Enable 2021 compatibility lints for all in-tree code

This just applies the suggested fixes from the compatibility warnings,
leaving any that are in practice spurious in. This is primarily intended to
provide a starting point to identify possible fixes to the migrations (e.g., by
avoiding spurious warnings).

A secondary commit cleans these up where they are false positives (as is true in
many of the cases).

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -906,8 +906,11 @@ impl ThinLTOKeysMap {
     ) -> Self {
         let keys = iter::zip(modules, names)
             .map(|(module, name)| {
-                let key = build_string(|rust_str| unsafe {
-                    llvm::LLVMRustComputeLTOCacheKey(rust_str, module.identifier, data.0);
+                let key = build_string(|rust_str| {
+                    let _ = &data;
+                    unsafe {
+                        llvm::LLVMRustComputeLTOCacheKey(rust_str, module.identifier, data.0);
+                    }
                 })
                 .expect("Invalid ThinLTO module key");
                 (name.clone().into_string().unwrap(), key)

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -77,7 +77,7 @@ macro_rules! throw_validation_failure {
 ///
 macro_rules! try_validation {
     ($e:expr, $where:expr,
-    $( $( $p:pat )|+ => { $( $what_fmt:expr ),+ } $( expected { $( $expected_fmt:expr ),+ } )? ),+ $(,)?
+    $( $( $p:pat_param )|+ => { $( $what_fmt:expr ),+ } $( expected { $( $expected_fmt:expr ),+ } )? ),+ $(,)?
     ) => {{
         match $e {
             Ok(x) => x,

compiler/rustc_infer/src/infer/at.rs

@@ -195,10 +195,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
             let mut fields = at.infcx.combine_fields(trace, at.param_env);
-            fields
-                .sub(a_is_expected)
-                .relate(a, b)
-                .map(move |_| InferOk { value: (), obligations: fields.obligations })
+            fields.sub(a_is_expected).relate(a, b).map(move |_| {
+                let _ = &fields;
+                InferOk { value: (), obligations: fields.obligations }
+            })
         })
     }
 
@@ -212,10 +212,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
             let mut fields = at.infcx.combine_fields(trace, at.param_env);
-            fields
-                .equate(a_is_expected)
-                .relate(a, b)
-                .map(move |_| InferOk { value: (), obligations: fields.obligations })
+            fields.equate(a_is_expected).relate(a, b).map(move |_| {
+                let _ = &fields;
+                InferOk { value: (), obligations: fields.obligations }
+            })
         })
     }
 
@@ -227,10 +227,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
             let mut fields = at.infcx.combine_fields(trace, at.param_env);
-            fields
-                .lub(a_is_expected)
-                .relate(a, b)
-                .map(move |t| InferOk { value: t, obligations: fields.obligations })
+            fields.lub(a_is_expected).relate(a, b).map(move |t| {
+                let _ = &fields;
+                InferOk { value: t, obligations: fields.obligations }
+            })
         })
     }
 
@@ -242,10 +242,10 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
             let mut fields = at.infcx.combine_fields(trace, at.param_env);
-            fields
-                .glb(a_is_expected)
-                .relate(a, b)
-                .map(move |t| InferOk { value: t, obligations: fields.obligations })
+            fields.glb(a_is_expected).relate(a, b).map(move |t| {
+                let _ = &fields;
+                InferOk { value: t, obligations: fields.obligations }
+            })
         })
     }
 }

compiler/rustc_interface/src/util.rs

@@ -125,6 +125,7 @@ pub fn scoped_thread<F: FnOnce() -> R + Send, R: Send>(cfg: thread::Builder, f:
     let result_ptr = Ptr(&mut result as *mut _ as *mut ());
 
     let thread = cfg.spawn(move || {
+        let _ = (&run, &result_ptr);
         let run = unsafe { (*(run.0 as *mut Option<F>)).take().unwrap() };
         let result = unsafe { &mut *(result_ptr.0 as *mut Option<R>) };
         *result = Some(run());

compiler/rustc_mir_build/src/lints.rs

@@ -41,6 +41,7 @@ crate fn check<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
         let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
         let sp = tcx.sess.source_map().guess_head_span(tcx.hir().span_with_body(hir_id));
         tcx.struct_span_lint_hir(UNCONDITIONAL_RECURSION, hir_id, sp, |lint| {
+            let _ = &vis;
             let mut db = lint.build("function cannot return without recursing");
             db.span_label(sp, "cannot return without recursing");
             // offer some help to the programmer.

compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

@@ -40,7 +40,10 @@ where
             info!("fully_perform({:?})", self);
         }
 
-        scrape_region_constraints(infcx, || (self.closure)(infcx))
+        scrape_region_constraints(infcx, || {
+            let _ = &self;
+            (self.closure)(infcx)
+        })
     }
 }
 

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -394,6 +394,7 @@ fn report_conflicting_impls(
     // now because the struct_lint methods don't return back the DiagnosticBuilder
     // that's passed in.
     let decorate = |err: LintDiagnosticBuilder<'_>| {
+        let _ = &overlap;
         let msg = format!(
             "conflicting implementations of trait `{}`{}{}",
             overlap.trait_desc,

compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs

@@ -104,19 +104,22 @@ impl ChildrenExt for Children {
                 let self_ty = trait_ref.self_ty();
 
                 // FIXME: should postpone string formatting until we decide to actually emit.
-                with_no_trimmed_paths(|| OverlapError {
-                    with_impl: possible_sibling,
-                    trait_desc: trait_ref.print_only_trait_path().to_string(),
-                    // Only report the `Self` type if it has at least
-                    // some outer concrete shell; otherwise, it's
-                    // not adding much information.
-                    self_desc: if self_ty.has_concrete_skeleton() {
-                        Some(self_ty.to_string())
-                    } else {
-                        None
-                    },
-                    intercrate_ambiguity_causes: overlap.intercrate_ambiguity_causes,
-                    involves_placeholder: overlap.involves_placeholder,
+                with_no_trimmed_paths(|| {
+                    let _ = &overlap;
+                    OverlapError {
+                        with_impl: possible_sibling,
+                        trait_desc: trait_ref.print_only_trait_path().to_string(),
+                        // Only report the `Self` type if it has at least
+                        // some outer concrete shell; otherwise, it's
+                        // not adding much information.
+                        self_desc: if self_ty.has_concrete_skeleton() {
+                            Some(self_ty.to_string())
+                        } else {
+                            None
+                        },
+                        intercrate_ambiguity_causes: overlap.intercrate_ambiguity_causes,
+                        involves_placeholder: overlap.involves_placeholder,
+                    }
                 })
             };
 

compiler/rustc_typeck/src/check/compare_method.rs

@@ -1192,6 +1192,7 @@ fn compare_type_predicate_entailment<'tcx>(
         normalize_cause.clone(),
     );
     tcx.infer_ctxt().enter(|infcx| {
+        let _ = &impl_ty_own_bounds;
         let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
         let infcx = &inh.infcx;
 

compiler/rustc_typeck/src/check/method/probe.rs

@@ -441,6 +441,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // this creates one big transaction so that all type variables etc
         // that we create during the probe process are removed later
         self.probe(|_| {
+            let _ = &steps;
             let mut probe_cx = ProbeContext::new(
                 self,
                 span,

src/librustdoc/doctest.rs

@@ -964,6 +964,7 @@ impl Tester for Collector {
                 test_type: test::TestType::DocTest,
             },
             testfn: test::DynTestFn(Box::new(move || {
+                let _ = &config;
                 let report_unused_externs = |uext| {
                     unused_externs.lock().unwrap().push(uext);
                 };

src/tools/clippy/clippy_utils/src/lib.rs

@@ -990,7 +990,10 @@ pub fn can_move_expr_to_closure(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) ->
         captures: HirIdMap::default(),
     };
     v.visit_expr(expr);
-    v.allow_closure.then(|| v.captures)
+    v.allow_closure.then(|| {
+        let _ = &v;
+        v.captures
+    })
 }
 
 /// Returns the method names and argument list of nested method call expressions that make up