Rollup merge of #140272 - Kivooeo:new-fix-four, r=est31

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

**Description**

This PR improves the error message when using `||` in an if let chain expression, addressing #140263.

**Changes**

1. Creates a dedicated error message specifically for `||` usage in let chains
2. Points the primary span directly at the `||` operator
3. Removes confusing secondary notes about "let statements" and unsupported contexts
5. Adds UI tests verifying the new error message and valid cases

**Before**
```rust
error: expected expression, found let statement
 --> src/main.rs:2:8
  |
2 |     if let true = true || false {}
  |        ^^^^^^^^^^^^^^^
  |
  = note: only supported directly in conditions of if and while expressions
note: || operators are not supported in let chain expressions
 --> src/main.rs:2:24
  |
2 |     if let true = true || false {}
  |
```

**After**
```rust
error: `||` operators are not supported in let chain conditions
 --> src/main.rs:2:24
  |
2 |     if let true = true || false {}
  |                        ^^
```

**Implementation details**
1. Added new `OrInLetChain` diagnostic in errors.rs

2. Modified `CondChecker` in expr.rs to prioritize the `||` error

3. Updated fluent message definitions to use clearer wording

**Related issue**
Fixes #140263

cc ```@ehuss``` (issue author)

2 files changed, 15 insertions, 4 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 35cf4c1b00d..6a6fb0eb9b5 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/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,
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 370eb3f402d..f3b53971b29 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/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 {