Rollup merge of #89920 - hudson-ayers:location-detail-control, r=davidtwco

Implement -Z location-detail flag

This PR implements the `-Z location-detail` flag as described in https://github.com/rust-lang/rfcs/pull/2091 .

`-Z location-detail=val` controls what location details are tracked when using `caller_location`. This allows users to control what location details are printed as part of panic messages, by allowing them to exclude any combination of filenames, line numbers, and column numbers. This option is intended to provide users with a way to mitigate the size impact of `#[track_caller]`.

Some measurements of the savings of this approach on an embedded binary can be found here: https://github.com/rust-lang/rust/issues/70579#issuecomment-942556822 .

Closes #70580 (unless people want to leave that open as a place for discussion of further improvements).

This is my first real PR to rust, so any help correcting mistakes / understanding side effects / improving my tests is appreciated :)

I have one question: RFC 2091 specified this as a debugging option (I think that is what -Z implies?). Does that mean this can never be stabilized without a separate MCP? If so, do I need to submit an MCP now, or is the initial RFC specifying this option sufficient for this to be merged as is, and then an MCP would be needed for eventual stabilization?

1 files changed, 43 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/compiler-flags/location-detail.md b/src/doc/unstable-book/src/compiler-flags/location-detail.md
new file mode 100644
index 00000000000..08d937cc282
--- /dev/null
+++ b/src/doc/unstable-book/src/compiler-flags/location-detail.md
@@ -0,0 +1,43 @@
+# `location-detail`
+
+The tracking issue for this feature is: [#70580](https://github.com/rust-lang/rust/issues/70580).
+
+------------------------
+
+Option `-Z location-detail=val` controls what location details are tracked when
+using `caller_location`. This allows users to control what location details
+are printed as part of panic messages, by allowing them to exclude any combination
+of filenames, line numbers, and column numbers. This option is intended to provide
+users with a way to mitigate the size impact of `#[track_caller]`.
+
+This option supports a comma separated list of location details to be included. Valid options
+within this list are:
+
+- `file` - the filename of the panic will be included in the panic output
+- `line` - the source line of the panic will be included in the panic output
+- `column` - the source column of the panic will be included in the panic output
+
+Any combination of these three options are supported. If this option is not specified,
+all three are included by default.
+
+An example of a panic output when using `-Z location-detail=line`:
+```text
+panicked at 'Process blink had a fault', <redacted>:323:0
+```
+
+The code size savings from this option are two-fold. First, the `&'static str` values
+for each path to a file containing a panic are removed from the binary. For projects
+with deep directory structures and many files with panics, this can add up. This category
+of savings can only be realized by excluding filenames from the panic output. Second,
+savings can be realized by allowing multiple panics to be fused into a single panicking
+branch. It is often the case that within a single file, multiple panics with the same
+panic message exist -- e.g. two calls to `Option::unwrap()` in a single line, or
+two calls to `Result::expect()` on adjacent lines. If column and line information
+are included in the `Location` struct passed to the panic handler, these branches cannot
+be fused, as the output is different depending on which panic occurs. However if line
+and column information is identical for all panics, these branches can be fused, which
+can lead to substantial code size savings, especially for small embedded binaries with
+many panics.
+
+The savings from this option are amplified when combined with the use of `-Zbuild-std`, as
+otherwise paths for panics within the standard library are still included in your binary.