1 //! Contains utilities for generating suggestions for borrowck errors related to unsatisfied
2 //! outlives constraints.
4 use rustc_data_structures::fx::FxHashSet;
5 use rustc_errors::Diagnostic;
6 use rustc_middle::ty::RegionVid;
7 use smallvec::SmallVec;
8 use std::collections::BTreeMap;
10 use crate::MirBorrowckCtxt;
12 use super::{ErrorConstraintInfo, RegionName, RegionNameSource};
14 /// The different things we could suggest.
15 enum SuggestedConstraint {
16 /// Outlives(a, [b, c, d, ...]) => 'a: 'b + 'c + 'd + ...
17 Outlives(RegionName, SmallVec<[RegionName; 2]>),
20 Equal(RegionName, RegionName),
22 /// 'a: 'static i.e. 'a = 'static and the user should just use 'static
26 /// Collects information about outlives constraints that needed to be added for a given MIR node
27 /// corresponding to a function definition.
29 /// Adds a help note suggesting adding a where clause with the needed constraints.
31 pub struct OutlivesSuggestionBuilder {
32 /// The list of outlives constraints that need to be added. Specifically, we map each free
33 /// region to all other regions that it must outlive. I will use the shorthand `fr:
34 /// outlived_frs`. Not all of these regions will already have names necessarily. Some could be
35 /// implicit free regions that we inferred. These will need to be given names in the final
36 /// suggestion message.
37 constraints_to_add: BTreeMap<RegionVid, Vec<RegionVid>>,
40 impl OutlivesSuggestionBuilder {
41 /// Returns `true` iff the `RegionNameSource` is a valid source for an outlives
44 // FIXME: Currently, we only report suggestions if the `RegionNameSource` is an early-bound
45 // region or a named region, avoiding using regions with synthetic names altogether. This
46 // allows us to avoid giving impossible suggestions (e.g. adding bounds to closure args).
47 // We can probably be less conservative, since some inferred free regions are namable (e.g.
48 // the user can explicitly name them. To do this, we would allow some regions whose names
49 // come from `MatchedAdtAndSegment`, being careful to filter out bad suggestions, such as
50 // naming the `'self` lifetime in methods, etc.
51 fn region_name_is_suggestable(name: &RegionName) -> bool {
53 RegionNameSource::NamedEarlyBoundRegion(..)
54 | RegionNameSource::NamedFreeRegion(..)
55 | RegionNameSource::Static => true,
57 // Don't give suggestions for upvars, closure return types, or other unnameable
59 RegionNameSource::SynthesizedFreeEnvRegion(..)
60 | RegionNameSource::AnonRegionFromArgument(..)
61 | RegionNameSource::AnonRegionFromUpvar(..)
62 | RegionNameSource::AnonRegionFromOutput(..)
63 | RegionNameSource::AnonRegionFromYieldTy(..)
64 | RegionNameSource::AnonRegionFromAsyncFn(..)
65 | RegionNameSource::AnonRegionFromImplSignature(..) => {
66 debug!("Region {:?} is NOT suggestable", name);
72 /// Returns a name for the region if it is suggestable. See `region_name_is_suggestable`.
73 fn region_vid_to_name(
75 mbcx: &MirBorrowckCtxt<'_, '_>,
77 ) -> Option<RegionName> {
78 mbcx.give_region_a_name(region).filter(Self::region_name_is_suggestable)
81 /// Compiles a list of all suggestions to be printed in the final big suggestion.
82 fn compile_all_suggestions(
84 mbcx: &MirBorrowckCtxt<'_, '_>,
85 ) -> SmallVec<[SuggestedConstraint; 2]> {
86 let mut suggested = SmallVec::new();
88 // Keep track of variables that we have already suggested unifying so that we don't print
89 // out silly duplicate messages.
90 let mut unified_already = FxHashSet::default();
92 for (fr, outlived) in &self.constraints_to_add {
93 let Some(fr_name) = self.region_vid_to_name(mbcx, *fr) else {
97 let outlived = outlived
99 // if there is a `None`, we will just omit that constraint
100 .filter_map(|fr| self.region_vid_to_name(mbcx, *fr).map(|rname| (fr, rname)))
101 .collect::<Vec<_>>();
103 // No suggestable outlived lifetimes.
104 if outlived.is_empty() {
108 // There are three types of suggestions we can make:
109 // 1) Suggest a bound: 'a: 'b
110 // 2) Suggest replacing 'a with 'static. If any of `outlived` is `'static`, then we
111 // should just replace 'a with 'static.
112 // 3) Suggest unifying 'a with 'b if we have both 'a: 'b and 'b: 'a
116 .any(|(_, outlived_name)| matches!(outlived_name.source, RegionNameSource::Static))
118 suggested.push(SuggestedConstraint::Static(fr_name));
120 // We want to isolate out all lifetimes that should be unified and print out
121 // separate messages for them.
123 let (unified, other): (Vec<_>, Vec<_>) = outlived.into_iter().partition(
124 // Do we have both 'fr: 'r and 'r: 'fr?
126 self.constraints_to_add
128 .map(|r_outlived| r_outlived.as_slice().contains(fr))
133 for (r, bound) in unified.into_iter() {
134 if !unified_already.contains(fr) {
135 suggested.push(SuggestedConstraint::Equal(fr_name.clone(), bound));
136 unified_already.insert(r);
140 if !other.is_empty() {
142 other.iter().map(|(_, rname)| rname.clone()).collect::<SmallVec<_>>();
143 suggested.push(SuggestedConstraint::Outlives(fr_name, other))
151 /// Add the outlives constraint `fr: outlived_fr` to the set of constraints we need to suggest.
152 pub(crate) fn collect_constraint(&mut self, fr: RegionVid, outlived_fr: RegionVid) {
153 debug!("Collected {:?}: {:?}", fr, outlived_fr);
155 // Add to set of constraints for final help note.
156 self.constraints_to_add.entry(fr).or_default().push(outlived_fr);
159 /// Emit an intermediate note on the given `Diagnostic` if the involved regions are
161 pub(crate) fn intermediate_suggestion(
163 mbcx: &MirBorrowckCtxt<'_, '_>,
164 errci: &ErrorConstraintInfo<'_>,
165 diag: &mut Diagnostic,
167 // Emit an intermediate note.
168 let fr_name = self.region_vid_to_name(mbcx, errci.fr);
169 let outlived_fr_name = self.region_vid_to_name(mbcx, errci.outlived_fr);
171 if let (Some(fr_name), Some(outlived_fr_name)) = (fr_name, outlived_fr_name)
172 && !matches!(outlived_fr_name.source, RegionNameSource::Static)
175 "consider adding the following bound: `{fr_name}: {outlived_fr_name}`",
180 /// If there is a suggestion to emit, add a diagnostic to the buffer. This is the final
181 /// suggestion including all collected constraints.
182 pub(crate) fn add_suggestion(&self, mbcx: &mut MirBorrowckCtxt<'_, '_>) {
183 // No constraints to add? Done.
184 if self.constraints_to_add.is_empty() {
185 debug!("No constraints to suggest.");
189 // If there is only one constraint to suggest, then we already suggested it in the
190 // intermediate suggestion above.
191 if self.constraints_to_add.len() == 1
192 && self.constraints_to_add.values().next().unwrap().len() == 1
194 debug!("Only 1 suggestion. Skipping.");
198 // Get all suggestable constraints.
199 let suggested = self.compile_all_suggestions(mbcx);
201 // If there are no suggestable constraints...
202 if suggested.is_empty() {
203 debug!("Only 1 suggestable constraint. Skipping.");
207 // If there is exactly one suggestable constraints, then just suggest it. Otherwise, emit a
208 // list of diagnostics.
209 let mut diag = if suggested.len() == 1 {
210 mbcx.infcx.tcx.sess.diagnostic().struct_help(&match suggested.last().unwrap() {
211 SuggestedConstraint::Outlives(a, bs) => {
212 let bs: SmallVec<[String; 2]> = bs.iter().map(|r| format!("{}", r)).collect();
213 format!("add bound `{}: {}`", a, bs.join(" + "))
216 SuggestedConstraint::Equal(a, b) => {
217 format!("`{}` and `{}` must be the same: replace one with the other", a, b)
219 SuggestedConstraint::Static(a) => format!("replace `{}` with `'static`", a),
222 // Create a new diagnostic.
228 .struct_help("the following changes may resolve your lifetime errors");
231 for constraint in suggested {
233 SuggestedConstraint::Outlives(a, bs) => {
234 let bs: SmallVec<[String; 2]> =
235 bs.iter().map(|r| format!("{}", r)).collect();
236 diag.help(&format!("add bound `{}: {}`", a, bs.join(" + ")));
238 SuggestedConstraint::Equal(a, b) => {
240 "`{}` and `{}` must be the same: replace one with the other",
244 SuggestedConstraint::Static(a) => {
245 diag.help(&format!("replace `{}` with `'static`", a));
253 // We want this message to appear after other messages on the mir def.
254 let mir_span = mbcx.body.span;
255 diag.sort_span = mir_span.shrink_to_hi();
257 // Buffer the diagnostic
258 mbcx.buffer_non_error_diag(diag);