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.

2 files changed, 16 insertions, 1 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 9b5dd09372c..ad49227222b 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/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.
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index 88bd57d37cb..120a3c267f1 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -1,4 +1,5 @@
 use super::{ForceCollect, Parser, PathStyle, TrailingToken};
+use crate::parser::diagnostics::RemoveLet;
 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
 use rustc_ast::mut_visit::{noop_visit_pat, MutVisitor};
 use rustc_ast::ptr::P;
@@ -320,7 +321,13 @@ impl<'a> Parser<'a> {
         maybe_recover_from_interpolated_ty_qpath!(self, true);
         maybe_whole!(self, NtPat, |x| x);
 
-        let lo = self.token.span;
+        let mut lo = self.token.span;
+
+        if self.token.is_keyword(kw::Let) && self.look_ahead(1, |tok| tok.can_begin_pattern()) {
+            self.bump();
+            self.sess.emit_err(RemoveLet { span: lo });
+            lo = self.token.span;
+        }
 
         let pat = if self.check(&token::BinOp(token::And)) || self.token.kind == token::AndAnd {
             self.parse_pat_deref(expected)?