Don't evaluate predicates twice to check for impossible predicates

Author: oli-obk

compiler/rustc_middle/src/query/erase.rs

@@ -241,6 +241,7 @@ trivial! {
     Option<rustc_target::spec::PanicStrategy>,
     Option<usize>,
     Result<(), rustc_errors::ErrorGuaranteed>,
+    Result<(), traits::util::HasImpossiblePredicates>,
     Result<(), rustc_middle::traits::query::NoSolution>,
     Result<rustc_middle::traits::EvaluationResult, rustc_middle::traits::OverflowError>,
     rustc_ast::expand::allocator::AllocatorKind,

compiler/rustc_middle/src/query/mod.rs

@@ -39,6 +39,7 @@ use crate::traits::query::{
     OutlivesBound,
 };
 use crate::traits::specialization_graph;
+use crate::traits::util::HasImpossiblePredicates;
 use crate::traits::{
     CodegenObligationError, EvaluationResult, ImplSource, ObjectSafetyViolation, ObligationCause,
     OverflowError, WellFormedLoc,
@@ -1019,7 +1020,7 @@ rustc_queries! {
     }
 
     /// Run the const prop lints on the `mir_promoted` of an item.
-    query const_prop_lint(key: LocalDefId) {
+    query const_prop_lint(key: LocalDefId) -> Result<(), HasImpossiblePredicates> {
         desc { |tcx| "checking const prop lints for `{}`", tcx.def_path_str(key) }
         cache_on_disk_if { true }
     }

compiler/rustc_middle/src/traits/util.rs

@@ -46,3 +46,9 @@ impl<'tcx> Iterator for Elaborator<'tcx> {
         }
     }
 }
+
+/// Used as an error type to signal that an item may have an invalid body, because its
+/// where bounds are trivially not satisfyable.
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(HashStable, Encodable, Decodable)]
+pub struct HasImpossiblePredicates;

compiler/rustc_mir_transform/src/lib.rs

@@ -40,6 +40,7 @@ use rustc_middle::mir::{
     SourceInfo, Statement, StatementKind, TerminatorKind, START_BLOCK,
 };
 use rustc_middle::query::Providers;
+use rustc_middle::traits::util::HasImpossiblePredicates;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
 use rustc_span::{source_map::Spanned, sym, DUMMY_SP};
 use rustc_trait_selection::traits;
@@ -399,51 +400,12 @@ fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: LocalDefId) -> Body<'_> {
     body
 }
 
-fn const_prop_lint(tcx: TyCtxt<'_>, def: LocalDefId) {
+fn const_prop_lint(tcx: TyCtxt<'_>, def: LocalDefId) -> Result<(), HasImpossiblePredicates> {
     let (body, _) = tcx.mir_promoted(def);
     let body = body.borrow();
 
     let mir_borrowck = tcx.mir_borrowck(def);
 
-    // If there are impossible bounds on the body being const prop linted,
-    // the const eval logic used in const prop may ICE unexpectedly.
-    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())
-        && mir_borrowck.tainted_by_errors.is_none()
-        && body.tainted_by_errors.is_none()
-    {
-        const_prop_lint::ConstPropLint.run_lint(tcx, &body);
-    }
-}
-
-/// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
-/// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
-/// end up missing the source MIR due to stealing happening.
-fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &Steal<Body<'_>> {
-    if tcx.is_coroutine(def.to_def_id()) {
-        tcx.ensure_with_value().mir_coroutine_witnesses(def);
-    }
-    let mir_borrowck = tcx.mir_borrowck(def);
-    tcx.ensure().const_prop_lint(def);
-
-    let is_fn_like = tcx.def_kind(def).is_fn_like();
-    if is_fn_like {
-        // Do not compute the mir call graph without said call graph actually being used.
-        if pm::should_run_pass(tcx, &inline::Inline) {
-            tcx.ensure_with_value().mir_inliner_callees(ty::InstanceDef::Item(def.to_def_id()));
-        }
-    }
-
-    let (body, _) = tcx.mir_promoted(def);
-    let mut body = body.steal();
-    if let Some(error_reported) = mir_borrowck.tainted_by_errors {
-        body.tainted_by_errors = Some(error_reported);
-    }
-
     // Check if it's even possible to satisfy the 'where' clauses
     // for this item.
     //
@@ -478,6 +440,40 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
         .iter()
         .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
     if traits::impossible_predicates(tcx, traits::elaborate(tcx, predicates).collect()) {
+        Err(HasImpossiblePredicates)
+    } else {
+        if mir_borrowck.tainted_by_errors.is_none() && body.tainted_by_errors.is_none() {
+            const_prop_lint::ConstPropLint.run_lint(tcx, &body);
+        }
+        Ok(())
+    }
+}
+
+/// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
+/// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
+/// end up missing the source MIR due to stealing happening.
+fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &Steal<Body<'_>> {
+    if tcx.is_coroutine(def.to_def_id()) {
+        tcx.ensure_with_value().mir_coroutine_witnesses(def);
+    }
+    let mir_borrowck = tcx.mir_borrowck(def);
+    let has_impossible_predicates = tcx.const_prop_lint(def);
+
+    let is_fn_like = tcx.def_kind(def).is_fn_like();
+    if is_fn_like {
+        // Do not compute the mir call graph without said call graph actually being used.
+        if pm::should_run_pass(tcx, &inline::Inline) {
+            tcx.ensure_with_value().mir_inliner_callees(ty::InstanceDef::Item(def.to_def_id()));
+        }
+    }
+
+    let (body, _) = tcx.mir_promoted(def);
+    let mut body = body.steal();
+    if let Some(error_reported) = mir_borrowck.tainted_by_errors {
+        body.tainted_by_errors = Some(error_reported);
+    }
+
+    if let Err(HasImpossiblePredicates) = has_impossible_predicates {
         trace!("found unsatisfiable predicates for {:?}", body.source);
         // Clear the body to only contain a single `unreachable` statement.
         let bbs = body.basic_blocks.as_mut();