Rollup merge of #55781 - pnkfelix:issue-54382-more-precise-spans-for-temps-and-their-drops, r=davidtwco

More precise spans for temps and their drops

This PR has two main enhancements:

 1. when possible during code generation for a statement (like `expr();`), pass along the span of a statement, and then attribute the drops of temporaries from that statement to the statement's end-point (which will be the semicolon if it is a statement that is terminating by a semicolon).
 2. when evaluating a block expression into a MIR temp, use the span of the block's tail expression (rather than the span of whole block including its statements and curly-braces) for the span of the temp.

Each of these individually increases the precision of our diagnostic output; together they combine to make a much clearer picture about the control flow through the spans.

Fix #54382

3 files changed, 63 insertions, 0 deletions

diff --git a/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.nll.stderr b/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.nll.stderr
new file mode 100644
index 00000000000..c308562c0cc
--- /dev/null
+++ b/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.nll.stderr
@@ -0,0 +1,20 @@
+error[E0597]: `_thing1` does not live long enough
+  --> $DIR/issue-54382-use-span-of-tail-of-block.rs:7:29
+   |
+LL |             D("other").next(&_thing1)
+   |             ----------------^^^^^^^^-
+   |             |               |
+   |             |               borrowed value does not live long enough
+   |             a temporary with access to the borrow is created here ...
+LL |         }
+LL |     }
+   |     - `_thing1` dropped here while still borrowed
+LL | 
+LL |     ;
+   |     - ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `D`
+   |
+   = note: The temporary is part of an expression at the end of a block. Consider adding semicolon after the expression so its temporaries are dropped sooner, before the local variables declared by the block are dropped.
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.
diff --git a/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.rs b/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.rs
new file mode 100644
index 00000000000..99eafe0e9d1
--- /dev/null
+++ b/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.rs
@@ -0,0 +1,28 @@
+fn main() {
+    {
+        let mut _thing1 = D(Box::new("thing1"));
+        {
+            let _thing2 = D("thing2");
+            side_effects();
+            D("other").next(&_thing1)
+        }
+    }
+
+    ;
+}
+
+#[derive(Debug)]
+struct D<T: std::fmt::Debug>(T);
+
+impl<T: std::fmt::Debug>  Drop for D<T> {
+    fn drop(&mut self) {
+        println!("dropping {:?})", self);
+    }
+}
+
+impl<T: std::fmt::Debug> D<T> {
+    fn next<U: std::fmt::Debug>(&self, _other: U) -> D<U> { D(_other) }
+    fn end(&self) { }
+}
+
+fn side_effects() { }
diff --git a/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.stderr b/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.stderr
new file mode 100644
index 00000000000..eeba7d6bb44
--- /dev/null
+++ b/src/test/ui/nll/issue-54382-use-span-of-tail-of-block.stderr
@@ -0,0 +1,15 @@
+error[E0597]: `_thing1` does not live long enough
+  --> $DIR/issue-54382-use-span-of-tail-of-block.rs:7:30
+   |
+LL |             D("other").next(&_thing1)
+   |                              ^^^^^^^ borrowed value does not live long enough
+LL |         }
+LL |     }
+   |     - `_thing1` dropped here while still borrowed
+LL | 
+LL |     ;
+   |     - borrowed value needs to live until here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.