# Description After this pr, nushell is able to raise errors with a backtrace, which should make users easier to debug. To enable the feature, users need to set env variable via `$env.NU_BACKTRACE = 1`. But yeah it might not work perfectly, there are some corner cases which might not be handled. I think it should close #13379 in another way. ### About the change The implementation mostly contained with 2 parts: 1. introduce a new `ChainedError` struct as well as a new `ShellError::ChainedError` variant. If `eval_instruction` returned an error, it converts the error to `ShellError::ChainedError`. `ChainedError` struct is responsable to display errors properly. It needs to handle the following 2 cases: - if we run a function which runs `error make` internally, it needs to display the error itself along with caller span. - if we run a `error make` directly, or some commands directly returns an error, we just want nushell raise an error about `error make`. 2. Attach caller spans to `ListStream` and `ByteStream`, because they are lazy streams, and *only* contains the span that runs it directly(like `^false`, for example), so nushell needs to add all caller spans to the stream. For example: in `def a [] { ^false }; def b [] { a; 33 }; b`, when we run `b`, which runs `a`, which runs `^false`, the `ByteStream` only contains the span of `^false`, we need to make it contains the span of `a`, so nushell is able to get all spans if something bad happened. This behavior is happened after running `Instruction::Call`, if it returns a `ByteStream` and `ListStream`, it will call `push_caller_span` method to attach call spans. # User-Facing Changes It's better to demostrate how it works by examples, given the following definition: ```nushell > $env.NU_BACKTRACE = 1 > def a [x] { if $x == 3 { error make {msg: 'a custom error'}}} > def a_2 [x] { if $x == 3 { ^false } else { $x } } > def a_3 [x] { if $x == 3 { [1 2 3] | each {error make {msg: 'a custom error inside list stream'} } } } > def b [--list-stream --external] { if $external == true { # error with non-zero exit code, which is generated from external command. a_2 1; a_2 3; a_2 2 } else if $list_stream == true { # error generated by list-stream a_3 1; a_3 3; a_3 2 } else { # error generated by command directly a 1; a 2; a 3 } } ``` Run `b` directly shows the following error: <details> ```nushell Error: chained_error × oops ╭─[entry #27:1:1] 1 │ b · ┬ · ╰── error happened when running this ╰──── Error: chained_error × oops ╭─[entry #26:10:19] 9 │ # error generated by command directly 10 │ a 1; a 2; a 3 · ┬ · ╰── error happened when running this 11 │ } ╰──── Error: × a custom error ╭─[entry #6:1:26] 1 │ def a [x] { if $x == 3 { error make {msg: 'a custom error'}}} · ─────┬──── · ╰── originates from here ╰──── ``` </details> Run `b --list-stream` shows the following error <details> ```nushell Error: chained_error × oops ╭─[entry #28:1:1] 1 │ b --list-stream · ┬ · ╰── error happened when running this ╰──── Error: nu:🐚:eval_block_with_input × Eval block failed with pipeline input ╭─[entry #26:7:16] 6 │ # error generated by list-stream 7 │ a_3 1; a_3 3; a_3 2 · ─┬─ · ╰── source value 8 │ } else { ╰──── Error: nu:🐚:eval_block_with_input × Eval block failed with pipeline input ╭─[entry #23:1:29] 1 │ def a_3 [x] { if $x == 3 { [1 2 3] | each {error make {msg: 'a custom error inside list stream'} } } } · ┬ · ╰── source value ╰──── Error: × a custom error inside list stream ╭─[entry #23:1:44] 1 │ def a_3 [x] { if $x == 3 { [1 2 3] | each {error make {msg: 'a custom error inside list stream'} } } } · ─────┬──── · ╰── originates from here ╰──── ``` </details> Run `b --external` shows the following error: <details> ```nushell Error: chained_error × oops ╭─[entry #29:1:1] 1 │ b --external · ┬ · ╰── error happened when running this ╰──── Error: nu:🐚:eval_block_with_input × Eval block failed with pipeline input ╭─[entry #26:4:16] 3 │ # error with non-zero exit code, which is generated from external command. 4 │ a_2 1; a_2 3; a_2 2 · ─┬─ · ╰── source value 5 │ } else if $list_stream == true { ╰──── Error: nu:🐚:non_zero_exit_code × External command had a non-zero exit code ╭─[entry #7:1:29] 1 │ def a_2 [x] { if $x == 3 { ^false } else { $x } } · ──┬── · ╰── exited with code 1 ╰──── ``` </details> It also added a message to guide the usage of NU_BACKTRACE, see the last line in the following example: ```shell ls asdfasd Error: nu:🐚:io::not_found × I/O error ╰─▶ × Entity not found ╭─[entry #17:1:4] 1 │ ls asdfasd · ───┬─── · ╰── Entity not found ╰──── help: The error occurred at '/home/windsoilder/projects/nushell/asdfasd' set the `NU_BACKTRACE=1` environment variable to display a backtrace. ``` # Tests + Formatting Added some tests for the behavior. # After Submitting

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