Rollup merge of #125156 - zachs18:for_loops_over_fallibles_behind_refs, r=Nilstrieb

Expand `for_loops_over_fallibles` lint to lint on fallibles behind references.

Extends the scope of the (warn-by-default) lint `for_loops_over_fallibles` from just `for _ in x` where `x: Option<_>/Result<_, _>` to also cover `x: &(mut) Option<_>/Result<_>`

```rs
fn main() {
    // Current lints
    for _ in Some(42) {}
    for _ in Ok::<_, i32>(42) {}

    // New lints
    for _ in &Some(42) {}
    for _ in &mut Some(42) {}
    for _ in &Ok::<_, i32>(42) {}
    for _ in &mut Ok::<_, i32>(42) {}

    // Should not lint
    for _ in Some(42).into_iter() {}
    for _ in Some(42).iter() {}
    for _ in Some(42).iter_mut() {}
    for _ in Ok::<_, i32>(42).into_iter() {}
    for _ in Ok::<_, i32>(42).iter() {}
    for _ in Ok::<_, i32>(42).iter_mut() {}
}
```

<details><summary><code>cargo build</code> diff</summary>

```diff
diff --git a/old.out b/new.out
index 84215aa..ca195a7 100644
--- a/old.out
+++ b/new.out
`@@` -1,33 +1,93 `@@`
 warning: for loop over an `Option`. This is more readably written as an `if let` statement
  --> src/main.rs:3:14
   |
 3 |     for _ in Some(42) {}
   |              ^^^^^^^^
   |
   = note: `#[warn(for_loops_over_fallibles)]` on by default
 help: to check pattern in a loop use `while let`
   |
 3 |     while let Some(_) = Some(42) {}
   |     ~~~~~~~~~~~~~~~ ~~~
 help: consider using `if let` to clear intent
   |
 3 |     if let Some(_) = Some(42) {}
   |     ~~~~~~~~~~~~ ~~~

 warning: for loop over a `Result`. This is more readably written as an `if let` statement
  --> src/main.rs:4:14
   |
 4 |     for _ in Ok::<_, i32>(42) {}
   |              ^^^^^^^^^^^^^^^^
   |
 help: to check pattern in a loop use `while let`
   |
 4 |     while let Ok(_) = Ok::<_, i32>(42) {}
   |     ~~~~~~~~~~~~~ ~~~
 help: consider using `if let` to clear intent
   |
 4 |     if let Ok(_) = Ok::<_, i32>(42) {}
   |     ~~~~~~~~~~ ~~~

-warning: `for-loops-over-fallibles` (bin "for-loops-over-fallibles") generated 2 warnings
-    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.04s
+warning: for loop over a `&Option`. This is more readably written as an `if let` statement
+ --> src/main.rs:7:14
+  |
+7 |     for _ in &Some(42) {}
+  |              ^^^^^^^^^
+  |
+help: to check pattern in a loop use `while let`
+  |
+7 |     while let Some(_) = &Some(42) {}
+  |     ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+  |
+7 |     if let Some(_) = &Some(42) {}
+  |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `&mut Option`. This is more readably written as an `if let` statement
+ --> src/main.rs:8:14
+  |
+8 |     for _ in &mut Some(42) {}
+  |              ^^^^^^^^^^^^^
+  |
+help: to check pattern in a loop use `while let`
+  |
+8 |     while let Some(_) = &mut Some(42) {}
+  |     ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+  |
+8 |     if let Some(_) = &mut Some(42) {}
+  |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `&Result`. This is more readably written as an `if let` statement
+ --> src/main.rs:9:14
+  |
+9 |     for _ in &Ok::<_, i32>(42) {}
+  |              ^^^^^^^^^^^^^^^^^
+  |
+help: to check pattern in a loop use `while let`
+  |
+9 |     while let Ok(_) = &Ok::<_, i32>(42) {}
+  |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+  |
+9 |     if let Ok(_) = &Ok::<_, i32>(42) {}
+  |     ~~~~~~~~~~ ~~~
+
+warning: for loop over a `&mut Result`. This is more readably written as an `if let` statement
+  --> src/main.rs:10:14
+   |
+10 |     for _ in &mut Ok::<_, i32>(42) {}
+   |              ^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+10 |     while let Ok(_) = &mut Ok::<_, i32>(42) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+10 |     if let Ok(_) = &mut Ok::<_, i32>(42) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: `for-loops-over-fallibles` (bin "for-loops-over-fallibles") generated 6 warnings
+    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s

```

</details>

-----

Question:

* ~~Currently, the article `an` is used for `&Option`, and `&mut Option` in the lint diagnostic, since that's what `Option` uses. Is this okay or should it be changed? (likewise, `a` is used for `&Result` and `&mut Result`)~~ The article `a` is used for `&Option`, `&mut Option`, `&Result`, `&mut Result` and (as before) `Result`. Only `Option` uses `an` (as before).

`@rustbot` label +A-lint

11 files changed, 112 insertions, 15 deletions

diff --git a/compiler/rustc_builtin_macros/src/deriving/default.rs b/compiler/rustc_builtin_macros/src/deriving/default.rs
index bf92ddb3370..577523a1d5a 100644
--- a/compiler/rustc_builtin_macros/src/deriving/default.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/default.rs
@@ -3,7 +3,7 @@ use crate::deriving::generic::*;
 use crate::errors;
 use core::ops::ControlFlow;
 use rustc_ast as ast;
-use rustc_ast::visit::walk_list;
+use rustc_ast::visit::visit_opt;
 use rustc_ast::{attr, EnumDef, VariantData};
 use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
 use rustc_span::symbol::Ident;
@@ -224,7 +224,7 @@ impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonVariantDefaultAttr<'a, '
         self.visit_ident(v.ident);
         self.visit_vis(&v.vis);
         self.visit_variant_data(&v.data);
-        walk_list!(self, visit_anon_const, &v.disr_expr);
+        visit_opt!(self, visit_anon_const, &v.disr_expr);
         for attr in &v.attrs {
             rustc_ast::visit::walk_attribute(self, attr);
         }
diff --git a/compiler/rustc_hir_analysis/src/check/region.rs b/compiler/rustc_hir_analysis/src/check/region.rs
index 4540310937d..e51f95eed02 100644
--- a/compiler/rustc_hir_analysis/src/check/region.rs
+++ b/compiler/rustc_hir_analysis/src/check/region.rs
@@ -6,7 +6,7 @@
 //!
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
 
-use rustc_ast::visit::walk_list;
+use rustc_ast::visit::visit_opt;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -168,7 +168,7 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
                 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
             }
         }
-        walk_list!(visitor, visit_expr, &blk.expr);
+        visit_opt!(visitor, visit_expr, &blk.expr);
     }
 
     visitor.cx = prev_cx;
diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 6f6480a4964..fa2f4d61a8d 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -263,7 +263,7 @@ lint_extern_without_abi = extern declarations without an explicit ABI are deprec
     .help = the default ABI is {$default_abi}
 
 lint_for_loops_over_fallibles =
-    for loop over {$article} `{$ty}`. This is more readably written as an `if let` statement
+    for loop over {$article} `{$ref_prefix}{$ty}`. This is more readably written as an `if let` statement
     .suggestion = consider using `if let` to clear intent
     .remove_next = to iterate over `{$recv_snip}` remove the call to `next`
     .use_while_let = to check pattern in a loop use `while let`
diff --git a/compiler/rustc_lint/src/for_loops_over_fallibles.rs b/compiler/rustc_lint/src/for_loops_over_fallibles.rs
index a6876d8aae7..b05f5e7638b 100644
--- a/compiler/rustc_lint/src/for_loops_over_fallibles.rs
+++ b/compiler/rustc_lint/src/for_loops_over_fallibles.rs
@@ -52,14 +52,27 @@ impl<'tcx> LateLintPass<'tcx> for ForLoopsOverFallibles {
 
         let ty = cx.typeck_results().expr_ty(arg);
 
-        let &ty::Adt(adt, args) = ty.kind() else { return };
+        let (adt, args, ref_mutability) = match ty.kind() {
+            &ty::Adt(adt, args) => (adt, args, None),
+            &ty::Ref(_, ty, mutability) => match ty.kind() {
+                &ty::Adt(adt, args) => (adt, args, Some(mutability)),
+                _ => return,
+            },
+            _ => return,
+        };
 
         let (article, ty, var) = match adt.did() {
+            did if cx.tcx.is_diagnostic_item(sym::Option, did) && ref_mutability.is_some() => ("a", "Option", "Some"),
             did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("an", "Option", "Some"),
             did if cx.tcx.is_diagnostic_item(sym::Result, did) => ("a", "Result", "Ok"),
             _ => return,
         };
 
+        let ref_prefix = match ref_mutability {
+            None => "",
+            Some(ref_mutability) => ref_mutability.ref_prefix_str(),
+        };
+
         let sub = if let Some(recv) = extract_iterator_next_call(cx, arg)
             && let Ok(recv_snip) = cx.sess().source_map().span_to_snippet(recv.span)
         {
@@ -85,7 +98,7 @@ impl<'tcx> LateLintPass<'tcx> for ForLoopsOverFallibles {
         cx.emit_span_lint(
             FOR_LOOPS_OVER_FALLIBLES,
             arg.span,
-            ForLoopsOverFalliblesDiag { article, ty, sub, question_mark, suggestion },
+            ForLoopsOverFalliblesDiag { article, ref_prefix, ty, sub, question_mark, suggestion },
         );
     }
 }
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 3bd6faca379..e16389cc5af 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -620,6 +620,7 @@ pub enum PtrNullChecksDiag<'a> {
 #[diag(lint_for_loops_over_fallibles)]
 pub struct ForLoopsOverFalliblesDiag<'a> {
     pub article: &'static str,
+    pub ref_prefix: &'static str,
     pub ty: &'static str,
     #[subdiagnostic]
     pub sub: ForLoopsOverFalliblesLoopSub<'a>,
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs
index 3fc719394bf..d948354bc49 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/build/matches/mod.rs
@@ -925,7 +925,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 for subpattern in prefix.iter() {
                     self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
                 }
-                for subpattern in slice {
+                if let Some(subpattern) = slice {
                     self.visit_primary_bindings(
                         subpattern,
                         pattern_user_ty.clone().subslice(from, to),
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index d1d0e336cfe..63d0d6c260d 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -12,7 +12,7 @@ use crate::{Module, ModuleOrUniformRoot, NameBinding, ParentScope, PathResult};
 use crate::{ResolutionError, Resolver, Segment, UseError};
 
 use rustc_ast::ptr::P;
-use rustc_ast::visit::{walk_list, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
+use rustc_ast::visit::{visit_opt, walk_list, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
 use rustc_ast::*;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_errors::{codes::*, Applicability, DiagArgValue, IntoDiagArg, StashKey};
@@ -3280,7 +3280,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     fn resolve_local(&mut self, local: &'ast Local) {
         debug!("resolving local ({:?})", local);
         // Resolve the type.
-        walk_list!(self, visit_ty, &local.ty);
+        visit_opt!(self, visit_ty, &local.ty);
 
         // Resolve the initializer.
         if let Some((init, els)) = local.kind.init_else_opt() {
@@ -3479,8 +3479,8 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     fn resolve_arm(&mut self, arm: &'ast Arm) {
         self.with_rib(ValueNS, RibKind::Normal, |this| {
             this.resolve_pattern_top(&arm.pat, PatternSource::Match);
-            walk_list!(this, visit_expr, &arm.guard);
-            walk_list!(this, visit_expr, &arm.body);
+            visit_opt!(this, visit_expr, &arm.guard);
+            visit_opt!(this, visit_expr, &arm.body);
         });
     }
 
diff --git a/library/core/tests/result.rs b/library/core/tests/result.rs
index a47ef7aa1eb..00a6fd75b4f 100644
--- a/library/core/tests/result.rs
+++ b/library/core/tests/result.rs
@@ -170,6 +170,7 @@ pub fn test_iter() {
 }
 
 #[test]
+#[allow(for_loops_over_fallibles)]
 pub fn test_iter_mut() {
     let mut ok: Result<isize, &'static str> = Ok(100);
     for loc in ok.iter_mut() {
diff --git a/src/tools/compiletest/src/json.rs b/src/tools/compiletest/src/json.rs
index 10726b98420..29e8809e5bd 100644
--- a/src/tools/compiletest/src/json.rs
+++ b/src/tools/compiletest/src/json.rs
@@ -282,7 +282,7 @@ fn push_expected_errors(
 
     // Add notes for the backtrace
     for span in primary_spans {
-        for frame in &span.expansion {
+        if let Some(frame) = &span.expansion {
             push_backtrace(expected_errors, frame, file_name);
         }
     }
@@ -315,7 +315,7 @@ fn push_backtrace(
         });
     }
 
-    for previous_expansion in &expansion.span.expansion {
+    if let Some(previous_expansion) = &expansion.span.expansion {
         push_backtrace(expected_errors, previous_expansion, file_name);
     }
 }
diff --git a/tests/ui/lint/for_loop_over_fallibles.rs b/tests/ui/lint/for_loop_over_fallibles.rs
index 52c3b8f2aae..89ac20c3521 100644
--- a/tests/ui/lint/for_loop_over_fallibles.rs
+++ b/tests/ui/lint/for_loop_over_fallibles.rs
@@ -41,3 +41,25 @@ fn _returns_result() -> Result<(), ()> {
 
     Ok(())
 }
+
+fn _by_ref() {
+    // Shared refs
+    for _ in &Some(1) {}
+    //~^ WARN for loop over a `&Option`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+    for _ in &Ok::<_, ()>(1) {}
+    //~^ WARN for loop over a `&Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+
+    // Mutable refs
+    for _ in &mut Some(1) {}
+    //~^ WARN for loop over a `&mut Option`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+    for _ in &mut Ok::<_, ()>(1) {}
+    //~^ WARN for loop over a `&mut Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+}
diff --git a/tests/ui/lint/for_loop_over_fallibles.stderr b/tests/ui/lint/for_loop_over_fallibles.stderr
index 96efdf85c49..f695de08257 100644
--- a/tests/ui/lint/for_loop_over_fallibles.stderr
+++ b/tests/ui/lint/for_loop_over_fallibles.stderr
@@ -97,5 +97,65 @@ help: consider using `if let` to clear intent
 LL |     if let Ok(_) = Ok::<_, ()>([0; 0]) {}
    |     ~~~~~~~~~~ ~~~
 
-warning: 6 warnings emitted
+warning: for loop over a `&Option`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:47:14
+   |
+LL |     for _ in &Some(1) {}
+   |              ^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Some(_) = &Some(1) {}
+   |     ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Some(_) = &Some(1) {}
+   |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `&Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:51:14
+   |
+LL |     for _ in &Ok::<_, ()>(1) {}
+   |              ^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = &Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = &Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over a `&mut Option`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:57:14
+   |
+LL |     for _ in &mut Some(1) {}
+   |              ^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Some(_) = &mut Some(1) {}
+   |     ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Some(_) = &mut Some(1) {}
+   |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `&mut Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:61:14
+   |
+LL |     for _ in &mut Ok::<_, ()>(1) {}
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = &mut Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = &mut Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: 10 warnings emitted