Fix multiple spaces after `let`

This patch ensures that a single space is used also after `let` like
other keywords.

CHANGELOG.md

@@ -8,10 +8,26 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased changes
 ### Changed
  - Fix a bug that caused "single space after keyword" to not apply after the `function`
-   keyword in non-standard function definitions. ([#113])
+   keyword in non-standard function definitions ([#113]). This bug is classified as a
+   [spec-bug] and the fix will result in diffs like the following:
+   ```diff
+   -function()
+   +function ()
+        # ...
+    end
+   ```
+ - Fix a bug that caused "single space after keyword" to not apply after `let` ([#117]).
+   This bug is classified as a [spec-bug] and the fix will result in diffs like the
+   following when `let` is followed by multiple spaces in the source:
+   ```diff
+   -let  a = 1
+   +let a = 1
+        a
+    end
+   ```
  - Fix a bug that caused multiline variable blocks in `let` to not indent correctly ([#97],
    [#116]). This bug is classified as a [spec-bug] and the fix will result in diffs like the
-   following:
+   following whenever multiline variable blocks exist in the source:
    ```diff
     let a = 1,
    -    b = 2

src/runestone.jl

@@ -1107,6 +1107,42 @@ function no_spaces_around_colon_etc(ctx::Context, node::Node)
     return no_spaces_around_x(ctx, node, is_x)
 end
 
+function space_after_let(ctx, node)
+    @assert kind(node) === K"let" && !is_leaf(node)
+    p = position(ctx.fmt_io)
+    kids = verified_kids(node)
+    let_node = kids[1]
+    @assert kind(let_node) === K"let"
+    accept_node!(ctx, let_node)
+    vars_idx = 2
+    vars_node = kids[vars_idx]
+    @assert kind(vars_node) === K"block"
+    vars_kids = verified_kids(vars_node)
+    if length(vars_kids) == 0
+        @assert span(vars_node) == 0
+        seek(ctx.fmt_io, p)
+        # Empty block, but where are spaces and comments?
+        return nothing
+    end
+    # First node *must* be a space (?)
+    vars_kid = vars_kids[1]
+    @assert kind(vars_kid) === K"Whitespace"
+    if span(vars_kid) == 1
+        seek(ctx.fmt_io, p)
+        return nothing
+    else
+        replace_bytes!(ctx, " ", span(vars_kid))
+        ws = Node(JuliaSyntax.SyntaxHead(K"Whitespace", JuliaSyntax.TRIVIA_FLAG), 1)
+        vars_kids′ = copy(vars_kids)
+        vars_kids′[1] = ws
+        vars_node′ = make_node(vars_node, vars_kids′)
+        kids′ = copy(kids)
+        kids′[vars_idx] = vars_node′
+        seek(ctx.fmt_io, p)
+        return make_node(node, kids′)
+    end
+end
+
 # Single space around keywords:
 # Both sides of: `where`, `do` (if followed by arguments)
 # Right hand side of: `mutable`, `struct`, `abstract`, `primitive`, `type`, `function` (if
@@ -1114,6 +1150,9 @@ end
 # global, const
 function spaces_around_keywords(ctx::Context, node::Node)
     is_leaf(node) && return nothing
+    if kind(node) === K"let"
+        return space_after_let(ctx, node)
+    end
     keyword_set = KSet"""
     where do mutable struct abstract primitive type function if elseif catch while return
     local global const module baremodule

test/runtests.jl

@@ -552,6 +552,8 @@ end
         @test format_string("function f()\n    return$(sp)\nend") == "function f()\n    return\nend"
         @test format_string("module$(sp)A\nend") == "module A\nend"
         @test format_string("module$(sp)(A)\nend") == "module (A)\nend"
+        @test format_string("let$(sp)x = 1\nend") == "let x = 1\nend"
+        @test format_string("let$(sp)\nend") == "let\nend"
         for word in ("local", "global"), rhs in ("a", "a, b", "a = 1", "a, b = 1, 2")
             word == "const" && rhs in ("a", "a, b") && continue
             @test format_string("$(word)$(sp)$(rhs)") == "$(word) $(rhs)"