Implement let-else formatting

src/expr.rs

@@ -575,6 +575,17 @@ fn rewrite_block(
     label: Option<ast::Label>,
     context: &RewriteContext<'_>,
     shape: Shape,
+) -> Option<String> {
+    rewrite_block_inner(block, attrs, label, true, context, shape)
+}
+
+fn rewrite_block_inner(
+    block: &ast::Block,
+    attrs: Option<&[ast::Attribute]>,
+    label: Option<ast::Label>,
+    allow_single_line: bool,
+    context: &RewriteContext<'_>,
+    shape: Shape,
 ) -> Option<String> {
     let prefix = block_prefix(context, block, shape)?;
 
@@ -586,7 +597,7 @@ fn rewrite_block(
 
     let result = rewrite_block_with_visitor(context, &prefix, block, attrs, label, shape, true);
     if let Some(ref result_str) = result {
-        if result_str.lines().count() <= 3 {
+        if allow_single_line && result_str.lines().count() <= 3 {
             if let rw @ Some(_) =
                 rewrite_single_line_block(context, &prefix, block, attrs, label, shape)
             {
@@ -598,6 +609,16 @@ fn rewrite_block(
     result
 }
 
+/// Rewrite the divergent block of a `let-else` statement.
+pub(crate) fn rewrite_let_else_block(
+    block: &ast::Block,
+    allow_single_line: bool,
+    context: &RewriteContext<'_>,
+    shape: Shape,
+) -> Option<String> {
+    rewrite_block_inner(block, None, None, allow_single_line, context, shape)
+}
+
 // Rewrite condition if the given expression has one.
 pub(crate) fn rewrite_cond(
     context: &RewriteContext<'_>,
@@ -1004,6 +1025,49 @@ impl<'a> ControlFlow<'a> {
     }
 }
 
+/// Rewrite the `else` keyword with surrounding comments.
+///
+/// force_newline_else: whether or not to rewrite the `else` keyword on a newline.
+/// is_last: true if this is an `else` and `false` if this is an `else if` block.
+/// context: rewrite context
+/// span: Span between the end of the last expression and the start of the else block,
+///       which contains the `else` keyword
+/// shape: Shape
+pub(crate) fn rewrite_else_kw_with_comments(
+    force_newline_else: bool,
+    is_last: bool,
+    context: &RewriteContext<'_>,
+    span: Span,
+    shape: Shape,
+) -> String {
+    let else_kw_lo = context.snippet_provider.span_before(span, "else");
+    let before_else_kw = mk_sp(span.lo(), else_kw_lo);
+    let before_else_kw_comment = extract_comment(before_else_kw, context, shape);
+
+    let else_kw_hi = context.snippet_provider.span_after(span, "else");
+    let after_else_kw = mk_sp(else_kw_hi, span.hi());
+    let after_else_kw_comment = extract_comment(after_else_kw, context, shape);
+
+    let newline_sep = &shape.indent.to_string_with_newline(context.config);
+    let before_sep = match context.config.control_brace_style() {
+        _ if force_newline_else => newline_sep.as_ref(),
+        ControlBraceStyle::AlwaysNextLine | ControlBraceStyle::ClosingNextLine => {
+            newline_sep.as_ref()
+        }
+        ControlBraceStyle::AlwaysSameLine => " ",
+    };
+    let after_sep = match context.config.control_brace_style() {
+        ControlBraceStyle::AlwaysNextLine if is_last => newline_sep.as_ref(),
+        _ => " ",
+    };
+
+    format!(
+        "{}else{}",
+        before_else_kw_comment.as_ref().map_or(before_sep, |s| &**s),
+        after_else_kw_comment.as_ref().map_or(after_sep, |s| &**s),
+    )
+}
+
 impl<'a> Rewrite for ControlFlow<'a> {
     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
         debug!("ControlFlow::rewrite {:?} {:?}", self, shape);
@@ -1070,41 +1134,14 @@ impl<'a> Rewrite for ControlFlow<'a> {
                 }
             };
 
-            let between_kwd_else_block = mk_sp(
-                self.block.span.hi(),
-                context
-                    .snippet_provider
-                    .span_before(mk_sp(self.block.span.hi(), else_block.span.lo()), "else"),
-            );
-            let between_kwd_else_block_comment =
-                extract_comment(between_kwd_else_block, context, shape);
-
-            let after_else = mk_sp(
-                context
-                    .snippet_provider
-                    .span_after(mk_sp(self.block.span.hi(), else_block.span.lo()), "else"),
-                else_block.span.lo(),
+            let else_kw = rewrite_else_kw_with_comments(
+                false,
+                last_in_chain,
+                context,
+                self.block.span.between(else_block.span),
+                shape,
             );
-            let after_else_comment = extract_comment(after_else, context, shape);
-
-            let between_sep = match context.config.control_brace_style() {
-                ControlBraceStyle::AlwaysNextLine | ControlBraceStyle::ClosingNextLine => {
-                    &*alt_block_sep
-                }
-                ControlBraceStyle::AlwaysSameLine => " ",
-            };
-            let after_sep = match context.config.control_brace_style() {
-                ControlBraceStyle::AlwaysNextLine if last_in_chain => &*alt_block_sep,
-                _ => " ",
-            };
-
-            result.push_str(&format!(
-                "{}else{}",
-                between_kwd_else_block_comment
-                    .as_ref()
-                    .map_or(between_sep, |s| &**s),
-                after_else_comment.as_ref().map_or(after_sep, |s| &**s),
-            ));
+            result.push_str(&else_kw);
             result.push_str(&rewrite?);
         }
 

src/items.rs

@@ -18,7 +18,8 @@ use crate::config::lists::*;
 use crate::config::{BraceStyle, Config, IndentStyle, Version};
 use crate::expr::{
     is_empty_block, is_simple_block_stmt, rewrite_assign_rhs, rewrite_assign_rhs_with,
-    rewrite_assign_rhs_with_comments, RhsAssignKind, RhsTactics,
+    rewrite_assign_rhs_with_comments, rewrite_else_kw_with_comments, rewrite_let_else_block,
+    RhsAssignKind, RhsTactics,
 };
 use crate::lists::{definitive_tactic, itemize_list, write_list, ListFormatting, Separator};
 use crate::macros::{rewrite_macro, MacroPosition};
@@ -44,7 +45,7 @@ fn type_annotation_separator(config: &Config) -> &str {
 }
 
 // Statements of the form
-// let pat: ty = init;
+// let pat: ty = init; or let pat: ty = init else { .. };
 impl Rewrite for ast::Local {
     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
         debug!(
@@ -54,7 +55,7 @@ impl Rewrite for ast::Local {
 
         skip_out_of_file_lines_range!(context, self.span);
 
-        if contains_skip(&self.attrs) || matches!(self.kind, ast::LocalKind::InitElse(..)) {
+        if contains_skip(&self.attrs) {
             return None;
         }
 
@@ -112,7 +113,7 @@ impl Rewrite for ast::Local {
 
         result.push_str(&infix);
 
-        if let Some((init, _els)) = self.kind.init_else_opt() {
+        if let Some((init, else_block)) = self.kind.init_else_opt() {
             // 1 = trailing semicolon;
             let nested_shape = shape.sub_width(1)?;
 
@@ -123,14 +124,97 @@ impl Rewrite for ast::Local {
                 &RhsAssignKind::Expr(&init.kind, init.span),
                 nested_shape,
             )?;
-            // todo else
+
+            if let Some(block) = else_block {
+                let else_kw_span = init.span.between(block.span);
+                let force_newline_else =
+                    !same_line_else_kw_and_brace(&result, context, else_kw_span, nested_shape);
+                let else_kw = rewrite_else_kw_with_comments(
+                    force_newline_else,
+                    true,
+                    context,
+                    else_kw_span,
+                    shape,
+                );
+                result.push_str(&else_kw);
+
+                let allow_single_line = allow_single_line_let_else_block(&result, block);
+
+                let mut rw_else_block =
+                    rewrite_let_else_block(block, allow_single_line, context, shape)?;
+
+                if allow_single_line && !rw_else_block.contains('\n') {
+                    let available_space = shape.width.saturating_sub(result.len());
+                    if available_space <= rw_else_block.len() {
+                        // writing this on one line would exceed the available width
+                        rw_else_block = rewrite_let_else_block(block, false, context, shape)?;
+                    }
+                }
+
+                result.push_str(&rw_else_block);
+            };
         }
 
         result.push(';');
         Some(result)
     }
 }
 
+/// When the initializer expression is multi-lined, then the else keyword and opening brace of the
+/// block ( i.e. "else {") should be put on the same line as the end of the initializer expression
+/// if all the following are true:
+///
+/// 1. The initializer expression ends with one or more closing parentheses, square brackets,
+///    or braces
+/// 2. There is nothing else on that line
+/// 3. That line is not indented beyond the indent on the first line of the let keyword
+fn same_line_else_kw_and_brace(
+    init_str: &str,
+    context: &RewriteContext<'_>,
+    else_kw_span: Span,
+    init_shape: Shape,
+) -> bool {
+    if !init_str.contains('\n') {
+        // initializer expression is single lined. The "else {" can only be placed on the same line
+        // as the initializer expression if there is enough room for it.
+        // 7 = ` else {`
+        return init_shape.width.saturating_sub(init_str.len()) >= 7;
+    }
+
+    // 1. The initializer expression ends with one or more `)`, `]`, `}`.
+    if !init_str.ends_with([')', ']', '}']) {
+        return false;
+    }
+
+    // 2. There is nothing else on that line
+    // For example, there are no comments
+    let else_kw_snippet = context.snippet(else_kw_span).trim();
+    if else_kw_snippet != "else" {
+        return false;
+    }
+
+    // 3. The last line of the initializer expression is not indented beyond the `let` keyword
+    let indent = init_shape.indent.to_string(context.config);
+    init_str
+        .lines()
+        .last()
+        .expect("initializer expression is multi-lined")
+        .strip_prefix(indent.as_ref())
+        .map_or(false, |l| !l.starts_with(char::is_whitespace))
+}
+
+fn allow_single_line_let_else_block(result: &str, block: &ast::Block) -> bool {
+    if result.contains('\n') {
+        return false;
+    }
+
+    if block.stmts.len() <= 1 {
+        return true;
+    }
+
+    false
+}
+
 // FIXME convert to using rewrite style rather than visitor
 // FIXME format modules in this style
 #[allow(dead_code)]