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-rw-r--r--src/tools/rust-analyzer/docs/dev/README.md10
1 files changed, 9 insertions, 1 deletions
diff --git a/src/tools/rust-analyzer/docs/dev/README.md b/src/tools/rust-analyzer/docs/dev/README.md
index 002b8ba2a66..12e6d829a08 100644
--- a/src/tools/rust-analyzer/docs/dev/README.md
+++ b/src/tools/rust-analyzer/docs/dev/README.md
@@ -178,7 +178,15 @@ RA_PROFILE=foo|bar|baz   // enabled only selected entries
 RA_PROFILE=*@3>10        // dump everything, up to depth 3, if it takes more than 10 ms
 ```
 
-In particular, I have `export RA_PROFILE='*>10'` in my shell profile.
+Some rust-analyzer contributors have `export RA_PROFILE='*>10'` in my shell profile.
+
+For machine-readable JSON output, we have the `RA_PROFILE_JSON` env variable. We support
+filtering only by span name:
+
+```
+RA_PROFILE=* // dump everything
+RA_PROFILE_JSON="vfs_load|parallel_prime_caches|discover_command" // dump selected spans
+```
 
 We also have a "counting" profiler which counts number of instances of popular structs.
 It is enabled by `RA_COUNT=1`.