Only warn about unused mut in user-written code

varkor · varkor · commit 113141b6f5d1 · 2018-10-03T14:14:11.000+01:00
diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
@@ -4058,16 +4058,16 @@ impl<'a> LoweringContext<'a> {
                 // expand <head>
                 let head = self.lower_expr(head);
                 let head_sp = head.span;
+                let desugared_span = self.allow_internal_unstable(
+                    CompilerDesugaringKind::ForLoop,
+                    head.span,
+                );
 
                 let iter = self.str_to_ident("iter");
 
                 let next_ident = self.str_to_ident("__next");
-                let next_sp = self.allow_internal_unstable(
-                    CompilerDesugaringKind::ForLoop,
-                    head_sp,
-                );
                 let next_pat = self.pat_ident_binding_mode(
-                    next_sp,
+                    desugared_span,
                     next_ident,
                     hir::BindingAnnotation::Mutable,
                 );
@@ -4096,8 +4096,11 @@ impl<'a> LoweringContext<'a> {
                 };
 
                 // `mut iter`
-                let iter_pat =
-                    self.pat_ident_binding_mode(head_sp, iter, hir::BindingAnnotation::Mutable);
+                let iter_pat = self.pat_ident_binding_mode(
+                    desugared_span,
+                    iter,
+                    hir::BindingAnnotation::Mutable
+                );
 
                 // `match ::std::iter::Iterator::next(&mut iter) { ... }`
                 let match_expr = {
@@ -4126,8 +4129,12 @@ impl<'a> LoweringContext<'a> {
                 let next_expr = P(self.expr_ident(head_sp, next_ident, next_pat.id));
 
                 // `let mut __next`
-                let next_let =
-                    self.stmt_let_pat(head_sp, None, next_pat, hir::LocalSource::ForLoopDesugar);
+                let next_let = self.stmt_let_pat(
+                    desugared_span,
+                    None,
+                    next_pat,
+                    hir::LocalSource::ForLoopDesugar,
+                );
 
                 // `let <pat> = __next`
                 let pat = self.lower_pat(pat);
diff --git a/src/librustc_borrowck/borrowck/unused.rs b/src/librustc_borrowck/borrowck/unused.rs
@@ -76,10 +76,14 @@ impl<'a, 'tcx> UnusedMutCx<'a, 'tcx> {
             }
 
             let (hir_id, span) = ids[0];
-            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
+            if span.compiler_desugaring_kind().is_some() {
+                // If the `mut` arises as part of a desugaring, we should ignore it.
+                continue;
+            }
 
             // Ok, every name wasn't used mutably, so issue a warning that this
             // didn't need to be mutable.
+            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
             tcx.struct_span_lint_hir(UNUSED_MUT,
                                      hir_id,
                                      span,
diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
@@ -316,14 +316,18 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
             }
 
             let span = local_decl.source_info.span;
-            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
+            if span.compiler_desugaring_kind().is_some() {
+                // If the `mut` arises as part of a desugaring, we should ignore it.
+                continue;
+            }
 
             let mut err = tcx.struct_span_lint_node(
                 UNUSED_MUT,
                 vsi[local_decl.source_info.scope].lint_root,
                 span,
                 "variable does not need to be mutable",
             );
+            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
             err.span_suggestion_short_with_applicability(
                 mut_span,
                 "remove this `mut`",
diff --git a/src/librustc_mir/shim.rs b/src/librustc_mir/shim.rs
@@ -140,7 +140,8 @@ enum CallKind {
 fn temp_decl(mutability: Mutability, ty: Ty, span: Span) -> LocalDecl {
     let source_info = SourceInfo { scope: OUTERMOST_SOURCE_SCOPE, span };
     LocalDecl {
-        mutability, ty,
+        mutability,
+        ty,
         user_ty: None,
         name: None,
         source_info,
diff --git a/src/test/ui/mut/no-mut-lint-for-desugared-mut.rs b/src/test/ui/mut/no-mut-lint-for-desugared-mut.rs
@@ -0,0 +1,8 @@
+// run-pass
+
+#![deny(unused_mut)]
+#![allow(unreachable_code)]
+
+fn main() {
+    for _ in { return (); 0..3 } {} // ok
+}
