Move unstatisfaction check earlier.

Author: cjgillot

compiler/rustc_mir_transform/src/lib.rs

@@ -390,6 +390,50 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
         body.tainted_by_errors = Some(error_reported);
     }
 
+    // Check if it's even possible to satisfy the 'where' clauses
+    // for this item.
+    //
+    // This branch will never be taken for any normal function.
+    // However, it's possible to `#!feature(trivial_bounds)]` to write
+    // a function with impossible to satisfy clauses, e.g.:
+    // `fn foo() where String: Copy {}`
+    //
+    // We don't usually need to worry about this kind of case,
+    // since we would get a compilation error if the user tried
+    // to call it. However, since we optimize even without any
+    // calls to the function, we need to make sure that it even
+    // makes sense to try to evaluate the body.
+    //
+    // If there are unsatisfiable where clauses, then all bets are
+    // off, and we just give up.
+    //
+    // We manually filter the predicates, skipping anything that's not
+    // "global". We are in a potentially generic context
+    // (e.g. we are evaluating a function without substituting generic
+    // parameters, so this filtering serves two purposes:
+    //
+    // 1. We skip evaluating any predicates that we would
+    // never be able prove are unsatisfiable (e.g. `<T as Foo>`
+    // 2. We avoid trying to normalize predicates involving generic
+    // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
+    // the normalization code (leading to cycle errors), since
+    // it's usually never invoked in this way.
+    let predicates = tcx
+        .predicates_of(body.source.def_id())
+        .predicates
+        .iter()
+        .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
+    if traits::impossible_predicates(tcx, traits::elaborate(tcx, predicates).collect()) {
+        trace!("found unsatisfiable predicates for {:?}", body.source);
+        // Clear the body to only contain a single `unreachable` statement.
+        let bbs = body.basic_blocks.as_mut();
+        bbs.raw.truncate(1);
+        bbs[START_BLOCK].statements.clear();
+        bbs[START_BLOCK].terminator_mut().kind = TerminatorKind::Unreachable;
+        body.var_debug_info.clear();
+        body.local_decls.raw.truncate(body.arg_count + 1);
+    }
+
     run_analysis_to_runtime_passes(tcx, &mut body);
 
     tcx.alloc_steal_mir(body)
@@ -482,56 +526,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         WithMinOptLevel(1, x)
     }
 
-    // Check if it's even possible to satisfy the 'where' clauses
-    // for this item.
-    // This branch will never be taken for any normal function.
-    // However, it's possible to `#!feature(trivial_bounds)]` to write
-    // a function with impossible to satisfy clauses, e.g.:
-    // `fn foo() where String: Copy {}`
-    //
-    // We don't usually need to worry about this kind of case,
-    // since we would get a compilation error if the user tried
-    // to call it. However, since we optimize even without any
-    // calls to the function, we need to make sure that it even
-    // makes sense to try to evaluate the body.
-    //
-    // If there are unsatisfiable where clauses, then all bets are
-    // off, and we just give up.
-    //
-    // We manually filter the predicates, skipping anything that's not
-    // "global". We are in a potentially generic context
-    // (e.g. we are evaluating a function without substituting generic
-    // parameters, so this filtering serves two purposes:
-    //
-    // 1. We skip evaluating any predicates that we would
-    // never be able prove are unsatisfiable (e.g. `<T as Foo>`
-    // 2. We avoid trying to normalize predicates involving generic
-    // parameters (e.g. `<T as Foo>::MyItem`). This can confuse
-    // the normalization code (leading to cycle errors), since
-    // it's usually never invoked in this way.
-    let predicates = tcx
-        .predicates_of(body.source.def_id())
-        .predicates
-        .iter()
-        .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
-    if traits::impossible_predicates(tcx, traits::elaborate(tcx, predicates).collect()) {
-        trace!("optimizations skipped for {:?}: found unsatisfiable predicates", body.source);
-        // Clear the body to only contain a single `unreachable` statement.
-        let bbs = body.basic_blocks.as_mut();
-        bbs.raw.truncate(1);
-        bbs[START_BLOCK].statements.clear();
-        bbs[START_BLOCK].terminator_mut().kind = TerminatorKind::Unreachable;
-        body.var_debug_info.clear();
-        body.local_decls.raw.truncate(body.arg_count + 1);
-        pm::run_passes(
-            tcx,
-            body,
-            &[&reveal_all::RevealAll, &dump_mir::Marker("PreCodegen")],
-            Some(MirPhase::Runtime(RuntimePhase::Optimized)),
-        );
-        return;
-    }
-
     // The main optimizations that we do on MIR.
     pm::run_passes(
         tcx,