refactor: plan 6.5 frontend hardening and concise diagnostics

.sampo/changesets/borrow-local-scopes.md

@@ -0,0 +1,5 @@
+---
+cargo/nash-can: minor
+---
+
+Borrow module data and local binding maps during canonicalization instead of cloning the full environment at each scope. Preserve shadowing, diagnostics, and error recovery.

.sampo/changesets/bounded-trait-lookup.md

@@ -0,0 +1,6 @@
+---
+cargo/nash-can: patch
+cargo/nash-solve: patch
+---
+
+Use ordered map ranges for trait candidate lookup, evidence construction, entailment, and missing-implementation suggestions. Preserve candidate order without a second index.

.sampo/changesets/concise-source-aware-diagnostics.md

@@ -0,0 +1,12 @@
+---
+cargo/nash-parse: minor
+cargo/nash-constrain: minor
+cargo/nash-solve: patch
+cargo/nash-report: minor
+cargo/nash-language-server: minor
+cargo/nash-cli: minor
+---
+
+Use concise diagnostics with full expected/actual type comparisons, expectation-origin labels, and stable codes independent of display titles. Retain parser opening positions for closing-delimiter reports. Support arbitrary secondary labels and related reports across source files.
+
+Extend diagnostic JSON with code, severity, labels, suggestions, and related reports. JSON messages now contain styled prose without embedded source drawings; consumers should render the structured labels. LSP diagnostic codes now use stable identifiers instead of titles and include secondary and related source locations.

.sampo/changesets/direct-ast-inference.md

@@ -0,0 +1,8 @@
+---
+cargo/nash-constrain: minor
+cargo/nash-solve: minor
+cargo/nash-driver: patch
+cargo/nash-report: patch
+---
+
+Infer directly from the canonical AST into the existing union-find and predicate engine. Remove the allocated constraint tree and intermediate inference Type, preserving schemes, evidence, rank ownership, recursive-group sequencing, and complete diagnostics. Pass canonical modules directly to the solver.

.sampo/changesets/parser-accumulators.md

@@ -0,0 +1,5 @@
+---
+cargo/nash-parse: patch
+---
+
+Accumulate function arguments and binary operators without cloning partial chains. Keep parser arena allocation linear in operator-chain length.

.sampo/changesets/parser-nesting.md

@@ -0,0 +1,6 @@
+---
+cargo/nash-parse: patch
+cargo/nash-report: patch
+---
+
+Report excessive expression, pattern, and type nesting before stack exhaustion. Parse flat sequences and nested comments with loops.

.sampo/changesets/parser-text-input.md

@@ -0,0 +1,6 @@
+---
+cargo/nash-parse: minor
+cargo/nash-driver: patch
+---
+
+Require UTF-8 text at the parser boundary instead of arbitrary bytes. Remove unchecked string conversions and pass source text directly from the driver.

.sampo/changesets/remove-interface-cache.md

@@ -0,0 +1,5 @@
+---
+cargo/nash-driver: minor
+---
+
+Remove unused disk interface-cache APIs, serialization, and cache metadata. Preserve in-memory exports, kind contracts, and fingerprints returned by compilation.

.sampo/changesets/source-coordinates.md

@@ -0,0 +1,10 @@
+---
+cargo/nash-region: minor
+cargo/nash-source: minor
+cargo/nash-parse: minor
+cargo/nash-can: patch
+cargo/nash-report: minor
+cargo/nash-language-server: patch
+---
+
+Use source-sized coordinates and diagnostic widths throughout parsing and reporting. Check LSP coordinate conversion instead of truncating. Reject oversized Unicode escapes without integer overflow, and make arbitrary lookahead offsets safe.

CLAUDE.md

@@ -23,15 +23,15 @@ We use **bumpalo** for arena allocation:
 ```rust
 let bump = Bump::new();
 let src: &str = bump.alloc_str(&file_contents);
-let mut parser = Parser::new(&bump, src.as_bytes());
+let mut parser = Parser::new(&bump, src);
 ```
 
 ### AST Type Guidelines
 
 **Inline small `Copy` types** - don't put them behind `&'a`:
 - Small enums (e.g., `VarType`, `Associativity`) - just store the value
 - Newtypes around primitives (e.g., `Precedence(u16)`) - just store the value
-- `Region` (8 bytes of integers) - same size as a pointer, no benefit to indirection
+- `Region` uses native-size source coordinates (32 bytes on 64-bit hosts); keep it inline in AST nodes.
 
 **Use `&'a T` for**:
 - Large types

Cargo.toml

@@ -10,7 +10,6 @@ license = "Apache-2.0"
 
 [workspace.dependencies]
 async-trait = "0.1"
-bincode = "1"
 bumpalo = { version = "3.19.1", features = ["collections"] }
 clap = { version = "4.5.60", features = ["derive"] }
 color-print = "0.3.7"

SPEC.md

@@ -8,7 +8,7 @@ component specs in [`docs/`](docs/); chunked implementation plans in
 ## Pipeline
 
 ```
-parse -> canonicalize -> kinds -> constrain/solve (+ traits) -> nitpick
+parse -> canonicalize -> kinds -> direct inference (+ traits) -> nitpick
       -> macro expansion (loop) -> Core IR -> optimize -> UPLC
 ```
 
@@ -24,10 +24,10 @@ produce UPLC programs; all dependencies inline into each program.
 | `nash-parse` | parser + Elm error hierarchy | extend ([plans/01](plans/01-syntax.md)) |
 | `nash-ast` | canonical AST | extend |
 | `nash-can` | canonicalization, interfaces | extend |
-| `nash-constrain` | constraint generation, kinds | extend |
-| `nash-solve` | solver, traits, defaulting | extend |
+| `nash-constrain` | union-find types, canonical instantiation, type errors | done |
+| `nash-solve` | direct AST inference, traits, defaulting | extend |
 | `nash-nitpick` | exhaustiveness and redundancy | done ([plans/05](plans/05-nitpick.md)) |
-| `nash-report` | diagnostics (Elm prose -> miette) | done ([plans/06](plans/06-diagnostics.md)) |
+| `nash-report` | concise diagnostics (terminal, JSON, LSP) | done ([plans/06](plans/06-diagnostics.md)) |
 | `nash-ir` | Core IR + passes | new ([plans/07](plans/07-codegen.md), [08](plans/08-optimizer.md)) |
 | `nash-codegen` | Can -> Core -> UPLC | new ([plans/07](plans/07-codegen.md)) |
 | `nash-test` | test runner, fuzzing, shrinking | new ([plans/10](plans/10-testing.md)) |
@@ -46,7 +46,8 @@ Done:
 
 - [x] Parser (Elm `Parse/*` port, full syntax error hierarchy)
 - [x] Canonicalization (Elm `Canonicalize/*` port, SCC, interfaces)
-- [x] Type inference (Elm `Type/*` port: constraints, rank-based solver, records, aliases)
+- [x] Type inference (direct AST inference, rank-based solver, records, aliases)
+- [x] Frontend hardening and direct-inference parity ([verification](docs/frontend-hardening-verification.md))
 - [x] Project config, driver, dependency-ordered builds, interface cache
 - [x] `nash check`
 - [x] UPLC runtime (`nash-plutus`): conformance suite passes
@@ -60,7 +61,7 @@ Planned, in execution order (each links to its plan):
 - [x] 03 Traits: qualified types, resolution, superclasses, defaults, multi-param, orphan rules, literal traits + defaulting, evidence — [plans/03-traits.md](plans/03-traits.md) (default imports deferred to Plan 12)
 - [x] 04 Representation: remove row polymorphism and Elm supertypes, builtin type inventory, record encoding — [plans/04-representation.md](plans/04-representation.md)
 - [x] 05 Exhaustiveness (`Nitpick/PatternMatches` port) — [plans/05-nitpick.md](plans/05-nitpick.md)
-- [x] 06 Diagnostics (`nash-report`, Elm `Reporting/*` port onto miette) — [plans/06-diagnostics.md](plans/06-diagnostics.md)
+- [x] 06 Diagnostics (`nash-report`, concise source labels, stable codes, JSON/LSP) — [plans/06-diagnostics.md](plans/06-diagnostics.md); [concise diagnostics refactor](plans/diagnostics-refactor.md) complete
 - [ ] 07 Codegen: Core IR, monomorphization, decision trees, recursion, Data casts, UPLC lowering — [plans/07-codegen.md](plans/07-codegen.md)
 - [ ] 08 Optimizer: inlining, builtin force caching, DCE, case-of-known-ctor/constant folding — [plans/08-optimizer.md](plans/08-optimizer.md)
 - [ ] 09 Validators + `nash build` — [plans/09-validators-build.md](plans/09-validators-build.md)
@@ -80,7 +81,7 @@ Later: LSP features, web playground, package registry (pubgrub), TypeScript code
 | `AST/Source.hs` | `crates/nash-source/src/lib.rs` |
 | `AST/Canonical.hs` | `crates/nash-ast/src/lib.rs` |
 | `Canonicalize/*` | `crates/nash-can/src/*` |
-| `Type/Type.hs`, `Type/Constrain/*` | `crates/nash-constrain/src/*` |
+| `Type/Type.hs`, `Type/Constrain/*` | `crates/nash-constrain/src/*`, `crates/nash-solve/src/solve/*` |
 | `Type/{Solve,Unify,Occurs}.hs` | `crates/nash-solve/src/*` |
 | `Nitpick/PatternMatches.hs` | `crates/nash-nitpick` |
 | `Reporting/{Doc,Report,Render,Suggest}.hs`, `Reporting/Error/*` | `crates/nash-report` |

crates/nash-can/src/entailment.rs

@@ -350,9 +350,7 @@ impl<'a> Resolver<'_, 'a> {
         let mut selected = None;
         for (key, info) in self
             .tables
-            .impls
-            .iter()
-            .filter(|(key, _)| Some(key.trait_) == wanted.trait_)
+            .impls_for(wanted.trait_.ok_or(Failure::Missing)?)
         {
             if let nash_ast::head::Match::Yes(arguments) = nash_ast::head::matches(
                 &mut nash_ast::head::Canonical,