Auto merge of #52991 - nikomatsakis:nll-escaping-into-return, r=pnkfelix

avoid computing liveness for locals that escape into statics

Fixes #52713

I poked at this on the plane and I think it's working -- but I want to do a bit more investigation and double check. The idea is to identify those local variables where the entire value will "escape" into the return -- for them, we don't need to compute liveness, since we know that the outlives relations from the return type will force those regions to be equal to free regions. This should help with html5ever in particular.

- [x] test performance
- [x] verify correctness
- [x] add comments

r? @pnkfelix
cc @lqd

4 files changed, 45 insertions, 36 deletions

diff --git a/src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr b/src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr
index 95acdab3e80..52f1547bce6 100644
--- a/src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr
+++ b/src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr
@@ -1,30 +1,24 @@
 error[E0597]: borrowed value does not live long enough
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:15:21
    |
-LL |   fn gimme_static_mut_let() -> &'static mut u32 {
-   |  _______________________________________________-
-LL | |     let ref mut x = 1234543; //~ ERROR
-   | |                     ^^^^^^^ temporary value does not live long enough
-LL | |     x
-LL | | }
-   | | -
-   | | |
-   | |_temporary value only lives until here
-   |   borrow later used here
+LL |     let ref mut x = 1234543; //~ ERROR
+   |                     ^^^^^^^ temporary value does not live long enough
+LL |     x
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
 
 error[E0597]: borrowed value does not live long enough
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:20:25
    |
-LL |   fn gimme_static_mut_let_nested() -> &'static mut u32 {
-   |  ______________________________________________________-
-LL | |     let (ref mut x, ) = (1234543, ); //~ ERROR
-   | |                         ^^^^^^^^^^^ temporary value does not live long enough
-LL | |     x
-LL | | }
-   | | -
-   | | |
-   | |_temporary value only lives until here
-   |   borrow later used here
+LL |     let (ref mut x, ) = (1234543, ); //~ ERROR
+   |                         ^^^^^^^^^^^ temporary value does not live long enough
+LL |     x
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
 
 error[E0597]: borrowed value does not live long enough
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:25:11
diff --git a/src/test/ui/nll/get_default.nll.stderr b/src/test/ui/nll/get_default.nll.stderr
index b955a51e38d..580dce3c0fe 100644
--- a/src/test/ui/nll/get_default.nll.stderr
+++ b/src/test/ui/nll/get_default.nll.stderr
@@ -63,9 +63,18 @@ LL |         match map.get() {
 LL |             Some(v) => {
 LL |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
-...
-LL |                 return v;
-   |                        - borrow later used here
+   |
+note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 41:1...
+  --> $DIR/get_default.rs:41:1
+   |
+LL | / fn err(map: &mut Map) -> &String {
+LL | |     loop {
+LL | |         match map.get() {
+LL | |             Some(v) => {
+...  |
+LL | |     }
+LL | | }
+   | |_^
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:51:17
diff --git a/src/test/ui/nll/get_default.stderr b/src/test/ui/nll/get_default.stderr
index 75194bf55bc..2f8eab907c7 100644
--- a/src/test/ui/nll/get_default.stderr
+++ b/src/test/ui/nll/get_default.stderr
@@ -63,9 +63,18 @@ LL |         match map.get() {
 LL |             Some(v) => {
 LL |                 map.set(String::new()); // Both AST and MIR error here
    |                 ^^^ mutable borrow occurs here
-...
-LL |                 return v;
-   |                        - borrow later used here
+   |
+note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 41:1...
+  --> $DIR/get_default.rs:41:1
+   |
+LL | / fn err(map: &mut Map) -> &String {
+LL | |     loop {
+LL | |         match map.get() {
+LL | |             Some(v) => {
+...  |
+LL | |     }
+LL | | }
+   | |_^
 
 error[E0502]: cannot borrow `*map` as mutable because it is also borrowed as immutable (Mir)
   --> $DIR/get_default.rs:51:17
diff --git a/src/test/ui/nll/return-ref-mut-issue-46557.stderr b/src/test/ui/nll/return-ref-mut-issue-46557.stderr
index f40e38c63f5..f441085f242 100644
--- a/src/test/ui/nll/return-ref-mut-issue-46557.stderr
+++ b/src/test/ui/nll/return-ref-mut-issue-46557.stderr
@@ -1,16 +1,13 @@
 error[E0597]: borrowed value does not live long enough
   --> $DIR/return-ref-mut-issue-46557.rs:17:21
    |
-LL |   fn gimme_static_mut() -> &'static mut u32 {
-   |  ___________________________________________-
-LL | |     let ref mut x = 1234543; //~ ERROR borrowed value does not live long enough [E0597]
-   | |                     ^^^^^^^ temporary value does not live long enough
-LL | |     x
-LL | | }
-   | | -
-   | | |
-   | |_temporary value only lives until here
-   |   borrow later used here
+LL |     let ref mut x = 1234543; //~ ERROR borrowed value does not live long enough [E0597]
+   |                     ^^^^^^^ temporary value does not live long enough
+LL |     x
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
 
 error: aborting due to previous error