Rollup merge of #108427 - y21:for-else-diagnostic, r=compiler-errors

Recover from for-else and while-else

This recovers from attempts at writing for-else or while-else loops, which might help users coming from e.g. Python.
```rs
for _ in 0..0 {
  // ...
} else {
  // ...
}
```
Combined with trying to store it in a let binding, the current diagnostic can be a bit confusing. It mentions let-else and suggests wrapping the loop in parentheses, which the user probably doesn't want. let-else doesn't make sense for `for` and `while` loops, as they are of type `()` (which already is an irrefutable pattern and doesn't need let-else).
<details>
<summary>Current diagnostic</summary>

```rs
error: right curly brace `}` before `else` in a `let...else` statement not allowed
 --> src/main.rs:4:5
  |
4 |     } else {
  |     ^
  |
help: wrap the expression in parentheses
  |
2 ~     let _x = (for _ in 0..0 {
3 |
4 ~     }) else {
  |
```
</details>

Some questions:
- Can the wording for the error message be improved? Would "for...else loops are not allowed" fit better?
- Should we be more "conservative" in case we want to support this in the future (i.e. say "for...else loops are **currently** not allowed/supported")?
- Is there a better way than storing a `&'static str` for the loop type? It is used for substituting the placeholder in the locale file (since it can emit either `for...else` or `while...else`). Maybe there is an enum I could use that I couldn't find

compiler/rustc_parse/locales/en-US.ftl

@@ -151,6 +151,10 @@ parse_missing_in_in_for_loop = missing `in` in `for` loop
 parse_missing_expression_in_for_loop = missing expression to iterate on in `for` loop
     .suggestion = try adding an expression to the `for` loop
 
+parse_loop_else = `{$loop_kind}...else` loops are not supported
+    .note = consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    .loop_keyword = `else` is attached to this loop
+
 parse_missing_comma_after_match_arm = expected `,` following `match` arm
     .suggestion = missing a comma here to end this `match` arm
 

compiler/rustc_parse/src/errors.rs

@@ -451,6 +451,17 @@ pub(crate) struct MissingExpressionInForLoop {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_loop_else)]
+#[note]
+pub(crate) struct LoopElseNotSupported {
+    #[primary_span]
+    pub span: Span,
+    pub loop_kind: &'static str,
+    #[label(parse_loop_keyword)]
+    pub loop_kw: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_missing_comma_after_match_arm)]
 pub(crate) struct MissingCommaAfterMatchArm {

compiler/rustc_parse/src/parser/expr.rs

@@ -2503,9 +2503,27 @@ impl<'a> Parser<'a> {
         let (attrs, loop_block) = self.parse_inner_attrs_and_block()?;
 
         let kind = ExprKind::ForLoop(pat, expr, loop_block, opt_label);
+
+        self.recover_loop_else("for", lo)?;
+
         Ok(self.mk_expr_with_attrs(lo.to(self.prev_token.span), kind, attrs))
     }
 
+    /// Recovers from an `else` clause after a loop (`for...else`, `while...else`)
+    fn recover_loop_else(&mut self, loop_kind: &'static str, loop_kw: Span) -> PResult<'a, ()> {
+        if self.token.is_keyword(kw::Else) && self.may_recover() {
+            let else_span = self.token.span;
+            self.bump();
+            let else_clause = self.parse_else_expr()?;
+            self.sess.emit_err(errors::LoopElseNotSupported {
+                span: else_span.to(else_clause.span),
+                loop_kind,
+                loop_kw,
+            });
+        }
+        Ok(())
+    }
+
     fn error_missing_in_for_loop(&mut self) {
         let (span, sub): (_, fn(_) -> _) = if self.token.is_ident_named(sym::of) {
             // Possibly using JS syntax (#75311).
@@ -2530,6 +2548,9 @@ impl<'a> Parser<'a> {
             err.span_label(cond.span, "this `while` condition successfully parsed");
             err
         })?;
+
+        self.recover_loop_else("while", lo)?;
+
         Ok(self.mk_expr_with_attrs(
             lo.to(self.prev_token.span),
             ExprKind::While(cond, body, opt_label),
@@ -2541,6 +2562,7 @@ impl<'a> Parser<'a> {
     fn parse_expr_loop(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
         let loop_span = self.prev_token.span;
         let (attrs, body) = self.parse_inner_attrs_and_block()?;
+        self.recover_loop_else("loop", lo)?;
         Ok(self.mk_expr_with_attrs(
             lo.to(self.prev_token.span),
             ExprKind::Loop(body, opt_label, loop_span),

tests/ui/for/for-else-err.rs

@@ -0,0 +1,8 @@
+fn main() {
+    for _ in 0..1 {
+        //~^ NOTE `else` is attached to this loop
+    } else {
+        //~^ ERROR `for...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    }
+}

tests/ui/for/for-else-err.stderr

@@ -0,0 +1,17 @@
+error: `for...else` loops are not supported
+  --> $DIR/for-else-err.rs:4:7
+   |
+LL |       for _ in 0..1 {
+   |       --- `else` is attached to this loop
+LL |
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |     }
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+

tests/ui/for/for-else-let-else-err.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let _ = for _ in 0..1 {
+        //~^ NOTE `else` is attached to this loop
+    } else {
+        //~^ ERROR `for...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    };
+}

tests/ui/for/for-else-let-else-err.stderr

@@ -0,0 +1,17 @@
+error: `for...else` loops are not supported
+  --> $DIR/for-else-let-else-err.rs:4:7
+   |
+LL |       let _ = for _ in 0..1 {
+   |               --- `else` is attached to this loop
+LL |
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |     };
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+

tests/ui/let-else/let-else-brace-before-else.fixed

@@ -7,10 +7,6 @@ fn main() {
         //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
         return;
     };
-    let Some(1) = (loop { break Some(1) }) else {
-        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
-        return;
-    };
     let 2 = 1 + (match 1 { n => n }) else {
         //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
         return;

tests/ui/let-else/let-else-brace-before-else.rs

@@ -7,10 +7,6 @@ fn main() {
         //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
         return;
     };
-    let Some(1) = loop { break Some(1) } else {
-        //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
-        return;
-    };
     let 2 = 1 + match 1 { n => n } else {
         //~^ ERROR right curly brace `}` before `else` in a `let...else` statement not allowed
         return;

tests/ui/let-else/let-else-brace-before-else.stderr

@@ -10,18 +10,7 @@ LL |     let Some(1) = ({ Some(1) }) else {
    |                   +           +
 
 error: right curly brace `}` before `else` in a `let...else` statement not allowed
-  --> $DIR/let-else-brace-before-else.rs:10:40
-   |
-LL |     let Some(1) = loop { break Some(1) } else {
-   |                                        ^
-   |
-help: wrap the expression in parentheses
-   |
-LL |     let Some(1) = (loop { break Some(1) }) else {
-   |                   +                      +
-
-error: right curly brace `}` before `else` in a `let...else` statement not allowed
-  --> $DIR/let-else-brace-before-else.rs:14:34
+  --> $DIR/let-else-brace-before-else.rs:10:34
    |
 LL |     let 2 = 1 + match 1 { n => n } else {
    |                                  ^
@@ -32,7 +21,7 @@ LL |     let 2 = 1 + (match 1 { n => n }) else {
    |                 +                  +
 
 error: right curly brace `}` before `else` in a `let...else` statement not allowed
-  --> $DIR/let-else-brace-before-else.rs:18:40
+  --> $DIR/let-else-brace-before-else.rs:14:40
    |
 LL |     let Some(1) = unsafe { unsafe_fn() } else {
    |                                        ^
@@ -42,5 +31,5 @@ help: wrap the expression in parentheses
 LL |     let Some(1) = (unsafe { unsafe_fn() }) else {
    |                   +                      +
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 

tests/ui/loops/loop-else-break-with-value.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let Some(1) = loop {
+        //~^ NOTE `else` is attached to this loop
+        break Some(1)
+    } else {
+        //~^ ERROR `loop...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+        return;
+    };
+}

tests/ui/loops/loop-else-break-with-value.stderr

@@ -0,0 +1,18 @@
+error: `loop...else` loops are not supported
+  --> $DIR/loop-else-break-with-value.rs:5:7
+   |
+LL |       let Some(1) = loop {
+   |                     ---- `else` is attached to this loop
+...
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |         return;
+LL | |     };
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+

tests/ui/loops/loop-else-err.rs

@@ -0,0 +1,8 @@
+fn main() {
+    loop {
+        //~^ NOTE `else` is attached to this loop
+    } else {
+        //~^ ERROR `loop...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    }
+}

tests/ui/loops/loop-else-err.stderr

@@ -0,0 +1,17 @@
+error: `loop...else` loops are not supported
+  --> $DIR/loop-else-err.rs:4:7
+   |
+LL |       loop {
+   |       ---- `else` is attached to this loop
+LL |
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |     }
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+

tests/ui/loops/loop-else-let-else-err.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let _ = loop {
+        //~^ NOTE `else` is attached to this loop
+    } else {
+        //~^ ERROR `loop...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    };
+}

tests/ui/loops/loop-else-let-else-err.stderr

@@ -0,0 +1,17 @@
+error: `loop...else` loops are not supported
+  --> $DIR/loop-else-let-else-err.rs:4:7
+   |
+LL |       let _ = loop {
+   |               ---- `else` is attached to this loop
+LL |
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |     };
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+

tests/ui/while/while-else-err.rs

@@ -0,0 +1,8 @@
+fn main() {
+    while false {
+        //~^ NOTE `else` is attached to this loop
+    } else {
+        //~^ ERROR `while...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    };
+}

tests/ui/while/while-else-err.stderr

@@ -0,0 +1,17 @@
+error: `while...else` loops are not supported
+  --> $DIR/while-else-err.rs:4:7
+   |
+LL |       while false {
+   |       ----- `else` is attached to this loop
+LL |
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |     };
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+

tests/ui/while/while-else-let-else-err.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let _ = while false {
+        //~^ NOTE `else` is attached to this loop
+    } else {
+        //~^ ERROR `while...else` loops are not supported
+        //~| NOTE consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    };
+}

tests/ui/while/while-else-let-else-err.stderr

@@ -0,0 +1,17 @@
+error: `while...else` loops are not supported
+  --> $DIR/while-else-let-else-err.rs:4:7
+   |
+LL |       let _ = while false {
+   |               ----- `else` is attached to this loop
+LL |
+LL |       } else {
+   |  _______^
+LL | |
+LL | |
+LL | |     };
+   | |_____^
+   |
+   = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+
+error: aborting due to previous error
+