style-guide: Rewrite let-else section for clarity, without changing formatting

The section as written did not cover all cases, and left some of them
implicit. Rewrite it to systematically cover all cases. Place examples
immediately following the corresponding case.

In the process, reorder to move the simplest cases first: start with
single-line and add progressively more line breaks.

This does not change the meaning of the section at all, and in
particular does not change the defined style for let-else statements.

Author: joshtriplett

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

@@ -101,22 +101,69 @@ let Foo {
 
 #### else blocks (let-else statements)
 
-If a let statement contains an `else` component, also known as a let-else statement,
-then the `else` component should be formatted according to the same rules as the `else` block
-in [control flow expressions (i.e. if-else, and if-let-else expressions)](./expressions.md#control-flow-expressions).
-Apply the same formatting rules to the components preceding
-the `else` block (i.e. the `let pattern: Type = initializer_expr ...` portion)
-as described [above](#let-statements)
-
-Similarly to if-else expressions, if 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 with a preceding space if all the following are true:
+A let statement can contain an `else` component, making it a let-else statement.
+In this case, always apply the same formatting rules to the components preceding
+the `else` block (i.e. the `let pattern: Type = initializer_expr` portion)
+as described [for other let statements](#let-statements).
+
+The entire let-else statement may be formatted on a single line if all the
+following are true:
+
+* the entire statement is *short*
+* the `else` block contains only a single-line expression and no statements
+* the `else` block contains no comments
+* the let statement components preceding the `else` block can be formatted on a single line
+
+```rust
+let Some(1) = opt else { return };
+```
+
+Formatters may allow users to configure the value of the threshold
+used to determine whether a let-else statement is *short*.
+
+Otherwise, the let-else statement requires some line breaks.
+
+If breaking a let-else statement across multiple lines, never break between the
+`else` and the `{`, and always break before the `}`.
+
+If the let statement components preceding the `else` can be formatted on a
+single line, but the let-else does not qualify to be placed entirely on a
+single line, put the `else {` on the same line as the initializer expression,
+with a space between them, then break the line after the `{`. Indent the
+closing `}` to match the `let`, and indent the contained block one step
+further.
+
+```rust
+let Some(1) = opt else {
+    return;
+};
+
+let Some(1) = opt else {
+    // nope
+    return
+};
+```
+
+If the let statement components preceding the `else` can be formatted on a
+single line, but the `else {` does not fit on the same line, break the line
+before the `else`.
+
+```rust
+    let Some(x) = some_really_really_really_really_really_really_really_really_really_long_name
+    else {
+        return;
+    };
+```
+
+If the initializer expression is multi-line, 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, with a space between them, if all the following are
+true:
 
 * The initializer expression ends with one or more closing
   parentheses, square brackets, and/or braces
 * There is nothing else on that line
-* That line is not indented beyond the indent of the first line containing the `let` keyword
+* That line has the same indentation level as the initial `let` keyword.
 
 For example:
 
@@ -133,7 +180,9 @@ let Some(x) = y.foo(
 }
 ```
 
-Otherwise, the `else` keyword and opening brace should be placed on the next line after the end of the initializer expression, and should not be indented (the `else` keyword should be aligned with the `let` keyword).
+Otherwise, the `else` keyword and opening brace should be placed on the next
+line after the end of the initializer expression, and the `else` keyword should
+have the same indentation level as the `let` keyword.
 
 For example:
 
@@ -153,11 +202,6 @@ fn main() {
         return
     };
 
-    let Some(x) = some_really_really_really_really_really_really_really_really_really_long_name
-    else {
-        return;
-    };
-
     let Some(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) =
         bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
     else {
@@ -166,31 +210,6 @@ fn main() {
 }
 ```
 
-##### Single line let-else statements
-
-The entire let-else statement may be formatted on a single line if all the following are true:
-
-* the entire statement is *short*
-* the `else` block contains a single-line expression and no statements
-* the `else` block contains no comments
-* the let statement components preceding the `else` block can be formatted on a single line
-
-```rust
-let Some(1) = opt else { return };
-
-let Some(1) = opt else {
-    return;
-};
-
-let Some(1) = opt else {
-    // nope
-    return
-};
-```
-
-Formatters may allow users to configure the value of the threshold
-used to determine whether a let-else statement is *short*.
-
 ### Macros in statement position
 
 A macro use in statement position should use parentheses or square brackets as