Auto merge of rust-lang#78801 - sexxi-goose:min_capture, r=nikomatsakis

RFC-2229: Implement Precise Capture Analysis

### This PR introduces
- Feature gate for RFC-2229 (incomplete) `capture_disjoint_field`
- Rustc Attribute to print out the capture analysis `rustc_capture_analysis`
- Precise capture analysis

### Description of the analysis
1. If the feature gate is not set then all variables that are not local to the closure will be added to the list of captures. (This is for backcompat)
2. The rest of the analysis is based entirely on how the captured `Place`s are used within the closure. Precise information (i.e. projections) about the `Place` is maintained throughout.
3. To reduce the amount of information we need to keep track of, we do a minimization step. In this step, we determine a list such that no Place within this list represents an ancestor path to another entry in the list.  Check rust-lang/project-rfc-2229#9 for more detailed examples.
4. To keep the compiler functional as before we implement a Bridge between the results of this new analysis to existing data structures used for closure captures. Note the new capture analysis results are only part of MaybeTypeckTables that is the information is only available during typeck-ing.

### Known issues
- Statements like `let _ = x` will make the compiler ICE when used within a closure with the feature enabled. More generally speaking the issue is caused by `let` statements that create no bindings and are init'ed using a Place expression.

### Testing
We removed the code that would handle the case where the feature gate is not set, to enable the feature as default and did a bors try and perf run. More information here: rust-lang#78762

### Thanks
This has been slowly in the works for a while now.
I want to call out `@Azhng` `@ChrisPardy` `@null-sleep` `@jenniferwills` `@logmosier` `@roxelo` for working on this and the previous PRs that led up to this, `@nikomatsakis` for guiding us.

Closes rust-lang/project-rfc-2229#7
Closes rust-lang/project-rfc-2229#9
Closes rust-lang/project-rfc-2229#6
Closes rust-lang/project-rfc-2229#19

r? `@nikomatsakis`

compiler/rustc_feature/src/active.rs

@@ -616,6 +616,9 @@ declare_features! (
     /// Enables `#[cfg(panic = "...")]` config key.
     (active, cfg_panic, "1.49.0", Some(77443), None),
 
+    /// Allows capturing disjoint fields in a closure/generator (RFC 2229).
+    (active, capture_disjoint_fields, "1.49.0", Some(53488), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -639,6 +642,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::inline_const,
     sym::repr128,
     sym::unsized_locals,
+    sym::capture_disjoint_fields,
 ];
 
 /// Some features are not allowed to be used together at the same time, if

compiler/rustc_feature/src/builtin_attrs.rs

@@ -547,6 +547,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // ==========================================================================
 
     rustc_attr!(TEST, rustc_outlives, Normal, template!(Word)),
+    rustc_attr!(TEST, rustc_capture_analysis, Normal, template!(Word)),
     rustc_attr!(TEST, rustc_variance, Normal, template!(Word)),
     rustc_attr!(TEST, rustc_layout, Normal, template!(List: "field1, field2, ...")),
     rustc_attr!(TEST, rustc_regions, Normal, template!(Word)),

compiler/rustc_middle/src/ty/context.rs

@@ -415,6 +415,10 @@ pub struct TypeckResults<'tcx> {
     /// entire variable.
     pub closure_captures: ty::UpvarListMap,
 
+    /// Tracks the minimum captures required for a closure;
+    /// see `MinCaptureInformationMap` for more details.
+    pub closure_min_captures: ty::MinCaptureInformationMap<'tcx>,
+
     /// Stores the type, expression, span and optional scope span of all types
     /// that are live across the yield of this generator (if a generator).
     pub generator_interior_types: Vec<GeneratorInteriorTypeCause<'tcx>>,
@@ -442,6 +446,7 @@ impl<'tcx> TypeckResults<'tcx> {
             tainted_by_errors: None,
             concrete_opaque_types: Default::default(),
             closure_captures: Default::default(),
+            closure_min_captures: Default::default(),
             generator_interior_types: Default::default(),
         }
     }
@@ -676,6 +681,7 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for TypeckResults<'tcx> {
             tainted_by_errors,
             ref concrete_opaque_types,
             ref closure_captures,
+            ref closure_min_captures,
             ref generator_interior_types,
         } = *self;
 
@@ -709,6 +715,7 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for TypeckResults<'tcx> {
             tainted_by_errors.hash_stable(hcx, hasher);
             concrete_opaque_types.hash_stable(hcx, hasher);
             closure_captures.hash_stable(hcx, hasher);
+            closure_min_captures.hash_stable(hcx, hasher);
             generator_interior_types.hash_stable(hcx, hasher);
         })
     }

compiler/rustc_middle/src/ty/mod.rs

@@ -6,6 +6,7 @@ pub use self::IntVarValue::*;
 pub use self::Variance::*;
 
 use crate::hir::exports::ExportMap;
+use crate::hir::place::Place as HirPlace;
 use crate::ich::StableHashingContext;
 use crate::middle::cstore::CrateStoreDyn;
 use crate::middle::resolve_lifetime::ObjectLifetimeDefault;
@@ -685,6 +686,12 @@ pub struct UpvarId {
     pub closure_expr_id: LocalDefId,
 }
 
+impl UpvarId {
+    pub fn new(var_hir_id: hir::HirId, closure_def_id: LocalDefId) -> UpvarId {
+        UpvarId { var_path: UpvarPath { hir_id: var_hir_id }, closure_expr_id: closure_def_id }
+    }
+}
+
 #[derive(Clone, PartialEq, Debug, TyEncodable, TyDecodable, Copy, HashStable)]
 pub enum BorrowKind {
     /// Data must be immutable and is aliasable.
@@ -767,6 +774,56 @@ pub struct UpvarBorrow<'tcx> {
     pub region: ty::Region<'tcx>,
 }
 
+/// Given the closure DefId this map provides a map of root variables to minimum
+/// set of `CapturedPlace`s that need to be tracked to support all captures of that closure.
+pub type MinCaptureInformationMap<'tcx> = FxHashMap<DefId, RootVariableMinCaptureList<'tcx>>;
+
+/// Part of `MinCaptureInformationMap`; Maps a root variable to the list of `CapturedPlace`.
+/// Used to track the minimum set of `Place`s that need to be captured to support all
+/// Places captured by the closure starting at a given root variable.
+///
+/// This provides a convenient and quick way of checking if a variable being used within
+/// a closure is a capture of a local variable.
+pub type RootVariableMinCaptureList<'tcx> = FxIndexMap<hir::HirId, MinCaptureList<'tcx>>;
+
+/// Part of `MinCaptureInformationMap`; List of `CapturePlace`s.
+pub type MinCaptureList<'tcx> = Vec<CapturedPlace<'tcx>>;
+
+/// A `Place` and the corresponding `CaptureInfo`.
+#[derive(PartialEq, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+pub struct CapturedPlace<'tcx> {
+    pub place: HirPlace<'tcx>,
+    pub info: CaptureInfo<'tcx>,
+}
+
+/// Part of `MinCaptureInformationMap`; describes the capture kind (&, &mut, move)
+/// for a particular capture as well as identifying the part of the source code
+/// that triggered this capture to occur.
+#[derive(PartialEq, Clone, Debug, Copy, TyEncodable, TyDecodable, HashStable)]
+pub struct CaptureInfo<'tcx> {
+    /// Expr Id pointing to use that resulted in selecting the current capture kind
+    ///
+    /// If the user doesn't enable feature `capture_disjoint_fields` (RFC 2229) then, it is
+    /// possible that we don't see the use of a particular place resulting in expr_id being
+    /// None. In such case we fallback on uvpars_mentioned for span.
+    ///
+    /// Eg:
+    /// ```rust,no_run
+    /// let x = 5;
+    ///
+    /// let c = || {
+    ///     let _ = x
+    /// };
+    /// ```
+    ///
+    /// In this example, if `capture_disjoint_fields` is **not** set, then x will be captured,
+    /// but we won't see it being used during capture analysis, since it's essentially a discard.
+    pub expr_id: Option<hir::HirId>,
+
+    /// Capture mode that was selected
+    pub capture_kind: UpvarCapture<'tcx>,
+}
+
 pub type UpvarListMap = FxHashMap<DefId, FxIndexMap<hir::HirId, UpvarId>>;
 pub type UpvarCaptureMap<'tcx> = FxHashMap<UpvarId, UpvarCapture<'tcx>>;
 

compiler/rustc_mir/src/borrow_check/diagnostics/mod.rs

@@ -383,16 +383,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     self.describe_field_from_ty(&ty, field, variant_index)
                 }
                 ty::Closure(def_id, _) | ty::Generator(def_id, _, _) => {
-                    // `tcx.upvars_mentioned(def_id)` returns an `Option`, which is `None` in case
-                    // the closure comes from another crate. But in that case we wouldn't
-                    // be borrowck'ing it, so we can just unwrap:
-                    let (&var_id, _) = self
-                        .infcx
-                        .tcx
-                        .upvars_mentioned(def_id)
-                        .unwrap()
-                        .get_index(field.index())
-                        .unwrap();
+                    // We won't be borrowck'ing here if the closure came from another crate,
+                    // so it's safe to call `expect_local`.
+                    //
+                    // We know the field exists so it's safe to call operator[] and `unwrap` here.
+                    let (&var_id, _) =
+                        self.infcx.tcx.typeck(def_id.expect_local()).closure_captures[&def_id]
+                            .get_index(field.index())
+                            .unwrap();
 
                     self.infcx.tcx.hir().name(var_id).to_string()
                 }
@@ -967,9 +965,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let expr = &self.infcx.tcx.hir().expect_expr(hir_id).kind;
         debug!("closure_span: hir_id={:?} expr={:?}", hir_id, expr);
         if let hir::ExprKind::Closure(.., body_id, args_span, _) = expr {
-            for ((upvar_hir_id, upvar), place) in
-                self.infcx.tcx.upvars_mentioned(def_id)?.iter().zip(places)
+            for (upvar_hir_id, place) in
+                self.infcx.tcx.typeck(def_id.expect_local()).closure_captures[&def_id]
+                    .keys()
+                    .zip(places)
             {
+                let span = self.infcx.tcx.upvars_mentioned(local_did)?[upvar_hir_id].span;
                 match place {
                     Operand::Copy(place) | Operand::Move(place)
                         if target_place == place.as_ref() =>
@@ -991,7 +992,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         let usage_span =
                             match self.infcx.tcx.typeck(local_did).upvar_capture(upvar_id) {
                                 ty::UpvarCapture::ByValue(Some(span)) => span,
-                                _ => upvar.span,
+                                _ => span,
                             };
                         return Some((*args_span, generator_kind, usage_span));
                     }

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -387,8 +387,9 @@ fn make_mirror_unadjusted<'a, 'tcx>(
                 }
             };
             let upvars = cx
-                .tcx
-                .upvars_mentioned(def_id)
+                .typeck_results()
+                .closure_captures
+                .get(&def_id)
                 .iter()
                 .flat_map(|upvars| upvars.iter())
                 .zip(substs.upvar_tys())

compiler/rustc_passes/src/liveness.rs

@@ -317,19 +317,20 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
         // swap in a new set of IR maps for this body
         let mut maps = IrMaps::new(self.tcx);
         let hir_id = maps.tcx.hir().body_owner(body.id());
-        let def_id = maps.tcx.hir().local_def_id(hir_id);
+        let local_def_id = maps.tcx.hir().local_def_id(hir_id);
+        let def_id = local_def_id.to_def_id();
 
         // Don't run unused pass for #[derive()]
-        if let Some(parent) = self.tcx.parent(def_id.to_def_id()) {
+        if let Some(parent) = self.tcx.parent(def_id) {
             if let DefKind::Impl = self.tcx.def_kind(parent.expect_local()) {
                 if self.tcx.has_attr(parent, sym::automatically_derived) {
                     return;
                 }
             }
         }
 
-        if let Some(upvars) = maps.tcx.upvars_mentioned(def_id) {
-            for (&var_hir_id, _upvar) in upvars {
+        if let Some(captures) = maps.tcx.typeck(local_def_id).closure_captures.get(&def_id) {
+            for &var_hir_id in captures.keys() {
                 let var_name = maps.tcx.hir().name(var_hir_id);
                 maps.add_variable(Upvar(var_hir_id, var_name));
             }
@@ -340,7 +341,7 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
         intravisit::walk_body(&mut maps, body);
 
         // compute liveness
-        let mut lsets = Liveness::new(&mut maps, def_id);
+        let mut lsets = Liveness::new(&mut maps, local_def_id);
         let entry_ln = lsets.compute(&body, hir_id);
         lsets.log_liveness(entry_ln, body.id().hir_id);
 
@@ -397,10 +398,18 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
                 // construction site.
                 let mut call_caps = Vec::new();
                 let closure_def_id = self.tcx.hir().local_def_id(expr.hir_id);
-                if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
-                    call_caps.extend(upvars.iter().map(|(&var_id, upvar)| {
+                if let Some(captures) = self
+                    .tcx
+                    .typeck(closure_def_id)
+                    .closure_captures
+                    .get(&closure_def_id.to_def_id())
+                {
+                    // If closure captures is Some, upvars_mentioned must also be Some
+                    let upvars = self.tcx.upvars_mentioned(closure_def_id).unwrap();
+                    call_caps.extend(captures.keys().map(|var_id| {
+                        let upvar = upvars[var_id];
                         let upvar_ln = self.add_live_node(UpvarNode(upvar.span));
-                        CaptureInfo { ln: upvar_ln, var_hid: var_id }
+                        CaptureInfo { ln: upvar_ln, var_hid: *var_id }
                     }));
                 }
                 self.set_captures(expr.hir_id, call_caps);
@@ -564,6 +573,7 @@ struct Liveness<'a, 'tcx> {
     typeck_results: &'a ty::TypeckResults<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     upvars: Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>>,
+    closure_captures: Option<&'tcx FxIndexMap<hir::HirId, ty::UpvarId>>,
     successors: IndexVec<LiveNode, Option<LiveNode>>,
     rwu_table: RWUTable,
 
@@ -587,6 +597,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         let typeck_results = ir.tcx.typeck(body_owner);
         let param_env = ir.tcx.param_env(body_owner);
         let upvars = ir.tcx.upvars_mentioned(body_owner);
+        let closure_captures = typeck_results.closure_captures.get(&body_owner.to_def_id());
 
         let closure_ln = ir.add_live_node(ClosureNode);
         let exit_ln = ir.add_live_node(ExitNode);
@@ -600,6 +611,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             typeck_results,
             param_env,
             upvars,
+            closure_captures,
             successors: IndexVec::from_elem_n(None, num_live_nodes),
             rwu_table: RWUTable::new(num_live_nodes * num_vars),
             closure_ln,
@@ -850,14 +862,13 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         // if they are live on the entry to the closure, since only the closure
         // itself can access them on subsequent calls.
 
-        if let Some(upvars) = self.upvars {
+        if let Some(closure_captures) = self.closure_captures {
             // Mark upvars captured by reference as used after closure exits.
-            for (&var_hir_id, upvar) in upvars.iter().rev() {
-                let upvar_id = ty::UpvarId {
-                    var_path: ty::UpvarPath { hir_id: var_hir_id },
-                    closure_expr_id: self.body_owner,
-                };
-                match self.typeck_results.upvar_capture(upvar_id) {
+            // Since closure_captures is Some, upvars must exists too.
+            let upvars = self.upvars.unwrap();
+            for (&var_hir_id, upvar_id) in closure_captures {
+                let upvar = upvars[&var_hir_id];
+                match self.typeck_results.upvar_capture(*upvar_id) {
                     ty::UpvarCapture::ByRef(_) => {
                         let var = self.variable(var_hir_id, upvar.span);
                         self.acc(self.exit_ln, var, ACC_READ | ACC_USE);
@@ -869,7 +880,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
         let succ = self.propagate_through_expr(&body.value, self.exit_ln);
 
-        if self.upvars.is_none() {
+        if self.closure_captures.is_none() {
             // Either not a closure, or closure without any captured variables.
             // No need to determine liveness of captured variables, since there
             // are none.
@@ -1341,7 +1352,21 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
         acc: u32,
     ) -> LiveNode {
         match path.res {
-            Res::Local(hid) => self.access_var(hir_id, hid, succ, acc, path.span),
+            Res::Local(hid) => {
+                let in_upvars = self.upvars.map_or(false, |u| u.contains_key(&hid));
+                let in_captures = self.closure_captures.map_or(false, |c| c.contains_key(&hid));
+
+                match (in_upvars, in_captures) {
+                    (false, _) | (true, true) => self.access_var(hir_id, hid, succ, acc, path.span),
+                    (true, false) => {
+                        // This case is possible when with RFC-2229, a wild pattern
+                        // is used within a closure.
+                        // eg: `let _ = x`. The closure doesn't capture x here,
+                        // even though it's mentioned in the closure.
+                        succ
+                    }
+                }
+            }
             _ => succ,
         }
     }
@@ -1531,11 +1556,15 @@ impl<'tcx> Liveness<'_, 'tcx> {
     }
 
     fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
-        let upvars = match self.upvars {
+        let closure_captures = match self.closure_captures {
             None => return,
-            Some(upvars) => upvars,
+            Some(closure_captures) => closure_captures,
         };
-        for (&var_hir_id, upvar) in upvars.iter() {
+
+        // If closure_captures is Some(), upvars must be Some() too.
+        let upvars = self.upvars.unwrap();
+        for &var_hir_id in closure_captures.keys() {
+            let upvar = upvars[&var_hir_id];
             let var = self.variable(var_hir_id, upvar.span);
             let upvar_id = ty::UpvarId {
                 var_path: ty::UpvarPath { hir_id: var_hir_id },

compiler/rustc_span/src/symbol.rs

@@ -318,6 +318,7 @@ symbols! {
         call_mut,
         call_once,
         caller_location,
+        capture_disjoint_fields,
         cdylib,
         ceilf32,
         ceilf64,
@@ -909,6 +910,7 @@ symbols! {
         rustc_args_required_const,
         rustc_attrs,
         rustc_builtin_macro,
+        rustc_capture_analysis,
         rustc_clean,
         rustc_const_stable,
         rustc_const_unstable,