push_str(&rhs);
         }

src/rewrite.rs

@@ -28,7 +28,14 @@ pub trait Rewrite {
 pub struct RewriteContext<'a> {
     pub codemap: &'a CodeMap,
     pub config: &'a Config,
+
+    // Indentation due to nesting of blocks.
     pub block_indent: usize,
+    // *Extra* indentation due to overflowing to the next line, e.g.,
+    // let foo =
+    //     bar();
+    // The extra 4 spaces when formatting `bar()` is overflow_indent.
+    pub overflow_indent: usize,
 }
 
 impl<'a> RewriteContext<'a> {
@@ -37,6 +44,16 @@ impl<'a> RewriteContext<'a> {
             codemap: self.codemap,
             config: self.config,
             block_indent: self.block_indent + self.config.tab_spaces,
+            overflow_indent: self.overflow_indent,
+        }
+    }
+
+    pub fn overflow_context(&self) -> RewriteContext<'a> {
+        RewriteContext {
+            codemap: self.codemap,
+            config: self.config,
+            block_indent: self.block_indent,
+            overflow_indent: self.overflow_indent + self.config.tab_spaces,
         }
     }
 

src/visitor.rs

@@ -338,6 +338,7 @@ impl<'a> FmtVisitor<'a> {
             codemap: self.codemap,
             config: self.config,
             block_indent: self.block_indent,
+            overflow_indent: 0,
         };
         // 1 = ";"
         match vp.rewrite(&context, self.config.max_width - offset - 1, offset) {
@@ -369,6 +370,7 @@ impl<'a> FmtVisitor<'a> {
             codemap: self.codemap,
             config: self.config,
             block_indent: self.block_indent,
+            overflow_indent: 0,
         }
     }
 }

tests/target/match.rs

@@ -53,3 +53,12 @@ fn foo() {
         Blurb     =>     {                  }
     };
 }
+
+// Test that a match on an overflow line is laid out properly.
+fn main() {
+    let sub_span =
+        match self.span.sub_span_after_keywooooooooooooooooooooord(use_item.span, keywords::As) {
+            Some(sub_span) => Some(sub_span),
+            None => sub_span,
+        };
+}