Do not bother optimizing impossible functions.

compiler/rustc_mir_transform/src/const_prop.rs

@@ -18,7 +18,6 @@ use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisi
 use rustc_span::{def_id::DefId, Span, DUMMY_SP};
 use rustc_target::abi::{self, Align, HasDataLayout, Size, TargetDataLayout};
 use rustc_target::spec::abi::Abi as CallAbi;
-use rustc_trait_selection::traits;
 
 use crate::MirPass;
 use rustc_const_eval::interpret::{
@@ -84,42 +83,6 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
             return;
         }
 
-        // 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 can do const propagation
-        // 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(def_id.to_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!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
-            return;
-        }
-
         trace!("ConstProp starting for {:?}", def_id);
 
         let dummy_body = &Body::new(

compiler/rustc_mir_transform/src/lib.rs

@@ -30,11 +30,12 @@ use rustc_middle::mir::visit::Visitor as _;
 use rustc_middle::mir::{
     traversal, AnalysisPhase, Body, ClearCrossCrate, ConstQualifs, Constant, LocalDecl, MirPass,
     MirPhase, Operand, Place, ProjectionElem, Promoted, RuntimePhase, Rvalue, SourceInfo,
-    Statement, StatementKind, TerminatorKind,
+    Statement, StatementKind, TerminatorKind, START_BLOCK,
 };
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
 use rustc_span::sym;
+use rustc_trait_selection::traits;
 
 #[macro_use]
 mod pass_manager;
@@ -389,6 +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)