Address review comments

- Add doc comment to new type
- Restore "only supported directly in conditions of `if` and `while` expressions" note
- Rename variant with clearer name

compiler/rustc_parse/messages.ftl

@@ -196,6 +196,7 @@ parse_expected_else_block = expected `{"{"}`, found {$first_tok}
     .suggestion = add an `if` if this is the condition of a chained `else if` statement
 
 parse_expected_expression_found_let = expected expression, found `let` statement
+    .note = only supported directly in conditions of `if` and `while` expressions
     .not_supported_or = `||` operators are not supported in let chain expressions
     .not_supported_parentheses = `let`s wrapped in parentheses are not supported in a context with let chains
 

compiler/rustc_parse/src/errors.rs

@@ -392,6 +392,7 @@ pub(crate) struct IfExpressionMissingCondition {
 
 #[derive(Diagnostic)]
 #[diag(parse_expected_expression_found_let)]
+#[note]
 pub(crate) struct ExpectedExpressionFoundLet {
     #[primary_span]
     pub span: Span,

compiler/rustc_parse/src/parser/expr.rs

@@ -2461,7 +2461,7 @@ impl<'a> Parser<'a> {
         let is_recovered = if !restrictions.contains(Restrictions::ALLOW_LET) {
             Some(self.sess.emit_err(errors::ExpectedExpressionFoundLet {
                 span: self.token.span,
-                reason: ForbiddenLetReason::GenericForbidden,
+                reason: ForbiddenLetReason::OtherForbidden,
             }))
         } else {
             None
@@ -3427,7 +3427,7 @@ impl<'a> Parser<'a> {
 #[derive(Clone, Copy, Subdiagnostic)]
 pub(crate) enum ForbiddenLetReason {
     /// `let` is not valid and the source environment is not important
-    GenericForbidden,
+    OtherForbidden,
     /// A let chain with the `||` operator
     #[note(parse_not_supported_or)]
     NotSupportedOr(#[primary_span] Span),
@@ -3439,6 +3439,15 @@ pub(crate) enum ForbiddenLetReason {
     NotSupportedParentheses(#[primary_span] Span),
 }
 
+/// Visitor to check for invalid/unstable use of `ExprKind::Let` that can't
+/// easily be caught in parsing. For example:
+///
+/// ```rust,ignore (example)
+/// // Only know that the let isn't allowed once the `||` token is reached
+/// if let Some(x) = y || true {}
+/// // Only know that the let isn't allowed once the second `=` token is reached.
+/// if let Some(x) = y && z = 1 {}
+/// ```
 struct CondChecker<'a> {
     parser: &'a Parser<'a>,
     forbid_let_reason: Option<ForbiddenLetReason>,
@@ -3495,14 +3504,14 @@ impl MutVisitor for CondChecker<'_> {
             | ExprKind::Tup(_)
             | ExprKind::Paren(_) => {
                 let forbid_let_reason = self.forbid_let_reason;
-                self.forbid_let_reason = Some(GenericForbidden);
+                self.forbid_let_reason = Some(OtherForbidden);
                 noop_visit_expr(e, self);
                 self.forbid_let_reason = forbid_let_reason;
             }
             ExprKind::Cast(ref mut op, _)
             | ExprKind::Type(ref mut op, _) => {
                 let forbid_let_reason = self.forbid_let_reason;
-                self.forbid_let_reason = Some(GenericForbidden);
+                self.forbid_let_reason = Some(OtherForbidden);
                 self.visit_expr(op);
                 self.forbid_let_reason = forbid_let_reason;
             }

tests/ui/expr/if/bad-if-let-suggestion.stderr

@@ -3,6 +3,8 @@ error: expected expression, found `let` statement
    |
 LL |     if let x = 1 && i = 2 {}
    |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
 
 error[E0425]: cannot find value `i` in this scope
   --> $DIR/bad-if-let-suggestion.rs:5:21

tests/ui/mir/issue-92893.stderr

@@ -3,6 +3,8 @@ error: expected expression, found `let` statement
    |
 LL | struct Bug<A = [(); (let a = (), 1).1]> {
    |                      ^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
 
 error: aborting due to previous error
 

tests/ui/rfcs/rfc-2294-if-let-guard/feature-gate.stderr

@@ -4,6 +4,7 @@ error: expected expression, found `let` statement
 LL |         () if (let 0 = 1) => {}
    |                ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:10:16
    |
@@ -16,6 +17,7 @@ error: expected expression, found `let` statement
 LL |         () if (((let 0 = 1))) => {}
    |                  ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:13:18
    |
@@ -28,6 +30,7 @@ error: expected expression, found `let` statement
 LL |         () if (let 0 = 1) && true => {}
    |                ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:24:16
    |
@@ -40,6 +43,7 @@ error: expected expression, found `let` statement
 LL |         () if true && (let 0 = 1) => {}
    |                        ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:27:24
    |
@@ -52,6 +56,7 @@ error: expected expression, found `let` statement
 LL |         () if (let 0 = 1) && (let 0 = 1) => {}
    |                ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:30:16
    |
@@ -64,6 +69,7 @@ error: expected expression, found `let` statement
 LL |         () if (let 0 = 1) && (let 0 = 1) => {}
    |                               ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:30:31
    |
@@ -76,6 +82,7 @@ error: expected expression, found `let` statement
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |                                          ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:34:42
    |
@@ -88,6 +95,7 @@ error: expected expression, found `let` statement
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |                                                       ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:34:42
    |
@@ -100,6 +108,7 @@ error: expected expression, found `let` statement
 LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) => {}
    |                                                                    ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/feature-gate.rs:34:42
    |
@@ -111,12 +120,16 @@ error: expected expression, found `let` statement
    |
 LL |     use_expr!((let 0 = 1 && 0 == 0));
    |                ^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
 
 error: expected expression, found `let` statement
   --> $DIR/feature-gate.rs:62:16
    |
 LL |     use_expr!((let 0 = 1));
    |                ^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
 
 error: no rules expected the token `let`
   --> $DIR/feature-gate.rs:70:15

tests/ui/rfcs/rfc-2294-if-let-guard/macro-expanded.stderr

@@ -7,6 +7,7 @@ LL |     ($e:expr) => { let Some(x) = $e }
 LL |         () if m!(Some(5)) => {}
    |               ----------- in this macro invocation
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
    = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error

tests/ui/rfcs/rfc-2294-if-let-guard/parens.stderr

@@ -4,6 +4,7 @@ error: expected expression, found `let` statement
 LL |         () if (let 0 = 1) => {}
    |                ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/parens.rs:10:16
    |
@@ -16,6 +17,7 @@ error: expected expression, found `let` statement
 LL |         () if (((let 0 = 1))) => {}
    |                  ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/parens.rs:12:18
    |
@@ -28,6 +30,7 @@ error: expected expression, found `let` statement
 LL |         () if (let 0 = 1) => {}
    |                ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/parens.rs:20:16
    |
@@ -40,6 +43,7 @@ error: expected expression, found `let` statement
 LL |         () if (((let 0 = 1))) => {}
    |                  ^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `let`s wrapped in parentheses are not supported in a context with let chains
   --> $DIR/parens.rs:22:18
    |

tests/ui/rfcs/rfc-2497-if-let-chains/ast-validate-guards.stderr

@@ -4,6 +4,7 @@ error: expected expression, found `let` statement
 LL |         Ok(opt) if let Some(4) = opt || false  => {}
    |                    ^^^^^^^^^^^^^^^^^
    |
+   = note: only supported directly in conditions of `if` and `while` expressions
 note: `||` operators are not supported in let chain expressions
   --> $DIR/ast-validate-guards.rs:5:38
    |

tests/ui/rfcs/rfc-2497-if-let-chains/avoid-invalid-mir.stderr

@@ -3,6 +3,8 @@ error: expected expression, found `let` statement
    |
 LL |     !let y = 42;
    |      ^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
 
 error: aborting due to previous error