location-detail: disable all location details when passed none

Prior to this fix, `-Z location-detail` provided no mechanism for
disabling all location details. This commit also adds a test case
to verify that this option continues to work as intended, and
clarifies the documentation of this option.

compiler/rustc_session/src/options.rs

@@ -393,8 +393,7 @@ mod desc {
         "either a boolean (`yes`, `no`, `on`, `off`, etc), `thin`, `fat`, or omitted";
     pub const parse_linker_plugin_lto: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or the path to the linker plugin";
-    pub const parse_location_detail: &str =
-        "comma separated list of location details to track: `file`, `line`, or `column`";
+    pub const parse_location_detail: &str = "either `none`, or a comma separated list of location details to track: `file`, `line`, or `column`";
     pub const parse_switch_with_opt_path: &str =
         "an optional path to the profiling data output directory";
     pub const parse_merge_functions: &str = "one of: `disabled`, `trampolines`, or `aliases`";
@@ -549,6 +548,9 @@ mod parse {
             ld.line = false;
             ld.file = false;
             ld.column = false;
+            if v == "none" {
+                return true;
+            }
             for s in v.split(',') {
                 match s {
                     "file" => ld.file = true,
@@ -1360,8 +1362,9 @@ options! {
     llvm_time_trace: bool = (false, parse_bool, [UNTRACKED],
         "generate JSON tracing data file from LLVM data (default: no)"),
     location_detail: LocationDetail = (LocationDetail::all(), parse_location_detail, [TRACKED],
-        "comma separated list of location details to be tracked when using caller_location \
-        valid options are `file`, `line`, and `column` (default: all)"),
+        "what location details should be tracked when using caller_location, either \
+        `none`, or a comma separated list of location details, for which \
+        valid options are `file`, `line`, and `column` (default: `file,line,column`)"),
     ls: bool = (false, parse_bool, [UNTRACKED],
         "list the symbols defined by a library crate (default: no)"),
     macro_backtrace: bool = (false, parse_bool, [UNTRACKED],

src/doc/unstable-book/src/compiler-flags/location-detail.md

@@ -17,8 +17,9 @@ within this list are:
 - `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.
+Any combination of these three options are supported. Alternatively, you can pass
+`none` to this option, which results in no location details being tracked.
+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

src/test/rustdoc-ui/z-help.stdout

@@ -69,7 +69,7 @@
     -Z                               link-only=val -- link the `.rlink` file generated by `-Z no-link` (default: no)
     -Z                            llvm-plugins=val -- a list LLVM plugins to enable (space separated)
     -Z                         llvm-time-trace=val -- generate JSON tracing data file from LLVM data (default: no)
-    -Z                         location-detail=val -- comma separated list of location details to be tracked when using caller_location valid options are `file`, `line`, and `column` (default: all)
+    -Z                         location-detail=val -- what location details should be tracked when using caller_location, either `none`, or a comma separated list of location details, for which valid options are `file`, `line`, and `column` (default: `file,line,column`)
     -Z                                      ls=val -- list the symbols defined by a library crate (default: no)
     -Z                         macro-backtrace=val -- show macro backtraces (default: no)
     -Z                         merge-functions=val -- control the operation of the MergeFunctions LLVM pass, taking the same values as the target option of the same name

src/test/ui/panics/location-detail-panic-no-location-info.rs

@@ -0,0 +1,8 @@
+// run-fail
+// check-run-results
+// compile-flags: -Zlocation-detail=none
+// exec-env:RUST_BACKTRACE=0
+
+fn main() {
+    panic!("no location info");
+}

src/test/ui/panics/location-detail-panic-no-location-info.run.stderr

@@ -0,0 +1,2 @@
+thread 'main' panicked at 'no location info', <redacted>:0:0
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace