Unrolled build for rust-lang#139456

Rollup merge of rust-lang#139456 - calebcartwright:style-let-chains-final, r=joshtriplett

style guide: add let-chain rules

Reopens rust-lang#110568

refs rust-lang#53667 and I suppose rust-lang#132833 as well

This reflects the style rules that the style team had already agreed upon back in 2023, with the addition of literals in the lhs being permissible for single line formatting, and the removal of unnecessary language/example snippets around non-`&&` operators that was a small hiccup in the original PR.

It also reflects current formatting behavior implemented in rustfmt (though note that the adjustment to include literals has been implemented & merged, but is still pending a sync to nightly)

Author: rust-timer

src/doc/style-guide/src/expressions.md

@@ -521,8 +521,11 @@ self.pre_comment.as_ref().map_or(
 
 ## Control flow expressions
 
-This section covers `if`, `if let`, `loop`, `while`, `while let`, and `for`
-expressions.
+This section covers `for` and `loop` expressions, as well as `if` and `while`
+expressions with their sub-expression variants. This includes those with a
+single `let` sub-expression (i.e. `if let` and `while let`)
+as well as "let-chains": those with one or more `let` sub-expressions and
+one or more bool-type conditions (i.e.  `if a && let Some(b) = c`).
 
 Put the keyword, any initial clauses, and the opening brace of the block all on
 a single line, if they fit. Apply the usual rules for [block
@@ -548,10 +551,11 @@ if let ... {
 }
 ```
 
-If the control line needs to be broken, prefer to break before the `=` in `*
-let` expressions and before `in` in a `for` expression; block-indent the
-following line. If the control line is broken for any reason, put the opening
-brace on its own line, not indented. Examples:
+If the control line needs to be broken, then prefer breaking after the `=` for any
+`let` sub-expression in an `if` or `while` expression that does not fit,
+and before `in` in a `for` expression; the following line should be block indented.
+If the control line is broken for any reason, then the opening brace should be on its
+own line and not indented. Examples:
 
 ```rust
 while let Some(foo)
@@ -572,6 +576,68 @@ if a_long_expression
 {
     ...
 }
+
+if let Some(a) = b
+    && another_long_expression
+    && a_third_long_expression
+{
+    // ...
+}
+
+if let Some(relatively_long_thing)
+    = a_long_expression
+    && another_long_expression
+    && a_third_long_expression
+{
+    // ...
+}
+
+if some_expr
+    && another_long_expression
+    && let Some(relatively_long_thing) =
+        a_long_long_long_long_long_long_really_reallllllllllyyyyyyy_long_expression
+    && a_third_long_expression
+{
+    // ...
+}
+```
+
+A let-chain control line is allowed to be formatted on a single line provided
+it only consists of two clauses, with the first, left-hand side operand being a literal or an
+`ident` (which can optionally be preceded by any number of unary prefix operators),
+and the second, right-hand side operand being a single-line `let` clause. Otherwise,
+the control line must be broken and formatted according to the above rules. For example:
+
+```rust
+if a && let Some(b) = foo() {
+    // ...
+}
+
+if true && let Some(b) = foo() {
+    // ...
+}
+
+let operator = if !from_hir_call && let Some(p) = parent {
+    // ...
+};
+
+if let Some(b) = foo()
+    && a
+{
+    // ..
+}
+
+if foo()
+    && let Some(b) = bar
+{
+    // ...
+}
+
+if gen_pos != GenericArgPosition::Type
+    && let Some(b) = gen_args.bindings.first()
+{
+    // ..
+}
 ```
 
 Where the initial clause spans multiple lines and ends with one or more closing