1 //! Contains utilities for generating suggestions for borrowck errors related to unsatisified
2 //! outlives constraints.
4 use std::collections::BTreeMap;
6 use rustc_data_structures::fx::FxHashSet;
7 use rustc_errors::DiagnosticBuilder;
8 use rustc_middle::ty::RegionVid;
11 use smallvec::SmallVec;
13 use crate::borrow_check::MirBorrowckCtxt;
15 use super::{ErrorConstraintInfo, RegionName, RegionNameSource};
17 /// The different things we could suggest.
18 enum SuggestedConstraint {
19 /// Outlives(a, [b, c, d, ...]) => 'a: 'b + 'c + 'd + ...
20 Outlives(RegionName, SmallVec<[RegionName; 2]>),
23 Equal(RegionName, RegionName),
25 /// 'a: 'static i.e. 'a = 'static and the user should just use 'static
29 /// Collects information about outlives constraints that needed to be added for a given MIR node
30 /// corresponding to a function definition.
32 /// Adds a help note suggesting adding a where clause with the needed constraints.
34 pub struct OutlivesSuggestionBuilder {
35 /// The list of outlives constraints that need to be added. Specifically, we map each free
36 /// region to all other regions that it must outlive. I will use the shorthand `fr:
37 /// outlived_frs`. Not all of these regions will already have names necessarily. Some could be
38 /// implicit free regions that we inferred. These will need to be given names in the final
39 /// suggestion message.
40 constraints_to_add: BTreeMap<RegionVid, Vec<RegionVid>>,
43 impl OutlivesSuggestionBuilder {
44 /// Returns `true` iff the `RegionNameSource` is a valid source for an outlives
47 // FIXME: Currently, we only report suggestions if the `RegionNameSource` is an early-bound
48 // region or a named region, avoiding using regions with synthetic names altogether. This
49 // allows us to avoid giving impossible suggestions (e.g. adding bounds to closure args).
50 // We can probably be less conservative, since some inferred free regions are namable (e.g.
51 // the user can explicitly name them. To do this, we would allow some regions whose names
52 // come from `MatchedAdtAndSegment`, being careful to filter out bad suggestions, such as
53 // naming the `'self` lifetime in methods, etc.
54 fn region_name_is_suggestable(name: &RegionName) -> bool {
56 RegionNameSource::NamedEarlyBoundRegion(..)
57 | RegionNameSource::NamedFreeRegion(..)
58 | RegionNameSource::Static => true,
60 // Don't give suggestions for upvars, closure return types, or other unnamable
62 RegionNameSource::SynthesizedFreeEnvRegion(..)
63 | RegionNameSource::AnonRegionFromArgument(..)
64 | RegionNameSource::AnonRegionFromUpvar(..)
65 | RegionNameSource::AnonRegionFromOutput(..)
66 | RegionNameSource::AnonRegionFromYieldTy(..)
67 | RegionNameSource::AnonRegionFromAsyncFn(..) => {
68 debug!("Region {:?} is NOT suggestable", name);
74 /// Returns a name for the region if it is suggestable. See `region_name_is_suggestable`.
75 fn region_vid_to_name(
77 mbcx: &MirBorrowckCtxt<'_, '_>,
79 ) -> Option<RegionName> {
80 mbcx.give_region_a_name(region).filter(Self::region_name_is_suggestable)
83 /// Compiles a list of all suggestions to be printed in the final big suggestion.
84 fn compile_all_suggestions(
86 mbcx: &MirBorrowckCtxt<'_, '_>,
87 ) -> SmallVec<[SuggestedConstraint; 2]> {
88 let mut suggested = SmallVec::new();
90 // Keep track of variables that we have already suggested unifying so that we don't print
91 // out silly duplicate messages.
92 let mut unified_already = FxHashSet::default();
94 for (fr, outlived) in &self.constraints_to_add {
95 let fr_name = if let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) {
101 let outlived = outlived
103 // if there is a `None`, we will just omit that constraint
104 .filter_map(|fr| self.region_vid_to_name(mbcx, *fr).map(|rname| (fr, rname)))
105 .collect::<Vec<_>>();
107 // No suggestable outlived lifetimes.
108 if outlived.is_empty() {
112 // There are three types of suggestions we can make:
113 // 1) Suggest a bound: 'a: 'b
114 // 2) Suggest replacing 'a with 'static. If any of `outlived` is `'static`, then we
115 // should just replace 'a with 'static.
116 // 3) Suggest unifying 'a with 'b if we have both 'a: 'b and 'b: 'a
118 if outlived.iter().any(|(_, outlived_name)| {
119 if let RegionNameSource::Static = outlived_name.source { true } else { false }
121 suggested.push(SuggestedConstraint::Static(fr_name));
123 // We want to isolate out all lifetimes that should be unified and print out
124 // separate messages for them.
126 let (unified, other): (Vec<_>, Vec<_>) = outlived.into_iter().partition(
127 // Do we have both 'fr: 'r and 'r: 'fr?
129 self.constraints_to_add
131 .map(|r_outlived| r_outlived.as_slice().contains(fr))
136 for (r, bound) in unified.into_iter() {
137 if !unified_already.contains(fr) {
138 suggested.push(SuggestedConstraint::Equal(fr_name.clone(), bound));
139 unified_already.insert(r);
143 if !other.is_empty() {
145 other.iter().map(|(_, rname)| rname.clone()).collect::<SmallVec<_>>();
146 suggested.push(SuggestedConstraint::Outlives(fr_name, other))
154 /// Add the outlives constraint `fr: outlived_fr` to the set of constraints we need to suggest.
155 crate fn collect_constraint(&mut self, fr: RegionVid, outlived_fr: RegionVid) {
156 debug!("Collected {:?}: {:?}", fr, outlived_fr);
158 // Add to set of constraints for final help note.
159 self.constraints_to_add.entry(fr).or_insert(Vec::new()).push(outlived_fr);
162 /// Emit an intermediate note on the given `Diagnostic` if the involved regions are
164 crate fn intermediate_suggestion(
166 mbcx: &MirBorrowckCtxt<'_, '_>,
167 errci: &ErrorConstraintInfo,
168 diag: &mut DiagnosticBuilder<'_>,
170 // Emit an intermediate note.
171 let fr_name = self.region_vid_to_name(mbcx, errci.fr);
172 let outlived_fr_name = self.region_vid_to_name(mbcx, errci.outlived_fr);
174 if let (Some(fr_name), Some(outlived_fr_name)) = (fr_name, outlived_fr_name) {
175 if let RegionNameSource::Static = outlived_fr_name.source {
176 diag.help(&format!("consider replacing `{}` with `'static`", fr_name));
179 "consider adding the following bound: `{}: {}`",
180 fr_name, outlived_fr_name
186 /// If there is a suggestion to emit, add a diagnostic to the buffer. This is the final
187 /// suggestion including all collected constraints.
188 crate fn add_suggestion(&self, mbcx: &mut MirBorrowckCtxt<'_, '_>) {
189 // No constraints to add? Done.
190 if self.constraints_to_add.is_empty() {
191 debug!("No constraints to suggest.");
195 // If there is only one constraint to suggest, then we already suggested it in the
196 // intermediate suggestion above.
197 if self.constraints_to_add.len() == 1
198 && self.constraints_to_add.values().next().unwrap().len() == 1
200 debug!("Only 1 suggestion. Skipping.");
204 // Get all suggestable constraints.
205 let suggested = self.compile_all_suggestions(mbcx);
207 // If there are no suggestable constraints...
208 if suggested.is_empty() {
209 debug!("Only 1 suggestable constraint. Skipping.");
213 // If there is exactly one suggestable constraints, then just suggest it. Otherwise, emit a
214 // list of diagnostics.
215 let mut diag = if suggested.len() == 1 {
216 mbcx.infcx.tcx.sess.diagnostic().struct_help(&match suggested.last().unwrap() {
217 SuggestedConstraint::Outlives(a, bs) => {
218 let bs: SmallVec<[String; 2]> = bs.iter().map(|r| format!("{}", r)).collect();
219 format!("add bound `{}: {}`", a, bs.join(" + "))
222 SuggestedConstraint::Equal(a, b) => {
223 format!("`{}` and `{}` must be the same: replace one with the other", a, b)
225 SuggestedConstraint::Static(a) => format!("replace `{}` with `'static`", a),
228 // Create a new diagnostic.
234 .struct_help("the following changes may resolve your lifetime errors");
237 for constraint in suggested {
239 SuggestedConstraint::Outlives(a, bs) => {
240 let bs: SmallVec<[String; 2]> =
241 bs.iter().map(|r| format!("{}", r)).collect();
242 diag.help(&format!("add bound `{}: {}`", a, bs.join(" + ")));
244 SuggestedConstraint::Equal(a, b) => {
246 "`{}` and `{}` must be the same: replace one with the other",
250 SuggestedConstraint::Static(a) => {
251 diag.help(&format!("replace `{}` with `'static`", a));
259 // We want this message to appear after other messages on the mir def.
260 let mir_span = mbcx.body.span;
261 diag.sort_span = mir_span.shrink_to_hi();
263 // Buffer the diagnostic
264 diag.buffer(&mut mbcx.errors_buffer);