Improve error message for || (or) in let chains

compiler/rustc_parse/messages.ftl

@@ -675,6 +675,8 @@ parse_note_pattern_alternatives_use_single_vert = alternatives in or-patterns ar
 
 parse_nul_in_c_str = null characters in C string literals are not supported
 
+parse_or_in_let_chain = `||` operators are not supported in let chain conditions
+
 parse_or_pattern_not_allowed_in_fn_parameters = top-level or-patterns are not allowed in function parameters
 parse_or_pattern_not_allowed_in_let_binding = top-level or-patterns are not allowed in `let` bindings
 parse_out_of_range_hex_escape = out of range hex escape

compiler/rustc_parse/src/errors.rs

@@ -478,6 +478,13 @@ pub(crate) struct ExpectedExpressionFoundLet {
     pub comparison: Option<MaybeComparison>,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_or_in_let_chain)]
+pub(crate) struct OrInLetChain {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Subdiagnostic, Clone, Copy)]
 #[multipart_suggestion(
     parse_maybe_missing_let,

compiler/rustc_parse/src/parser/expr.rs

@@ -4073,14 +4073,18 @@ impl MutVisitor for CondChecker<'_> {
         match e.kind {
             ExprKind::Let(_, _, _, ref mut recovered @ Recovered::No) => {
                 if let Some(reason) = self.forbid_let_reason {
-                    *recovered = Recovered::Yes(self.parser.dcx().emit_err(
-                        errors::ExpectedExpressionFoundLet {
+                    let error = match reason {
+                        NotSupportedOr(or_span) => {
+                            self.parser.dcx().emit_err(errors::OrInLetChain { span: or_span })
+                        }
+                        _ => self.parser.dcx().emit_err(errors::ExpectedExpressionFoundLet {
                             span,
                             reason,
                             missing_let: self.missing_let,
                             comparison: self.comparison,
-                        },
-                    ));
+                        }),
+                    };
+                    *recovered = Recovered::Yes(error);
                 } else if self.depth > 1 {
                     // Top level `let` is always allowed; only gate chains
                     match self.let_chains_policy {

tests/ui/parser/or-in-let-chain.edition2021.stderr

@@ -0,0 +1,28 @@
+error: `||` operators are not supported in let chain conditions
+  --> $DIR/or-in-let-chain.rs:6:24
+   |
+LL |     if let true = true || false {}
+   |                        ^^
+
+error: expected expression, found `let` statement
+  --> $DIR/or-in-let-chain.rs:9:9
+   |
+LL |     if (let true = true) || false {}
+   |         ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
+
+error: `||` operators are not supported in let chain conditions
+  --> $DIR/or-in-let-chain.rs:12:24
+   |
+LL |     if let true = true || false || true {}
+   |                        ^^
+
+error: `||` operators are not supported in let chain conditions
+  --> $DIR/or-in-let-chain.rs:15:33
+   |
+LL |     if let true = true && false || true {}
+   |                                 ^^
+
+error: aborting due to 4 previous errors
+

tests/ui/parser/or-in-let-chain.edition2024.stderr

@@ -0,0 +1,28 @@
+error: `||` operators are not supported in let chain conditions
+  --> $DIR/or-in-let-chain.rs:6:24
+   |
+LL |     if let true = true || false {}
+   |                        ^^
+
+error: expected expression, found `let` statement
+  --> $DIR/or-in-let-chain.rs:9:9
+   |
+LL |     if (let true = true) || false {}
+   |         ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if` and `while` expressions
+
+error: `||` operators are not supported in let chain conditions
+  --> $DIR/or-in-let-chain.rs:12:24
+   |
+LL |     if let true = true || false || true {}
+   |                        ^^
+
+error: `||` operators are not supported in let chain conditions
+  --> $DIR/or-in-let-chain.rs:15:33
+   |
+LL |     if let true = true && false || true {}
+   |                                 ^^
+
+error: aborting due to 4 previous errors
+

tests/ui/parser/or-in-let-chain.rs

@@ -0,0 +1,17 @@
+//@ revisions: edition2021 edition2024
+//@ [edition2021] edition: 2021
+//@ [edition2024] edition: 2024
+
+fn main() {
+    if let true = true || false {}
+    //~^ ERROR `||` operators are not supported in let chain conditions
+    // With parentheses
+    if (let true = true) || false {}
+    //~^ ERROR expected expression, found `let` statement
+    // Multiple || operators
+    if let true = true || false || true {}
+    //~^ ERROR `||` operators are not supported in let chain conditions
+    // Mixed operators (should still show error for ||)
+    if let true = true && false || true {}
+    //~^ ERROR `||` operators are not supported in let chain conditions
+}

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

@@ -3,7 +3,7 @@
 fn let_or_guard(x: Result<Option<i32>, ()>) {
     match x {
         Ok(opt) if let Some(4) = opt || false  => {}
-        //~^ ERROR expected expression, found `let` statement
+        //~^ ERROR `||` operators are not supported in let chain conditions
         _ => {}
     }
 }

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

@@ -1,11 +1,4 @@
-error: expected expression, found `let` statement
-  --> $DIR/ast-validate-guards.rs:5:20
-   |
-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
+error: `||` operators are not supported in let chain conditions
   --> $DIR/ast-validate-guards.rs:5:38
    |
 LL |         Ok(opt) if let Some(4) = opt || false  => {}

tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.feature.stderr

@@ -272,14 +272,7 @@ LL |     if (let 0 = 0)? {}
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: expected expression, found `let` statement
-  --> $DIR/disallowed-positions.rs:121:16
-   |
-LL |     if true || let 0 = 0 {}
-   |                ^^^^^^^^^
-   |
-   = note: only supported directly in conditions of `if` and `while` expressions
-note: `||` operators are not supported in let chain expressions
+error: `||` operators are not supported in let chain conditions
   --> $DIR/disallowed-positions.rs:121:13
    |
 LL |     if true || let 0 = 0 {}
@@ -485,14 +478,7 @@ LL |     while (let 0 = 0)? {}
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: expected expression, found `let` statement
-  --> $DIR/disallowed-positions.rs:212:19
-   |
-LL |     while true || let 0 = 0 {}
-   |                   ^^^^^^^^^
-   |
-   = note: only supported directly in conditions of `if` and `while` expressions
-note: `||` operators are not supported in let chain expressions
+error: `||` operators are not supported in let chain conditions
   --> $DIR/disallowed-positions.rs:212:16
    |
 LL |     while true || let 0 = 0 {}

tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.no_feature.stderr

@@ -272,14 +272,7 @@ LL |     if (let 0 = 0)? {}
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: expected expression, found `let` statement
-  --> $DIR/disallowed-positions.rs:121:16
-   |
-LL |     if true || let 0 = 0 {}
-   |                ^^^^^^^^^
-   |
-   = note: only supported directly in conditions of `if` and `while` expressions
-note: `||` operators are not supported in let chain expressions
+error: `||` operators are not supported in let chain conditions
   --> $DIR/disallowed-positions.rs:121:13
    |
 LL |     if true || let 0 = 0 {}
@@ -485,14 +478,7 @@ LL |     while (let 0 = 0)? {}
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: expected expression, found `let` statement
-  --> $DIR/disallowed-positions.rs:212:19
-   |
-LL |     while true || let 0 = 0 {}
-   |                   ^^^^^^^^^
-   |
-   = note: only supported directly in conditions of `if` and `while` expressions
-note: `||` operators are not supported in let chain expressions
+error: `||` operators are not supported in let chain conditions
   --> $DIR/disallowed-positions.rs:212:16
    |
 LL |     while true || let 0 = 0 {}

tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.nothing.stderr

@@ -272,14 +272,7 @@ LL |     if (let 0 = 0)? {}
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: expected expression, found `let` statement
-  --> $DIR/disallowed-positions.rs:121:16
-   |
-LL |     if true || let 0 = 0 {}
-   |                ^^^^^^^^^
-   |
-   = note: only supported directly in conditions of `if` and `while` expressions
-note: `||` operators are not supported in let chain expressions
+error: `||` operators are not supported in let chain conditions
   --> $DIR/disallowed-positions.rs:121:13
    |
 LL |     if true || let 0 = 0 {}
@@ -485,14 +478,7 @@ LL |     while (let 0 = 0)? {}
    |
    = note: only supported directly in conditions of `if` and `while` expressions
 
-error: expected expression, found `let` statement
-  --> $DIR/disallowed-positions.rs:212:19
-   |
-LL |     while true || let 0 = 0 {}
-   |                   ^^^^^^^^^
-   |
-   = note: only supported directly in conditions of `if` and `while` expressions
-note: `||` operators are not supported in let chain expressions
+error: `||` operators are not supported in let chain conditions
   --> $DIR/disallowed-positions.rs:212:16
    |
 LL |     while true || let 0 = 0 {}

tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.rs

@@ -119,7 +119,7 @@ fn nested_within_if_expr() {
     //~^ ERROR expected expression, found `let` statement
 
     if true || let 0 = 0 {}
-    //~^ ERROR expected expression, found `let` statement
+    //~^ ERROR `||` operators are not supported in let chain conditions
     if (true || let 0 = 0) {}
     //~^ ERROR expected expression, found `let` statement
     if true && (true || let 0 = 0) {}
@@ -210,7 +210,7 @@ fn nested_within_while_expr() {
     //~^ ERROR expected expression, found `let` statement
 
     while true || let 0 = 0 {}
-    //~^ ERROR expected expression, found `let` statement
+    //~^ ERROR `||` operators are not supported in let chain conditions
     while (true || let 0 = 0) {}
     //~^ ERROR expected expression, found `let` statement
     while true && (true || let 0 = 0) {}