NLL: need a (new) round of review of deltas between .stderr and .nll.stderr output

[pnkfelix]

We have made a lot of progress on NLL diagnostics

So much so that it behooves us to take a step back and reevaluate where we stand with respect to overall diagnostic quality.

As part of that, we should do another round of review of the differences between AST borrowck diagnostic output and NLL borrowck diagnostic output.

• see NLL: must review all .nll.stderr files #49862 for the previous round of review here

---

Update: I finished the review, and have put the cases where NLL is not up to par into "buckets." Each bucket has a linked comment on this issue (and, in some cases, a separate issue that is linked from that comment and from the description here).

Here are all the buckets (with links to each specific comment):

• When a bucket is no longer relevant (either because the issue has been fixed or we've decided its not actually an issue), it will be struckout (like so).

Bucket	Rust Issue	Pull Request	Interested Hackers
On Par, Assuming Faith In RFC 2005
Sort By Span	#51167	#52904	@pnkfelix
Better Diagnostic for Trait Object Capture		#54848	@davidtwco
Better Diagnostic When "Later" means "A Future Loop Iteration"		#52948	@davidtwco
Missing move suggestion	#51169	#55069	@matthewjasper
NLL Spans Show Hypothetical Code	#52877	#52907	@pnkfelix
NLL diagnostic for += is too desugared
NLL diagnostic is globally bad ... but locally justifiable?	#51191	#54720	@davidtwco
The "Later used here" diagnostic is not ideal for return values		#53177	@matthewjasper
Provide span for declaration of captured variables		#53164	@davidtwco
Test Weakness (might mean NLL is not being tested enough)			@pnkfelix
NLL is missing struct field suggestion		#54831	@davidtwco
NLL Diagnostic Redundancy
missing lifetime in suggestion	#52880	#52883	@estebank
NLL chooses not great span in its "lifetime required" message
NLL fails to suggest + bound on trait object		#53900 + #54853	@davidtwco
NLL fails to report lifetime mismatch error
"does not live long enough" when talking about a (thread-local) static		#53131	@davidtwco
NLL missing suggestion to extend lifetimes via let bindings	#49821	#54088	@matthewjasper
"borrowed content" instead of "dereference of raw pointer"		#54825	@davidtwco
NLL lacks various special case handling of closures		#53088 + #54976	@matthewjasper + @davidtwco
NLL didn't fix interaction of local dying with temps in dropck	#21114, #22323		@pnkfelix
NLL mentions lifetimes that are not included in printed span(s).		#52973	@davidtwco
NLL rejects FnMut closure where AST-borrowck accepted it	#53040	#54802	@davidtwco
Note and teach elision rules
Need Further Review

[pnkfelix]

see #49862 for the previous round of review here

[pnkfelix]

In the details, here's a list of all 226 of the current .nll.stderr files, along with a checkmark if I looked at it and a summary of my finding for it. After I go through the whole list I'll make buckets to put them into.

Details

• src/test/ui/asm-out-assign-imm.nll.stderr
  • NLL is stricly better
• src/test/ui/augmented-assignments.nll.stderr
  • not ideal; the MIR error is a little confusing because its describing micro-ops
    that are not immediately visible when one just sees x += x.
• src/test/ui/borrowck/borrowck-box-insensitivity.nll.stderr
  • This might represent an actual "regression" in NLL? Or at least
    we may be committing to no longer having the same level of
    box insensitivity in a number of the cases listed here.
  • See also MIR-borrowck: behavior around Box<T> is different from AST borrowck #45696
• src/test/ui/borrowck/borrowck-escaping-closure-error-1.nll.stderr
  • Would benefit from NLL: missing suggestion to use move keyword on closures #51169
• src/test/ui/borrowck/borrowck-escaping-closure-error-2.nll.stderr
  • Would benefit from NLL: missing suggestion to use move keyword on closures #51169
• src/test/ui/borrowck/borrowck-in-static.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/borrowck-move-error-with-note.nll.stderr
  • NLL is "better" here (at least if you are believer in the new match ergnomics).
• src/test/ui/borrowck/borrowck-move-out-of-vec-tail.nll.stderr
  • NLL is fine as long as you are okay with highlighting hypothetized code.
• src/test/ui/borrowck/borrowck-report-with-custom-diagnostic.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/borrowck-vec-pattern-nesting.nll.stderr
  • NLL is fine as long as you are okay with highlighting hypothetized code.
• src/test/ui/borrowck/issue-45983.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/issue-51117.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/issue-51415.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/issue-7573.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/mut-borrow-in-loop.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/mut-borrow-of-mut-ref.nll.stderr
  • NLL is giving a globally misleading hint, but it seems locally acceptable.
• src/test/ui/borrowck/mut-borrow-outside-loop.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/mutability-errors.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr
  • the "later used here" messaging for the short-lived temp is
    confusing and strictly worse than AST-borrowck. (A StorageEnd for
    a temp holding a &-borrow is not a use.)
  • We do have appropriate phrasing in at least one case in this test
    so at least this might be fixable by adjusting some rustc
    control-flow?
• src/test/ui/borrowck/regions-escape-bound-fn-2.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/regions-escape-bound-fn.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/regions-escape-unboxed-closure.nll.stderr
  • NLL is on par
• src/test/ui/borrowck/unboxed-closures-move-upvar-from-non-once-ref-closure.nll.stderr
  • AST-borrowck provides span of the actual captureed variable.
    (Should be easy to add this to NLL.)
• src/test/ui/closure-expected-type/expect-region-supply-region.nll.stderr
  • There are errors (lines 42-47) that are not being reported by NLL here. (Test weakness?)
• src/test/ui/closure-immutable-outer-variable.nll.stderr
  • NLL is on par
• src/test/ui/closure_context/issue-42065.nll.stderr
  • AST-borrowck has note that "closure cannot be invoked more than once because it moves dict
• src/test/ui/codemap_tests/huge_multispan_highlight.nll.stderr
  • NLL is on par
• src/test/ui/codemap_tests/issue-11715.nll.stderr
  • NLL is on par
• src/test/ui/codemap_tests/one_line.nll.stderr
  • NLL is on par
• src/test/ui/codemap_tests/overlapping_spans.nll.stderr
  • This is (still) no longer testing what I believe it was intended to test w.r.t. diagnostics.
  • See src/test/ui/const-eval/conditional_array_execution.rs:15 for a potential
    alternative test expression
• src/test/ui/codemap_tests/tab_3.nll.stderr
  • NLL is on par
• src/test/ui/command-line-diagnostics.nll.stderr
  • NLL is on par
• src/test/ui/const-eval/conditional_array_execution.nll.stderr
  • For some reason we are reporting line 19 two extra times under NLL?
• src/test/ui/const-eval/dont_promote_unstable_const_fn.nll.stderr
  • NLL reports a different set of "temporary value does not live long enough" errors
    (perhaps due to test weakness?)
• src/test/ui/const-eval/issue-43197.nll.stderr
  • For some reason we are reporting line 24 two extra times under NLL?
• src/test/ui/const-eval/issue-44578.nll.stderr
  • For some reason we are reporting line 35 two extra times under NLL?
• src/test/ui/did_you_mean/issue-31424.nll.stderr
  • still missing suggestion to "try removing &mut here"
• src/test/ui/did_you_mean/issue-34126.nll.stderr
  • NLL is on par
• src/test/ui/did_you_mean/issue-34337.nll.stderr
  • NLL is on par
• src/test/ui/did_you_mean/issue-35937.nll.stderr
  • NLL is on par
• src/test/ui/did_you_mean/issue-37139.nll.stderr
  • NLL is on par
• src/test/ui/did_you_mean/issue-38147-1.nll.stderr
  • NLL is on par
• src/test/ui/did_you_mean/issue-38147-2.nll.stderr
  • NLL is missing suggestion to change struct field type to &'a mut String
• src/test/ui/did_you_mean/issue-38147-3.nll.stderr
  • NLL is missing suggestion to change struct field type to &'a mut String
• src/test/ui/did_you_mean/issue-38147-4.nll.stderr
  • NLL is on par
• src/test/ui/did_you_mean/issue-39544.nll.stderr
  • NLL is losing the lifetime in &'z mut self of its suggestion
  • NLL is printing &mut Z instead of &'a mut Self in its suggestion (original was &'a Self)
• src/test/ui/did_you_mean/issue-40823.nll.stderr
  • NLL is on par
  • (its suggestion "consider changing this to be a mutable reference: &mut [1, 2, 3, 4]"
    is actually valid; I guess that does not get promoted to static storage.)
• src/test/ui/dropck/dropck-eyepatch-extern-crate.nll.stderr
  • We're missing lots of errors due to test weakness.
• src/test/ui/dropck/dropck-eyepatch-reorder.nll.stderr
  • We're missing lots of errors due to test weakness.
• src/test/ui/dropck/dropck-eyepatch.nll.stderr
  • We're missing lots of errors due to test weakness.
• src/test/ui/error-codes/E0017.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0388.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0389.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0499.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0502.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0503.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0504.nll.stderr
  • NLL still fails to mention the (special?) role of the closure here.
• src/test/ui/error-codes/E0505.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0597.nll.stderr
  • NLL is on par
• src/test/ui/error-codes/E0621-does-not-trigger-for-closures.nll.stderr
  • NLL is more succinct (and clearer?), but also uses "free region" jargon
• src/test/ui/existential_types/generic_type_does_not_live_long_enough.nll.stderr
  • NLL is missing "error[E0310]: the parameter type T may not live long enough"
    (is it masked by the earlier reported error under NLL?)
• src/test/ui/feature-gate-nll.nll.stderr
  • expected. Should change to use ignore-compare-mode-nll after that lands.
• src/test/ui/fn-closure-mutable-capture.nll.stderr
  • NLL is on par
• src/test/ui/generator/borrowing.nll.stderr
  • NLL is on par
• src/test/ui/generator/dropck.nll.stderr
  • NLL is on par
• src/test/ui/generator/ref-escapes-but-not-over-yield.nll.stderr
  • NLL leaves out the "borrowed value needs to live until here" message. (Acceptable?)
• src/test/ui/generator/yield-in-args.nll.stderr
  • NLL is on par
• src/test/ui/generator/yield-while-iterating.nll.stderr
  • NLL fails to highlight the use of x that is causing closure to borrow it
• src/test/ui/generator/yield-while-ref-reborrowed.nll.stderr
  • NLL fails to highlight the use of x that is causing closure to borrow it
• src/test/ui/hygiene/fields-move.nll.stderr
  • NLL is on par
• src/test/ui/hygiene/fields-numeric-borrowck.nll.stderr
  • NLL is on par
• src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr
  • NLL's message fails to suggest adding + <lifetime> to the impl Iterator return type
    (which might be a big point of this ui test?)
• src/test/ui/in-band-lifetimes/impl/dyn-trait.nll.stderr
  • NLL's message is succinct, but fails to mention the role of 'a (which is a more
    precise bound on the validity of x...)
• src/test/ui/in-band-lifetimes/mismatched.nll.stderr
  • NLL is on par
• src/test/ui/in-band-lifetimes/mismatched_trait.nll.stderr
  • NLL is on par
• src/test/ui/in-band-lifetimes/mut_while_borrow.nll.stderr
  • NLL is on par
• src/test/ui/issue-10291.nll.stderr
  • NLL uses "free region" jargon.
  • NLL also mentions 'x but does not have any span that includes that lifetime.
• src/test/ui/issue-10398.nll.stderr
  • NLL is on par
• src/test/ui/issue-11192.nll.stderr
  • NLL is on par
• src/test/ui/issue-11873.nll.stderr
  • NLL is on par
• src/test/ui/issue-12041.nll.stderr
  • NLL is on par
• src/test/ui/issue-12470.nll.stderr
  • NLL is on par
• src/test/ui/issue-12567.nll.stderr
  • NLL is missing the error from line 20?
• src/test/ui/issue-13058.nll.stderr
  • NLL is missing "explicit lifetime required in type of cont error which adds 'r to &T.
  • (perhaps this is an instance where fixing the "not reporting
    region error due to nll" would add value?)
• src/test/ui/issue-14285.nll.stderr
  • NLL is on par
• src/test/ui/issue-15034.nll.stderr
  • NLL is on par
• src/test/ui/issue-16683.nll.stderr
  • NLL's error looks very different from AST-borrowcks. Not clear which one is "better" UX.
• src/test/ui/issue-16922.nll.stderr
  • NLL is on par
• src/test/ui/issue-17263.nll.stderr
  • We're missing all the errors due to test weakness.
• src/test/ui/issue-17385.nll.stderr
  • NLL has redundant diagnostics ... once per match arm rather than once per match?
  • NLL is reporting span of whole match for a use match foo { ... }; would be better to
    just highlight input to match in common case.
• src/test/ui/issue-17545.nll.stderr
  • NLL is missing suggestion to pull id() up to a let binding to extend its lifetime.
• src/test/ui/issue-17728.nll.stderr
  • NLL is failing to report lifetime mismatch error
• src/test/ui/issue-17758.nll.stderr
  • NLL's error looks very different from AST-borrowcks. Not clear which one is "better" UX.a
  • at the very least, mentioning 'a would probably be a good idea.
• src/test/ui/issue-17954.ast.nll.stderr
  • NLL says that (thread-local) FOO is dropped, but that terminology
    doesn't make sense for statics
• src/test/ui/issue-18118.nll.stderr
  • NLL is on par
• src/test/ui/issue-18566.nll.stderr
  • NLL is on par
• src/test/ui/issue-18783.nll.stderr
  • NLL is on par
• src/test/ui/issue-20801.nll.stderr
  • NLL is saying "borrowed content" when AST says more specific "dreference of raw pointer"
• src/test/ui/issue-21600.nll.stderr
  • NLL is on par
• src/test/ui/issue-24267-flow-exit.nll.stderr
  • NLL is on par
• src/test/ui/issue-24357.nll.stderr
  • NLL is missing mention of "(into closure)" when it says value is moved
• src/test/ui/issue-25579.ast.nll.stderr
  • We're missing all the errors. Is this a weak test, or is this an demo of NLL feature?
• src/test/ui/issue-25793.nll.stderr
  • NLL is missing mention of "(into closure)" when it says value is moved
• src/test/ui/issue-2590.nll.stderr
  • NLL is on par
• src/test/ui/issue-26217.nll.stderr
  • NLL is failing to report lifetime mismatch error
  • NLL is reporting that main cannot have generic parameters.
    (But maybe should be revised in test to not do that.)
• src/test/ui/issue-27592.nll.stderr
  • NLL is on par
• src/test/ui/issue-30355.nll.stderr
  • NLL is on par
• src/test/ui/issue-30438-a.nll.stderr
  • NLL is missing suggestion to pull id() up to a let binding to extend its lifetime.
• src/test/ui/issue-30438-c.nll.stderr
  • NLL is on par
• src/test/ui/issue-3154.nll.stderr
  • NLL is on par
• src/test/ui/issue-33819.nll.stderr
  • NLL is on par
• src/test/ui/issue-36082.ast.nll.stderr
  • NLL is missing suggestion to pull id() up to a let binding to extend its lifetime.
• src/test/ui/issue-36400.nll.stderr
  • NLL is on par
• src/test/ui/issue-40288-2.nll.stderr
  • NLL is on par
• src/test/ui/issue-40288.nll.stderr
  • NLL is on par
• src/test/ui/issue-40402-ref-hints/issue-40402-1.nll.stderr
  • NLL is on par
• src/test/ui/issue-40402-ref-hints/issue-40402-2.nll.stderr
  • NLL is on par (assuming match ergonomics)
• src/test/ui/issue-40510-1.nll.stderr
  • These are totally different. No error in AST. No comments. What is this test?
• src/test/ui/issue-40510-3.nll.stderr
  • These are totally different. No error in AST. No comments. What is this test?
• src/test/ui/issue-41139.nll.stderr
  • NLL is on par
• src/test/ui/issue-41726.nll.stderr
  • NLL is on par
• src/test/ui/issue-42106.nll.stderr
  • NLL is on par
• src/test/ui/issue-42344.nll.stderr
  • NLL is on par
• src/test/ui/issue-42796.nll.stderr
  • NLL is on par
• src/test/ui/issue-4335.nll.stderr
  • NLL is missing suggestion to use move to force closure to take ownership of v
  • (also, NLL diagnostic order would be improved if we sorted by span.)
• src/test/ui/issue-45199.ast.nll.stderr
  • NLL is on par
• src/test/ui/issue-46023.ast.nll.stderr
  • NLL is on par
• src/test/ui/issue-46604.ast.nll.stderr
  • NLL is on par
• src/test/ui/issue-51244.nll.stderr
  • NLL is on par
• src/test/ui/issue-52049.nll.stderr
  • NLL is on par
• src/test/ui/issue-52126-assign-op-invariance.nll.stderr
  • NLL is on par
• src/test/ui/issue-52213.nll.stderr
  • NLL uses "free region" jargon.
  • NLL also mentions 'a and 'b but does not have any span that includes those lifetimes.
• src/test/ui/issue-6801.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/42701_one_named_and_one_anonymous.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-early-bound-in-struct.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-if-else-2.nll.stderr
  • NLL chooses not great span in its "lifetime 'a required." message
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-if-else-3.nll.stderr
  • NLL chooses not great span in its "lifetime 'a required." message
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-if-else-using-impl-2.nll.stderr
  • NLL chooses not great span in its "lifetime 'a required." message
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-if-else-using-impl-3.nll.stderr
  • NLL is okay
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-if-else-using-impl.nll.stderr
  • NLL chooses not great span in its "lifetime 'a required." message
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-if-else.nll.stderr
  • NLL chooses not great span in its "lifetime 'a required." message
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-return-type-is-anon.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex1-return-one-existing-name-self-is-anon.nll.stderr
  • NLL is okay
• src/test/ui/lifetime-errors/ex2a-push-one-existing-name-2.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex2a-push-one-existing-name-early-bound.nll.stderr
  • NLL reports type inference failure in "unrelated" spot. (Test weakness?)
• src/test/ui/lifetime-errors/ex2a-push-one-existing-name.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex2b-push-no-existing-names.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex2c-push-inference-variable.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex2d-push-inference-variable-2.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex2e-push-inference-variable-3.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-2.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-3.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs-2.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs-3.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs-4.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs-earlybound-regions.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs-latebound-regions.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-latebound-regions.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-one-is-struct-2.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-one-is-struct-3.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-one-is-struct-4.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-one-is-struct.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-return-type-is-anon.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-self-is-anon.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-using-fn-items.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-using-impl-items.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions-using-trait-objects.nll.stderr
  • NLL is on par
• src/test/ui/lifetime-errors/ex3-both-anon-regions.nll.stderr
  • NLL is on par
• src/test/ui/lifetimes/borrowck-let-suggestion.nll.stderr
  • NLL is on par
• src/test/ui/macros/span-covering-argument-1.nll.stderr
  • NLL is on par
• src/test/ui/moves-based-on-type-block-bad.nll.stderr
  • NLL is on par
• src/test/ui/moves-based-on-type-match-bindings.nll.stderr
  • NLL is on par
• src/test/ui/nll/get_default.nll.stderr
  • NLL is on par
• src/test/ui/region-borrow-params-issue-29793-small.nll.stderr
  • NLL is missing suggestion to use move to force closure to take ownership
  • NLL is missing other special handling of describing errors around closures
  • This test is a bit too long, in that it makes the diff hard to parse
• src/test/ui/regions-nested-fns-2.nll.stderr
  • NLL is missing suggestion to use move to force closure to take ownership
  • NLL is missing other special handling of describing errors around closures
• src/test/ui/rfc-2005-default-binding-mode/borrowck-issue-49631.nll.stderr
  • NLL is on par
• src/test/ui/rfc-2005-default-binding-mode/enum.nll.stderr
  • NLL is on par
• src/test/ui/rfc-2005-default-binding-mode/explicit-mut.nll.stderr
  • NLL is on par
• src/test/ui/span/borrowck-borrow-overloaded-auto-deref-mut.nll.stderr
  • NLL seems to be missing occurrences of the E0499 diagnostic
  • NLL has left lifetime out of its suggestino of &mut Own<Point>
• src/test/ui/span/borrowck-borrow-overloaded-deref-mut.nll.stderr
  • NLL has left lifetime out of its suggestino of &mut Own<Point>
• src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr
  • NLL is failing to highlight the occurrence of f within the closure
    that causes the erroneous move.
• src/test/ui/span/borrowck-call-method-from-mut-aliasable.nll.stderr
  • NLL is on par
• src/test/ui/span/borrowck-fn-in-const-b.nll.stderr
  • NLL is on par
• src/test/ui/span/borrowck-let-suggestion-suffixes.nll.stderr
  • NLL is missing the error about extent of young (perhaps test weakness?)
  • NLL is missing suggestion to use let binding to extend temps' lifetimes.
• src/test/ui/span/borrowck-object-mutability.nll.stderr
  • NLL is on par
• src/test/ui/span/borrowck-ref-into-rvalue.nll.stderr
  • NLL is on par
• src/test/ui/span/destructor-restrictions.nll.stderr
  • NLL is on par
• src/test/ui/span/dropck-object-cycle.nll.stderr
  • NLL is on par
• src/test/ui/span/dropck_arr_cycle_checked.nll.stderr
  • NLL is probably on par;
    NLL might be leaving out one of the errors. Or AST-borrowck might be redundantly reporting one.
  • (also, NLL diagnostic order would be improved if we sorted by span.)
• src/test/ui/span/dropck_direct_cycle_with_drop.nll.stderr
  • NLL is on par
• src/test/ui/span/dropck_misc_variants.nll.stderr
  • NLL is on par
• src/test/ui/span/dropck_vec_cycle_checked.nll.stderr
  • NLL is probably on par;
    NLL might be leaving out one of the errors. Or AST-borrowck might be redundantly reporting one.
  • (also, NLL diagnostic order would be improved if we sorted by span.)
• src/test/ui/span/issue-11925.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-15480.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-23338-locals-die-before-temps-of-body.nll.stderr
  • NLL was expected to fix this; see dropck: track order of destruction for r-value temporaries #22323
• src/test/ui/span/issue-24805-dropck-child-has-items-via-parent.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-24805-dropck-trait-has-items.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-24895-copy-clone-dropck.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-25199.nll.stderr
  • NLL has one less error, but that might be expected for this test.
    (not sure if there's a "test weakness" to be removed in this case)
• src/test/ui/span/issue-26656.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-29106.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-36537.nll.stderr
  • NLL is on par
• src/test/ui/span/issue-40157.nll.stderr
  • NLL is on par
• src/test/ui/span/issue28498-reject-ex1.nll.stderr
  • NLL has one less error, but that might be expected for this test.
    (not sure if there's a "test weakness" to be removed in this case)
  • Note: if you remove the line that causes the initial error, the
    subsequent one is revealed. So NLL remains sound; it just is a
    question of whether we always expect both errors to be reported
    when they occur in the same function body.
• src/test/ui/span/issue28498-reject-lifetime-param.nll.stderr
  • NLL has one less error, but that is expected for this test.
    See NLL lets borrowck observe drop order for let (a, b); #51036
• src/test/ui/span/issue28498-reject-passed-to-fn.nll.stderr
  • NLL has one less error, but that is expected for this test.
    See NLL lets borrowck observe drop order for let (a, b); #51036
• src/test/ui/span/issue28498-reject-trait-bound.nll.stderr
  • NLL has one less error, but that is expected for this test.
    See NLL lets borrowck observe drop order for let (a, b); #51036
• src/test/ui/span/mut-arg-hint.nll.stderr
  • NLL has left lifetime out of its suggestion of &mut std::string::String
• src/test/ui/span/mut-ptr-cant-outlive-ref.nll.stderr
  • NLL is on par
• src/test/ui/span/range-2.nll.stderr
  • NLL is probably on par
  • (NLL diagnostic order would be improved if we sorted by span.)
• src/test/ui/span/regionck-unboxed-closure-lifetimes.nll.stderr
  • NLL is on par
• src/test/ui/span/regions-close-over-borrowed-ref-in-obj.nll.stderr
  • NLL is on par
• src/test/ui/span/regions-close-over-type-parameter-2.nll.stderr
  • NLL is on par
  • but it would be nice if instead of "borrow later used here" we
    provided a more specific message about it being captured by a
    trait object (I presume its that object's destructor that is
    source of the use referenced here).
• src/test/ui/span/regions-escape-loop-via-variable.nll.stderr
  • NLL is on par
  • but it would be nice if we made it clear that "later use"
    here is due to a later loop iteration.
• src/test/ui/span/regions-escape-loop-via-vec.nll.stderr
  • NLL is on par
  • (NLL diagnostic order might be improved if we sorted by span, but
    AST-borrowck output is also not 100% sorted w.r.t. span here)
• src/test/ui/span/regions-infer-borrow-scope-within-loop.nll.stderr
  • NLL is on par
• src/test/ui/span/send-is-not-static-ensures-scoping.nll.stderr
  • NLL is on par
  • (NLL diagnostic order would be improved if we sorted by span.)
• src/test/ui/span/send-is-not-static-std-sync-2.nll.stderr
  • NLL is on par
• src/test/ui/span/send-is-not-static-std-sync.nll.stderr
  • NLL is on par
• src/test/ui/span/slice-borrow.nll.stderr
  • NLL is on par
• src/test/ui/span/vec-must-not-hide-type-from-dropck.nll.stderr
  • NLL is on par
• src/test/ui/span/vec_refs_data_with_early_death.nll.stderr
  • NLL is on par
  • (NLL diagnostic order would be improved if we sorted by span.)
• src/test/ui/span/wf-method-late-bound-regions.nll.stderr
  • NLL is on par
• src/test/ui/trivial-bounds-inconsistent-copy-reborrow.nll.stderr
  • NLL has left lifetime out of its suggestion of &mut &mut i32
• src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr
  • NLL is on par

[pnkfelix]

(I've almost finished the initial review; there are just 32 more diffs for me to look at. But I think it will have to wait until Monday.)

[pnkfelix]

I have finished my initial scan of all 226 files. (See details link in above comment.)

I have tagged 152 of the cases as "on par" with AST-borrowck. That leaves 74 cases that are not up to par.

Next, I need to take all of the 74 cases where NLL is not up to par with AST-borrowck, and organize those cases into buckets, so that we get a better idea of how many root issues remain to be resolved.

[pnkfelix]

I'm going to try making a separate comment for each bucket that I identified; that way each bucket will have a specific URL that one can link within this issue. I'm going to skip the cases where NLL is clearer at (or above) par when compared to AST-borrowck.

[pnkfelix]

On Par, Assuming Faith In RFC 2005

There are two cases where NLL is on par, as long as you are a believer in the new match ergonomics. I.e., the error produced by NLL and the suggestion it makes relies heavily on rust-lang/rfcs#2005 . In several cases we have stopped suggesting to the user that they inject a ref or ref mut, and instead we suggest that they stop dereferencing the match input itself (and then presumably the match ergonomics will often handle implicitly turning the pattern identifiers into references).

These are the cases:

rust/src/test/ui/borrowck/borrowck-move-error-with-note.nll.stderr

Lines 1 to 8 in fefe816

	error[E0507]: cannot move out of borrowed content
	--> $DIR/borrowck-move-error-with-note.rs:21:11
	|
	LL | match *f { //~ ERROR cannot move out of
	| ^^
	| |
	| cannot move out of borrowed content
	| help: consider removing this dereference operator: `f`

rust/src/test/ui/issue-40402-ref-hints/issue-40402-2.nll.stderr

Lines 1 to 10 in fefe816

	error[E0507]: cannot move out of borrowed content
	--> $DIR/issue-40402-2.rs:15:18
	|
	LL | let (a, b) = x[0]; //~ ERROR cannot move out of indexed content
	| - - ^^^^
	| | | |
	| | | cannot move out of borrowed content
	| | | help: consider using a reference instead: `&x[0]`
	| | move occurs because b has type `std::string::String`, which does not implement the `Copy` trait
	| move occurs because a has type `std::string::String`, which does not implement the `Copy` trait

[pnkfelix]

Sort By Span: #51167

There's a bunch where NLL is on par, but its output would be improved (or at least slightly improved) if we made the order of diagnostics correspond to the source order, i.e. #51167.

Update: Now has PR #52904

• https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/vec_refs_data_with_early_death.nll.stderr

• https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/regions-escape-loop-via-vec.nll.stderr

• https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/send-is-not-static-ensures-scoping.nll.stderr

• https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/range-2.nll.stderr

• https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/dropck_arr_cycle_checked.nll.stderr

• https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/dropck_vec_cycle_checked.nll.stderr

[pnkfelix]

Better Diagnostic for Trait Object Capture

In https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/test/ui/span/regions-close-over-type-parameter-2.nll.stderr NLL is on par but it would be nice if instead of "borrow later used here" we provided a more specific message about it being captured by a trait object (I presume its that object's destructor that is source of the use referenced here).

Has PR: #54848