WIP: mut_mut: generalize the from-expansion check

clippy_lints/src/mut_mut.rs

@@ -1,5 +1,4 @@
 use clippy_utils::diagnostics::{span_lint, span_lint_hir};
-use clippy_utils::higher;
 use rustc_hir::{self as hir, AmbigArg, intravisit};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::ty;
@@ -55,20 +54,11 @@ pub struct MutVisitor<'a, 'tcx> {
 
 impl<'tcx> intravisit::Visitor<'tcx> for MutVisitor<'_, 'tcx> {
     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'_>) {
-        if expr.span.in_external_macro(self.cx.sess().source_map()) {
+        if expr.span.from_expansion() {
             return;
         }
 
-        if let Some(higher::ForLoop { arg, body, .. }) = higher::ForLoop::hir(expr) {
-            // A `for` loop lowers to:
-            // ```rust
-            // match ::std::iter::Iterator::next(&mut iter) {
-            // //                                ^^^^
-            // ```
-            // Let's ignore the generated code.
-            intravisit::walk_expr(self, arg);
-            intravisit::walk_expr(self, body);
-        } else if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e) = expr.kind {
+        if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e) = expr.kind {
             if let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, _) = e.kind {
                 span_lint_hir(
                     self.cx,

tests/ui/mut_mut.rs

@@ -53,18 +53,25 @@ fn main() {
     }
 
     let mut z = inline!(&mut $(&mut 3u32));
-    //~^ mut_mut
 }
 
 fn issue939() {
     let array = [5, 6, 7, 8, 9];
     let mut args = array.iter().skip(2);
     for &arg in &mut args {
+        // make sure that we still check inside the block, even if we skip `&mut args` because of
+        // it coming from the expansion of `for-in` syntax
+        let a = &mut &mut 2;
+        //~^ mut_mut
         println!("{}", arg);
     }
 
     let args = &mut args;
     for arg in args {
+        // make sure that we still check inside the block, even if we skip `&mut args` because of
+        // it coming from the expansion of `for-in` syntax
+        let a = &mut &mut 2;
+        //~^ mut_mut
         println!(":{}", arg);
     }
 }

tests/ui/mut_mut.stderr

@@ -13,14 +13,6 @@ error: generally you want to avoid `&mut &mut _` if possible
 LL |     let mut x = &mut &mut 1u32;
    |                 ^^^^^^^^^^^^^^
 
-error: generally you want to avoid `&mut &mut _` if possible
-  --> tests/ui/mut_mut.rs:55:25
-   |
-LL |     let mut z = inline!(&mut $(&mut 3u32));
-   |                         ^
-   |
-   = note: this error originates in the macro `__inline_mac_fn_main` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error: this expression mutably borrows a mutable reference. Consider reborrowing
   --> tests/ui/mut_mut.rs:36:21
    |
@@ -57,5 +49,17 @@ error: generally you want to avoid `&mut &mut _` if possible
 LL |         let y: &mut &mut &mut u32 = &mut &mut &mut 2;
    |                     ^^^^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: generally you want to avoid `&mut &mut _` if possible
+  --> tests/ui/mut_mut.rs:64:17
+   |
+LL |         let a = &mut &mut 2;
+   |                 ^^^^^^^^^^^
+
+error: generally you want to avoid `&mut &mut _` if possible
+  --> tests/ui/mut_mut.rs:73:17
+   |
+LL |         let a = &mut &mut 2;
+   |                 ^^^^^^^^^^^
+
+error: aborting due to 10 previous errors