Ensure that early-bound function lifetimes are always 'local'

During borrowchecking, we treat any free (early-bound) regions on
the 'defining type' as `RegionClassification::External`. According
to the doc comments, we should only have 'external' regions when
checking a closure/generator.

However, a plain function can also have some if its regions
be considered 'early bound' - this occurs when the region is
constrained by an argument, appears in a `where` clause, or
in an opaque type. This was causing us to incorrectly mark these
regions as 'external', which caused some diagnostic code
to act as if we were referring to a 'parent' region from inside
a closure.

This PR marks all instantiated region variables as 'local'
when we're borrow-checking something other than a
closure/generator/inline-const.

Author: Aaron1011

compiler/rustc_borrowck/src/universal_regions.rs

@@ -180,8 +180,9 @@ pub enum RegionClassification {
     /// anywhere. There is only one, `'static`.
     Global,
 
-    /// An **external** region is only relevant for closures. In that
-    /// case, it refers to regions that are free in the closure type
+    /// An **external** region is only relevant for
+    /// closures, generators, and inline consts. In that
+    /// case, it refers to regions that are free in the type
     /// -- basically, something bound in the surrounding context.
     ///
     /// Consider this example:
@@ -198,8 +199,8 @@ pub enum RegionClassification {
     /// Here, the lifetimes `'a` and `'b` would be **external** to the
     /// closure.
     ///
-    /// If we are not analyzing a closure, there are no external
-    /// lifetimes.
+    /// If we are not analyzing a closure/generator/inline-const,
+    /// there are no external lifetimes.
     External,
 
     /// A **local** lifetime is one about which we know the full set
@@ -424,22 +425,30 @@ impl<'cx, 'tcx> UniversalRegionsBuilder<'cx, 'tcx> {
 
         let typeck_root_def_id = self.infcx.tcx.typeck_root_def_id(self.mir_def.did.to_def_id());
 
-        // If this is a closure or generator, then the late-bound regions from the enclosing
-        // function are actually external regions to us. For example, here, 'a is not local
-        // to the closure c (although it is local to the fn foo):
-        // fn foo<'a>() {
-        //     let c = || { let x: &'a u32 = ...; }
-        // }
-        if self.mir_def.did.to_def_id() != typeck_root_def_id {
+        // If this is is a 'root' body (not a closure/generator/inline const), then
+        // there are no extern regions, so the local regions start at the same
+        // position as the (empty) sub-list of extern regions
+        let first_local_index = if self.mir_def.did.to_def_id() == typeck_root_def_id {
+            first_extern_index
+        } else {
+            // If this is a closure, generator, or inline-const, then the late-bound regions from the enclosing
+            // function are actually external regions to us. For example, here, 'a is not local
+            // to the closure c (although it is local to the fn foo):
+            // fn foo<'a>() {
+            //     let c = || { let x: &'a u32 = ...; }
+            // }
             self.infcx
-                .replace_late_bound_regions_with_nll_infer_vars(self.mir_def.did, &mut indices)
-        }
-
-        let bound_inputs_and_output = self.compute_inputs_and_output(&indices, defining_ty);
+                .replace_late_bound_regions_with_nll_infer_vars(self.mir_def.did, &mut indices);
+            // Any regions created during the execution of `defining_ty` or during the above
+            // late-bound region replacement are all considered 'extern' regions
+            self.infcx.num_region_vars()
+        };
 
         // "Liberate" the late-bound regions. These correspond to
         // "local" free regions.
-        let first_local_index = self.infcx.num_region_vars();
+
+        let bound_inputs_and_output = self.compute_inputs_and_output(&indices, defining_ty);
+
         let inputs_and_output = self.infcx.replace_bound_regions_with_nll_infer_vars(
             FR,
             self.mir_def.did,

src/test/mir-opt/nll/named_lifetimes_basic.use_x.nll.0.mir

@@ -2,8 +2,8 @@
 
 | Free Region Mapping
 | '_#0r | Global | ['_#2r, '_#1r, '_#0r, '_#4r, '_#3r]
-| '_#1r | External | ['_#1r, '_#4r]
-| '_#2r | External | ['_#2r, '_#1r, '_#4r]
+| '_#1r | Local | ['_#1r, '_#4r]
+| '_#2r | Local | ['_#2r, '_#1r, '_#4r]
 | '_#3r | Local | ['_#4r, '_#3r]
 | '_#4r | Local | ['_#4r]
 |

src/test/ui/c-variadic/variadic-ffi-4.stderr

@@ -20,7 +20,7 @@ LL | pub unsafe extern "C" fn no_escape0<'f>(_: usize, ap: ...) -> VaListImpl<'f
    |                                     |
    |                                     lifetime `'f` defined here
 LL |     ap
-   |     ^^ returning this value requires that `'1` must outlive `'f`
+   |     ^^ function was supposed to return data with lifetime `'f` but it is returning data with lifetime `'1`
    |
    = note: requirement occurs because of the type VaListImpl<'_>, which makes the generic argument '_ invariant
    = note: the struct VaListImpl<'f> is invariant over the parameter 'f

src/test/ui/fn/implied-bounds-unnorm-associated-type.nll.stderr

@@ -6,7 +6,7 @@ LL | fn f<'a, 'b>(s: &'b str, _: <&'a &'b () as Trait>::Type) -> &'a str {
    |      |
    |      lifetime `'a` defined here
 LL |     s
-   |     ^ returning this value requires that `'b` must outlive `'a`
+   |     ^ function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
    |
    = help: consider adding the following bound: `'b: 'a`
 

src/test/ui/issues/issue-16683.nll.stderr

@@ -1,20 +1,13 @@
-error[E0521]: borrowed data escapes outside of associated function
+error: lifetime may not live long enough
   --> $DIR/issue-16683.rs:4:9
    |
 LL | trait T<'a> {
    |         -- lifetime `'a` defined here
 LL |     fn a(&'a self) -> &'a bool;
 LL |     fn b(&self) {
-   |          -----
-   |          |
-   |          `self` is a reference that is only valid in the associated function body
-   |          let's call the lifetime of this reference `'1`
+   |          - let's call the lifetime of this reference `'1`
 LL |         self.a();
-   |         ^^^^^^^^
-   |         |
-   |         `self` escapes the associated function body here
-   |         argument requires that `'1` must outlive `'a`
+   |         ^^^^^^^^ argument requires that `'1` must outlive `'a`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0521`.

src/test/ui/issues/issue-17758.nll.stderr

@@ -1,20 +1,13 @@
-error[E0521]: borrowed data escapes outside of associated function
+error: lifetime may not live long enough
   --> $DIR/issue-17758.rs:7:9
    |
 LL | trait Foo<'a> {
    |           -- lifetime `'a` defined here
 LL |     fn foo(&'a self);
 LL |     fn bar(&self) {
-   |            -----
-   |            |
-   |            `self` is a reference that is only valid in the associated function body
-   |            let's call the lifetime of this reference `'1`
+   |            - let's call the lifetime of this reference `'1`
 LL |         self.foo();
-   |         ^^^^^^^^^^
-   |         |
-   |         `self` escapes the associated function body here
-   |         argument requires that `'1` must outlive `'a`
+   |         ^^^^^^^^^^ argument requires that `'1` must outlive `'a`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0521`.

src/test/ui/issues/issue-52213.nll.stderr

@@ -7,7 +7,7 @@ LL | fn transmute_lifetime<'a, 'b, T>(t: &'a (T,)) -> &'b T {
    |                       lifetime `'a` defined here
 LL |     match (&t,) {
 LL |         ((u,),) => u,
-   |                    ^ returning this value requires that `'a` must outlive `'b`
+   |                    ^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
 

src/test/ui/match/match-ref-mut-invariance.nll.stderr

@@ -6,7 +6,7 @@ LL | impl<'b> S<'b> {
 LL |     fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
    |            -- lifetime `'a` defined here
 LL |         match self.0 { ref mut x => x }
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'a` must outlive `'b`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
    = note: requirement occurs because of a mutable reference to &i32

src/test/ui/match/match-ref-mut-let-invariance.nll.stderr

@@ -7,7 +7,7 @@ LL |     fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
    |            -- lifetime `'a` defined here
 LL |         let ref mut x = self.0;
 LL |         x
-   |         ^ returning this value requires that `'a` must outlive `'b`
+   |         ^ associated function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`
    = note: requirement occurs because of a mutable reference to &i32

src/test/ui/nll/issue-52113.stderr

@@ -7,7 +7,7 @@ LL | fn produce_err<'a, 'b: 'a>(data: &'b mut Vec<&'b u32>, value: &'a u32) -> i
    |                lifetime `'a` defined here
 ...
 LL |     x
-   |     ^ returning this value requires that `'a` must outlive `'b`
+   |     ^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
    |
    = help: consider adding the following bound: `'a: 'b`