Auto merge of #101362 - compiler-errors:unnecessary-let, r=cjgillot

Suggest removing unnecessary prefix let in patterns

Helps with #101291, though I think `@estebank` probably wants this:

> Finally, I think it'd be nice if we could detect that we don't know for sure and "just" swallow the rest of the expression (find the next ; accounting for nested braces) or the end of the item (easier).

... to be implemented before we close that issue out completely.

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -705,6 +705,14 @@ pub(crate) struct LeftArrowOperator {
     pub span: Span,
 }
 
+#[derive(SessionDiagnostic)]
+#[diag(parser::remove_let)]
+pub(crate) struct RemoveLet {
+    #[primary_span]
+    #[suggestion(applicability = "machine-applicable", code = "")]
+    pub span: Span,
+}
+
 // SnapshotParser is used to create a snapshot of the parser
 // without causing duplicate errors being emitted when the `Parser`
 // is dropped.