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Say why an opaque return type could not be read

The report named the functions and stopped there, which left every one of
them a separate investigation. It now carries the error the body actually
failed with, so the list sorts into causes:

    7   only calls to named functions are supported
    7   a closure body with statements is not implemented yet
    2   cannot infer the type of closure parameter `e`
    1   `match` in value position
    ...

Those are ordinary gaps in the lowering, not anything exotic about opaque
types -- which is the useful thing to know, and was invisible while the reason
was being swallowed by `.ok()`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
nandithebull committed 2026-09-19T19:27:13-07:00 Browse files
670d6a7 parent: f3e08c6
modified src/lower.rs +32 -2
@@ -348,6 +348,9 @@ pub struct Lowerer {
348348 /// Const generic parameters, by name, with their Nim type. Nim's `static`
349349 /// parameter is the same thing: a generic parameter that is a value.
350350 const_gens: HashMap<String, String>,
351+ /// Why the last opaque-return body failed to type, so the report can
352+ /// say rather than only naming the function.
353+ opaque_why: Option<String>,
351354 /// Methods a `trait` declares, by `(trait, method)`, with the trait's own
352355 /// generic parameters. Used to type a call on a value whose type is a
353356 /// generic parameter: `alternatives.choice(i)` where the parameter is
@@ -446,6 +449,7 @@ impl Lowerer {
446449 emitted_local: std::collections::HashSet::new(),
447450 emitted_assoc: std::collections::HashSet::new(),
448451 const_gens: HashMap::new(),
452+ opaque_why: None,
449453 trait_methods: HashMap::new(),
450454 bounds: HashMap::new(),
451455 const_names_at: std::cell::Cell::new(None),
@@ -2051,6 +2055,16 @@ impl Lowerer {
20512055 /// means nothing until `c` is bound, which is what `pat_bind` does for
20522056 /// the real lowering too.
20532057 fn result_ty(&mut self, e: &Expr) -> Option<Nim> {
2058+ let r = self.result_ty_inner(e);
2059+ if r.is_none() && self.opaque_why.is_none() {
2060+ if let Err(msg) = self.expr(e) {
2061+ self.opaque_why = Some(msg);
2062+ }
2063+ }
2064+ r
2065+ }
2066+
2067+ fn result_ty_inner(&mut self, e: &Expr) -> Option<Nim> {
20542068 match e {
20552069 Expr::Match(m) => {
20562070 let scrut = self.expr(&m.expr).ok()?;
@@ -2100,6 +2114,7 @@ impl Lowerer {
21002114 fn resolve_opaque(&mut self) -> Result<(), String> {
21012115 let opaque = Nim::Named(OPAQUE.into(), Vec::new());
21022116 let pending = std::mem::take(&mut self.opaque_fns);
2117+ let mut why: HashMap<String, String> = HashMap::new();
21032118 let mut left = pending.clone();
21042119 // One function's opaque type may be another's, so this iterates -- but
21052120 // it is bounded rather than run to exhaustion. Each round lowers every
@@ -2132,7 +2147,18 @@ impl Lowerer {
21322147 }
21332148 progressed = true;
21342149 }
2135- _ => still.push((key, sig, block, self_ty)),
2150+ _ => {
2151+ if let Some(w) = self.opaque_why.take() {
2152+ why.insert(
2153+ match &key {
2154+ OpaqueKey::Free(_, n) => n.clone(),
2155+ OpaqueKey::Method(t, n) => format!("{t}::{n}"),
2156+ },
2157+ w,
2158+ );
2159+ }
2160+ still.push((key, sig, block, self_ty))
2161+ }
21362162 }
21372163 }
21382164 left = still;
@@ -2154,7 +2180,10 @@ impl Lowerer {
21542180 ReturnType::Type(_, t) => quote::quote!(#t).to_string(),
21552181 ReturnType::Default => String::new(),
21562182 };
2157- format!(" {what}() -> {ret}")
2183+ match why.get(&what) {
2184+ Some(w) => format!(" {what}() -> {ret}\n {w}"),
2185+ None => format!(" {what}() -> {ret}"),
2186+ }
21582187 })
21592188 .collect();
21602189 names.sort();
@@ -2230,6 +2259,7 @@ impl Lowerer {
22302259 let saved_out = self.out.len();
22312260 let saved_indent = self.indent;
22322261 let saved_hoisted = self.hoisted.len();
2262+ self.opaque_why = None;
22332263 let found = self.result_ty(&tail);
22342264 self.out.truncate(saved_out);
22352265 self.indent = saved_indent;
@@ -348,6 +348,9 @@ pub struct Lowerer {
348 /// Const generic parameters, by name, with their Nim type. Nim's `static`348 /// Const generic parameters, by name, with their Nim type. Nim's `static`
349 /// parameter is the same thing: a generic parameter that is a value.349 /// parameter is the same thing: a generic parameter that is a value.
350 const_gens: HashMap<String, String>,350 const_gens: HashMap<String, String>,
351+ /// Why the last opaque-return body failed to type, so the report can
352+ /// say rather than only naming the function.
353+ opaque_why: Option<String>,
351 /// Methods a `trait` declares, by `(trait, method)`, with the trait's own354 /// Methods a `trait` declares, by `(trait, method)`, with the trait's own
352 /// generic parameters. Used to type a call on a value whose type is a355 /// generic parameters. Used to type a call on a value whose type is a
353 /// generic parameter: `alternatives.choice(i)` where the parameter is356 /// generic parameter: `alternatives.choice(i)` where the parameter is
@@ -446,6 +449,7 @@ impl Lowerer {
446 emitted_local: std::collections::HashSet::new(),449 emitted_local: std::collections::HashSet::new(),
447 emitted_assoc: std::collections::HashSet::new(),450 emitted_assoc: std::collections::HashSet::new(),
448 const_gens: HashMap::new(),451 const_gens: HashMap::new(),
452+ opaque_why: None,
449 trait_methods: HashMap::new(),453 trait_methods: HashMap::new(),
450 bounds: HashMap::new(),454 bounds: HashMap::new(),
451 const_names_at: std::cell::Cell::new(None),455 const_names_at: std::cell::Cell::new(None),
@@ -2051,6 +2055,16 @@ impl Lowerer {
2051 /// means nothing until `c` is bound, which is what `pat_bind` does for2055 /// means nothing until `c` is bound, which is what `pat_bind` does for
2052 /// the real lowering too.2056 /// the real lowering too.
2053 fn result_ty(&mut self, e: &Expr) -> Option<Nim> {2057 fn result_ty(&mut self, e: &Expr) -> Option<Nim> {
2058+ let r = self.result_ty_inner(e);
2059+ if r.is_none() && self.opaque_why.is_none() {
2060+ if let Err(msg) = self.expr(e) {
2061+ self.opaque_why = Some(msg);
2062+ }
2063+ }
2064+ r
2065+ }
2066+
2067+ fn result_ty_inner(&mut self, e: &Expr) -> Option<Nim> {
2054 match e {2068 match e {
2055 Expr::Match(m) => {2069 Expr::Match(m) => {
2056 let scrut = self.expr(&m.expr).ok()?;2070 let scrut = self.expr(&m.expr).ok()?;
@@ -2100,6 +2114,7 @@ impl Lowerer {
2100 fn resolve_opaque(&mut self) -> Result<(), String> {2114 fn resolve_opaque(&mut self) -> Result<(), String> {
2101 let opaque = Nim::Named(OPAQUE.into(), Vec::new());2115 let opaque = Nim::Named(OPAQUE.into(), Vec::new());
2102 let pending = std::mem::take(&mut self.opaque_fns);2116 let pending = std::mem::take(&mut self.opaque_fns);
2117+ let mut why: HashMap<String, String> = HashMap::new();
2103 let mut left = pending.clone();2118 let mut left = pending.clone();
2104 // One function's opaque type may be another's, so this iterates -- but2119 // One function's opaque type may be another's, so this iterates -- but
2105 // it is bounded rather than run to exhaustion. Each round lowers every2120 // it is bounded rather than run to exhaustion. Each round lowers every
@@ -2132,7 +2147,18 @@ impl Lowerer {
2132 }2147 }
2133 progressed = true;2148 progressed = true;
2134 }2149 }
2135- _ => still.push((key, sig, block, self_ty)),2150+ _ => {
2151+ if let Some(w) = self.opaque_why.take() {
2152+ why.insert(
2153+ match &key {
2154+ OpaqueKey::Free(_, n) => n.clone(),
2155+ OpaqueKey::Method(t, n) => format!("{t}::{n}"),
2156+ },
2157+ w,
2158+ );
2159+ }
2160+ still.push((key, sig, block, self_ty))
2161+ }
2136 }2162 }
2137 }2163 }
2138 left = still;2164 left = still;
@@ -2154,7 +2180,10 @@ impl Lowerer {
2154 ReturnType::Type(_, t) => quote::quote!(#t).to_string(),2180 ReturnType::Type(_, t) => quote::quote!(#t).to_string(),
2155 ReturnType::Default => String::new(),2181 ReturnType::Default => String::new(),
2156 };2182 };
2157- format!(" {what}() -> {ret}")2183+ match why.get(&what) {
2184+ Some(w) => format!(" {what}() -> {ret}\n {w}"),
2185+ None => format!(" {what}() -> {ret}"),
2186+ }
2158 })2187 })
2159 .collect();2188 .collect();
2160 names.sort();2189 names.sort();
@@ -2230,6 +2259,7 @@ impl Lowerer {
2230 let saved_out = self.out.len();2259 let saved_out = self.out.len();
2231 let saved_indent = self.indent;2260 let saved_indent = self.indent;
2232 let saved_hoisted = self.hoisted.len();2261 let saved_hoisted = self.hoisted.len();
2262+ self.opaque_why = None;
2233 let found = self.result_ty(&tail);2263 let found = self.result_ty(&tail);
2234 self.out.truncate(saved_out);2264 self.out.truncate(saved_out);
2235 self.indent = saved_indent;2265 self.indent = saved_indent;