Pretty-print let-else with added parenthesization when needed

[dtolnay]

Rustc used to produce invalid syntax for the following code, which is problematic because it means we cannot apply rustfmt to the output of -Zunpretty=expanded.

macro_rules! expr {
    ($e:expr) => { $e };
}

fn main() {
    let _ = expr!(loop {}) else { return; };
}

$ rustc repro.rs -Zunpretty=expanded | rustfmt
error: `loop...else` loops are not supported
 --> <stdin>:9:29
  |
9 | fn main() { let _ = loop {} else { return; }; }
  |                     ----    ^^^^^^^^^^^^^^^^
  |                     |
  |                     `else` is attached to this loop
  |
  = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run

[rustbot]

r? @nnethercote

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[compiler-errors]

r=me when CI is green

[dtolnay]

I initially tried testing this with:

    c2_let_expr_else_return!(
        [ loop {} ],
        "let _ = (loop {}) else { return; };",
        "let _ = loop {} else { return; }",
    );

but the curly brace diagnostic actually looks inside of the $expr. I am not sure whether that is intentional. (In my opinion it shouldn't do that.) I added a separate test in tests/ui/unpretty instead.

error: right curly brace `}` before `else` in a `let...else` statement not allowed
  --> tests/ui/macros/stringify.rs:704:17
   |
LL |         [ loop {} ],
   |                 ^
   |
help: wrap the expression in parentheses
   |
LL |         [ (loop {}) ],
   |           +       +

[dtolnay]

@bors r=compiler-errors

[bors]

📌 Commit 94cc82c has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 94cc82c with merge ecbe3fd...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing ecbe3fd to master...

[rust-timer]

Finished benchmarking commit (ecbe3fd): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-2.1%, 2.4%]	2

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 677.096s -> 676.356s (-0.11%)
Artifact size: 316.08 MiB -> 316.06 MiB (-0.01%)