[nll] better error message when returning refs to upvars

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

@@ -10,7 +10,9 @@
 
 use borrow_check::nll::constraints::{OutlivesConstraint};
 use borrow_check::nll::region_infer::RegionInferenceContext;
+use borrow_check::nll::region_infer::error_reporting::region_name::RegionNameSource;
 use borrow_check::nll::type_check::Locations;
+use borrow_check::nll::universal_regions::DefiningTy;
 use rustc::hir::def_id::DefId;
 use rustc::infer::error_reporting::nice_region_error::NiceRegionError;
 use rustc::infer::InferCtxt;
@@ -263,6 +265,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         debug!("report_error: fr_is_local={:?} outlived_fr_is_local={:?} category={:?}",
                fr_is_local, outlived_fr_is_local, category);
         match (category, fr_is_local, outlived_fr_is_local) {
+            (ConstraintCategory::Return, true, false) if self.is_closure_fn_mut(infcx, fr) =>
+                self.report_fnmut_error(mir, infcx, mir_def_id, fr, outlived_fr, span,
+                                        errors_buffer),
             (ConstraintCategory::Assignment, true, false) |
             (ConstraintCategory::CallArgument, true, false) =>
                 self.report_escaping_data_error(mir, infcx, mir_def_id, fr, outlived_fr,
@@ -274,6 +279,85 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         };
     }
 
+    /// Report a specialized error when `FnMut` closures return a reference to a captured variable.
+    /// This function expects `fr` to be local and `outlived_fr` to not be local.
+    ///
+    /// ```text
+    /// error: captured variable cannot escape `FnMut` closure body
+    ///   --> $DIR/issue-53040.rs:15:8
+    ///    |
+    /// LL |     || &mut v;
+    ///    |     -- ^^^^^^ creates a reference to a captured variable which escapes the closure body
+    ///    |     |
+    ///    |     inferred to be a `FnMut` closure
+    ///    |
+    ///    = note: `FnMut` closures only have access to their captured variables while they are
+    ///            executing...
+    ///    = note: ...therefore, returned references to captured variables will escape the closure
+    /// ```
+    fn report_fnmut_error(
+        &self,
+        mir: &Mir<'tcx>,
+        infcx: &InferCtxt<'_, '_, 'tcx>,
+        mir_def_id: DefId,
+        _fr: RegionVid,
+        outlived_fr: RegionVid,
+        span: Span,
+        errors_buffer: &mut Vec<Diagnostic>,
+    ) {
+        let mut diag = infcx.tcx.sess.struct_span_err(
+            span,
+            "captured variable cannot escape `FnMut` closure body",
+        );
+
+        // We should check if the return type of this closure is in fact a closure - in that
+        // case, we can special case the error further.
+        let return_type_is_closure = self.universal_regions.unnormalized_output_ty.is_closure();
+        let message = if return_type_is_closure {
+            "returns a closure that contains a reference to a captured variable, which then \
+             escapes the closure body"
+        } else {
+            "returns a reference to a captured variable which escapes the closure body"
+        };
+
+        diag.span_label(
+            span,
+            message,
+        );
+
+        match self.give_region_a_name(infcx, mir, mir_def_id, outlived_fr, &mut 1).source {
+            RegionNameSource::NamedEarlyBoundRegion(fr_span) |
+            RegionNameSource::NamedFreeRegion(fr_span) |
+            RegionNameSource::SynthesizedFreeEnvRegion(fr_span, _) |
+            RegionNameSource::CannotMatchHirTy(fr_span, _) |
+            RegionNameSource::MatchedHirTy(fr_span) |
+            RegionNameSource::MatchedAdtAndSegment(fr_span) |
+            RegionNameSource::AnonRegionFromUpvar(fr_span, _) |
+            RegionNameSource::AnonRegionFromOutput(fr_span, _, _) => {
+                diag.span_label(fr_span, "inferred to be a `FnMut` closure");
+            },
+            _ => {},
+        }
+
+        diag.note("`FnMut` closures only have access to their captured variables while they are \
+                   executing...");
+        diag.note("...therefore, they cannot allow references to captured variables to escape");
+
+        diag.buffer(errors_buffer);
+    }
+
+    /// Reports a error specifically for when data is escaping a closure.
+    ///
+    /// ```text
+    /// error: borrowed data escapes outside of function
+    ///   --> $DIR/lifetime-bound-will-change-warning.rs:44:5
+    ///    |
+    /// LL | fn test2<'a>(x: &'a Box<Fn()+'a>) {
+    ///    |              - `x` is a reference that is only valid in the function body
+    /// LL |     // but ref_obj will not, so warn.
+    /// LL |     ref_obj(x)
+    ///    |     ^^^^^^^^^^ `x` escapes the function body here
+    /// ```
     fn report_escaping_data_error(
         &self,
         mir: &Mir<'tcx>,
@@ -305,31 +389,46 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             span, &format!("borrowed data escapes outside of {}", escapes_from),
         );
 
-        if let Some((outlived_fr_name, outlived_fr_span)) = outlived_fr_name_and_span {
-            if let Some(name) = outlived_fr_name {
-                diag.span_label(
-                    outlived_fr_span,
-                    format!("`{}` is declared here, outside of the {} body", name, escapes_from),
-                );
-            }
+        if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
+            diag.span_label(
+                outlived_fr_span,
+                format!(
+                    "`{}` is declared here, outside of the {} body",
+                    outlived_fr_name, escapes_from
+                ),
+            );
         }
 
-        if let Some((fr_name, fr_span)) = fr_name_and_span {
-            if let Some(name) = fr_name {
-                diag.span_label(
-                    fr_span,
-                    format!("`{}` is a reference that is only valid in the {} body",
-                            name, escapes_from),
-                );
+        if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
+            diag.span_label(
+                fr_span,
+                format!(
+                    "`{}` is a reference that is only valid in the {} body",
+                    fr_name, escapes_from
+                ),
+            );
 
-                diag.span_label(span, format!("`{}` escapes the {} body here",
-                                               name, escapes_from));
-            }
+            diag.span_label(span, format!("`{}` escapes the {} body here", fr_name, escapes_from));
         }
 
         diag.buffer(errors_buffer);
     }
 
+    /// Reports a region inference error for the general case with named/synthesized lifetimes to
+    /// explain what is happening.
+    ///
+    /// ```text
+    /// error: unsatisfied lifetime constraints
+    ///   --> $DIR/regions-creating-enums3.rs:17:5
+    ///    |
+    /// LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
+    ///    |                -- -- lifetime `'b` defined here
+    ///    |                |
+    ///    |                lifetime `'a` defined here
+    /// LL |     ast::add(x, y)
+    ///    |     ^^^^^^^^^^^^^^ function was supposed to return data with lifetime `'a` but it
+    ///    |                    is returning data with lifetime `'b`
+    /// ```
     fn report_general_error(
         &self,
         mir: &Mir<'tcx>,
@@ -380,6 +479,15 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         diag.buffer(errors_buffer);
     }
 
+    /// Adds a suggestion to errors where a `impl Trait` is returned.
+    ///
+    /// ```text
+    /// help: to allow this impl Trait to capture borrowed data with lifetime `'1`, add `'_` as
+    ///       a constraint
+    ///    |
+    /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
+    ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    /// ```
     fn add_static_impl_trait_suggestion(
         &self,
         infcx: &InferCtxt<'_, '_, 'tcx>,
@@ -500,4 +608,22 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             .get(&(constraint.sup, constraint.sub));
         *opt_span_category.unwrap_or(&(constraint.category, mir.source_info(loc).span))
     }
+
+    /// Returns `true` if a closure is inferred to be an `FnMut` closure.
+    crate fn is_closure_fn_mut(
+        &self,
+        infcx: &InferCtxt<'_, '_, 'tcx>,
+        fr: RegionVid,
+    ) -> bool {
+        if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
+            if let ty::BoundRegion::BrEnv = free_region.bound_region {
+                if let DefiningTy::Closure(def_id, substs) = self.universal_regions.defining_ty {
+                    let closure_kind_ty = substs.closure_kind_ty(def_id, infcx.tcx);
+                    return Some(ty::ClosureKind::FnMut) == closure_kind_ty.to_opt_closure_kind();
+                }
+            }
+        }
+
+        false
+    }
 }

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/region_name.rs

@@ -27,8 +27,8 @@ use syntax_pos::symbol::InternedString;
 
 #[derive(Debug)]
 crate struct RegionName {
-    name: InternedString,
-    source: RegionNameSource,
+    crate name: InternedString,
+    crate source: RegionNameSource,
 }
 
 #[derive(Debug)]

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/var_name.rs

@@ -50,11 +50,12 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             .defining_ty
             .upvar_tys(tcx)
             .position(|upvar_ty| {
-                debug!(
-                    "get_upvar_index_for_region: upvar_ty = {:?}",
-                    upvar_ty,
-                );
-                tcx.any_free_region_meets(&upvar_ty, |r| r.to_region_vid() == fr)
+                debug!("get_upvar_index_for_region: upvar_ty={:?}", upvar_ty);
+                tcx.any_free_region_meets(&upvar_ty, |r| {
+                    let r = r.to_region_vid();
+                    debug!("get_upvar_index_for_region: r={:?} fr={:?}", r, fr);
+                    r == fr
+                })
             })?;
 
         let upvar_ty = self

src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr

@@ -20,24 +20,22 @@ LL |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than o
 LL |                    *y = 1;
    |                    ------ first borrow later used here
 
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/borrowck-describe-lvalue.rs:305:16
    |
 LL |              || {
-   |              --
-   |              ||
-   |              |return type of closure is [closure@$DIR/borrowck-describe-lvalue.rs:305:16: 311:18 x:&'2 mut i32]
-   |              lifetime `'1` represents this closure's body
-LL | /                || { //[mir]~ ERROR unsatisfied lifetime constraints
+   |               - inferred to be a `FnMut` closure
+LL | /                || { //[mir]~ ERROR captured variable cannot escape `FnMut` closure body
 LL | |                    let y = &mut x;
 LL | |                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
 LL | |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time
 LL | |                    *y = 1;
 LL | |                    drop(y);
 LL | |                 }
-   | |_________________^ returning this value requires that `'1` must outlive `'2`
+   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error[E0503]: cannot use `f.x` because it was mutably borrowed
   --> $DIR/borrowck-describe-lvalue.rs:53:9

src/test/ui/borrowck/borrowck-describe-lvalue.mir.stderr

@@ -20,24 +20,22 @@ LL |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than o
 LL |                    *y = 1;
    |                    ------ first borrow later used here
 
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/borrowck-describe-lvalue.rs:305:16
    |
 LL |              || {
-   |              --
-   |              ||
-   |              |return type of closure is [closure@$DIR/borrowck-describe-lvalue.rs:305:16: 311:18 x:&'2 mut i32]
-   |              lifetime `'1` represents this closure's body
-LL | /                || { //[mir]~ ERROR unsatisfied lifetime constraints
+   |               - inferred to be a `FnMut` closure
+LL | /                || { //[mir]~ ERROR captured variable cannot escape `FnMut` closure body
 LL | |                    let y = &mut x;
 LL | |                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
 LL | |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time
 LL | |                    *y = 1;
 LL | |                    drop(y);
 LL | |                 }
-   | |_________________^ returning this value requires that `'1` must outlive `'2`
+   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error[E0503]: cannot use `f.x` because it was mutably borrowed
   --> $DIR/borrowck-describe-lvalue.rs:53:9

src/test/ui/borrowck/borrowck-describe-lvalue.rs

@@ -302,7 +302,7 @@ fn main() {
         // FIXME(#49824) -- the free region error below should probably not be there
         let mut x = 0;
            || {
-               || { //[mir]~ ERROR unsatisfied lifetime constraints
+               || { //[mir]~ ERROR captured variable cannot escape `FnMut` closure body
                    let y = &mut x;
                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time

src/test/ui/issues/issue-40510-1.nll.stderr

@@ -1,15 +1,13 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-40510-1.rs:18:9
    |
 LL |     || {
-   |     --
-   |     ||
-   |     |return type of closure is &'2 mut std::boxed::Box<()>
-   |     lifetime `'1` represents this closure's body
+   |      - inferred to be a `FnMut` closure
 LL |         &mut x
-   |         ^^^^^^ returning this value requires that `'1` must outlive `'2`
+   |         ^^^^^^ returns a reference to a captured variable which escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-40510-3.nll.stderr

@@ -1,17 +1,15 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-40510-3.rs:18:9
    |
 LL |       || {
-   |       --
-   |       ||
-   |       |return type of closure is [closure@$DIR/issue-40510-3.rs:18:9: 20:10 x:&'2 mut std::vec::Vec<()>]
-   |       lifetime `'1` represents this closure's body
+   |        - inferred to be a `FnMut` closure
 LL | /         || {
 LL | |             x.push(())
 LL | |         }
-   | |_________^ returning this value requires that `'1` must outlive `'2`
+   | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-49824.nll.stderr

@@ -1,17 +1,15 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-49824.rs:22:9
    |
 LL |       || {
-   |       --
-   |       ||
-   |       |return type of closure is [closure@$DIR/issue-49824.rs:22:9: 24:10 x:&'2 mut i32]
-   |       lifetime `'1` represents this closure's body
+   |        - inferred to be a `FnMut` closure
 LL | /         || {
 LL | |             let _y = &mut x;
 LL | |         }
-   | |_________^ returning this value requires that `'1` must outlive `'2`
+   | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error
 

src/test/ui/nll/issue-53040.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(nll)]
+
+fn main() {
+    let mut v: Vec<()> = Vec::new();
+    || &mut v;
+}