[zzylol]

Summary

Found while reviewing this codebase's error-handling paths for unsupported SQL/PromQL constructs, prompted by wanting an "unsupported X" error to always clearly say what is unsupported.

The overall error-type shapes were already in good order: SqlError/LoweringError and PromqlError both carry layer/category information structurally (variant names like UnsupportedFeature/UnsupportedAggregate/TableNotFound vs. a wrapped DataFusionError), ResolveError/QueryExprError (asap-types) already use thiserror with informative messages naming the actual column/name involved, and DataFusionError's own Display embeds a category prefix ("Error during planning: ", "SQL error: ", "This feature is not implemented: ", "Schema error: ") that SqlError's Display preserves verbatim. bgp_jan2024_workload's Category taxonomy (Plan/Schema/Parse/NotImplemented/UnsupportedFeature/Other) already maps cleanly onto these existing variants.

But one real bug: five fallback arms in crates/frontend-promql/src/promql.rs (info_selector, lower_inner, expr_str, extract_matrix, num_expr) built their UnsupportedFeature/InvalidParameter message by formatting std::mem::discriminant(other) instead of other itself. Discriminant<Expr>'s Debug output is an opaque internal value — not the enum variant name, not the source text, not anything a person could act on. A query that hit any of these paths produced an error message that literally said nothing about what the person actually wrote.

promql_parser::parser::ast::Expr implements Display (it reconstructs PromQL source text), so this swaps format!("... {:?}", std::mem::discriminant(other)) for format!("... {other}") at all five sites — the message now shows the actual offending sub-expression.

Also fixes a related, less severe case in crates/frontend-sql/src/sql/mod.rs's lower_agg_intent fallback: it dumped a DataFusion Expr's full Debug representation (verbose internal AST shape, not the pretty-printed SQL) with no explanation of why it was rejected. Switched to Display plus a "why" clause ("measure is not an aggregate function call: {expr}"), matching the pattern every other lowering error in the same file already uses (e.g. expr_to_group_ref's "non-column GROUP BY expression: {other}").

Test plan

• cargo build --workspace --all-targets
• cargo test --workspace (including promql_conformance's 100 tests, sql_lowering's 60, and the bgp_jan2024_workload 200-query corpus tally — none pin the changed message text)
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings

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