Implement method signature suggestion for trait mismatches error

Author: reddevilmidzy

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -18,7 +18,7 @@ use rustc_middle::ty::{
     TypeSuperFoldable, TypeVisitable, TypeVisitableExt, TypeVisitor, TypingMode, Upcast,
 };
 use rustc_middle::{bug, span_bug};
-use rustc_span::{DUMMY_SP, Span};
+use rustc_span::{BytePos, DUMMY_SP, Span};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
@@ -1742,6 +1742,46 @@ fn compare_number_of_method_arguments<'tcx>(
             ),
         );
 
+        let sm = tcx.sess.source_map();
+        let find_param_bounds = |snippet: &str| {
+            let start = snippet.find('(')? + 1;
+            let mut depth = 1;
+            for (i, ch) in snippet[start..].char_indices() {
+                match ch {
+                    '(' => depth += 1,
+                    ')' => { depth -= 1; if depth == 0 { return Some((start, start + i)); } }
+                    _ => {}
+                }
+            }
+            None
+        };
+
+        let extract_params = |span: Span| sm.span_to_snippet(span).ok()
+            .and_then(|s| find_param_bounds(&s).map(|(lo, hi)| s[lo..hi].to_string()));
+
+        let impl_inputs_span = (!impl_m_sig.span.is_dummy()).then(|| {
+            let snippet = sm.span_to_snippet(impl_m_sig.span).ok()?;
+            let (lo, hi) = find_param_bounds(&snippet)?;
+            Some(impl_m_sig.span.with_lo(impl_m_sig.span.lo() + BytePos(lo as u32))
+                                .with_hi(impl_m_sig.span.lo() + BytePos(hi as u32)))
+        }).flatten();
+
+        let suggestion = trait_m.def_id.as_local()
+            .and_then(|id| extract_params(tcx.hir_expect_trait_item(id).expect_fn().0.span))
+            .or_else(|| {
+                let sig = trait_m.signature(tcx);
+                find_param_bounds(&sig).map(|(lo, hi)| sig[lo..hi].trim().to_string())
+            });
+
+        if let (Some(span), Some(suggestion)) = (impl_inputs_span, suggestion) {
+            err.span_suggestion_verbose(
+                span,
+                "modify the signature to match the trait definition",
+                suggestion,
+                Applicability::MaybeIncorrect,
+            );
+        }
+
         return Err(err.emit_unless_delay(delay));
     }
 

tests/ui/error-codes/E0050.stderr

@@ -6,6 +6,11 @@ LL |     fn foo(&self, x: u8) -> bool;
 ...
 LL |     fn foo(&self) -> bool { true }
    |            ^^^^^ expected 2 parameters, found 1
+   |
+help: modify the signature to match the trait definition
+   |
+LL |     fn foo(&self, x: u8) -> bool { true }
+   |                 +++++++
 
 error[E0050]: method `bar` has 1 parameter but the declaration in trait `Foo::bar` has 4
   --> $DIR/E0050.rs:11:12
@@ -15,6 +20,11 @@ LL |     fn bar(&self, x: u8, y: u8, z: u8);
 ...
 LL |     fn bar(&self) { }
    |            ^^^^^ expected 4 parameters, found 1
+   |
+help: modify the signature to match the trait definition
+   |
+LL |     fn bar(&self, x: u8, y: u8, z: u8) { }
+   |                 +++++++++++++++++++++
 
 error[E0050]: method `less` has 4 parameters but the declaration in trait `Foo::less` has 1
   --> $DIR/E0050.rs:12:13
@@ -24,6 +34,12 @@ LL |     fn less(&self);
 ...
 LL |     fn less(&self, x: u8, y: u8, z: u8) { }
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 1 parameter, found 4
+   |
+help: modify the signature to match the trait definition
+   |
+LL -     fn less(&self, x: u8, y: u8, z: u8) { }
+LL +     fn less(&self) { }
+   |
 
 error: aborting due to 3 previous errors
 

tests/ui/fn/issue-39259.stderr

@@ -17,6 +17,11 @@ LL |     fn call(&self) -> u32 {
    |             ^^^^^ expected 2 parameters, found 1
    |
    = note: `call` from trait: `extern "rust-call" fn(&Self, Args) -> <Self as FnOnce<Args>>::Output`
+help: modify the signature to match the trait definition
+   |
+LL -     fn call(&self) -> u32 {
+LL +     fn call(&Self, Args) -> u32 {
+   |
 
 error[E0277]: expected a `FnMut(u32)` closure, found `S`
   --> $DIR/issue-39259.rs:6:25

tests/ui/impl-trait/in-trait/method-signature-matches.too_few.stderr

@@ -6,6 +6,11 @@ LL |     fn come_on_a_little_more_effort(_: (), _: (), _: ()) -> impl Sized;
 ...
 LL |     fn come_on_a_little_more_effort() {}
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 3 parameters, found 0
+   |
+help: modify the signature to match the trait definition
+   |
+LL |     fn come_on_a_little_more_effort(_: (), _: (), _: ()) {}
+   |                                     +++++++++++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/in-trait/method-signature-matches.too_many.stderr

@@ -6,6 +6,12 @@ LL |     fn calm_down_please() -> impl Sized;
 ...
 LL |     fn calm_down_please(_: (), _: (), _: ()) {}
    |                            ^^^^^^^^^^^^^^^^ expected 0 parameters, found 3
+   |
+help: modify the signature to match the trait definition
+   |
+LL -     fn calm_down_please(_: (), _: (), _: ()) {}
+LL +     fn calm_down_please() {}
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/trait_type.stderr

@@ -19,6 +19,11 @@ LL |    fn fmt(&self) -> () { }
    |           ^^^^^ expected 2 parameters, found 1
    |
    = note: `fmt` from trait: `fn(&Self, &mut Formatter<'_>) -> Result<(), std::fmt::Error>`
+help: modify the signature to match the trait definition
+   |
+LL -    fn fmt(&self) -> () { }
+LL +    fn fmt(&Self, &mut Formatter<'_>) -> () { }
+   |
 
 error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
   --> $DIR/trait_type.rs:17:4

tests/ui/suggestions/bad-infer-in-trait-impl.stderr

@@ -12,6 +12,12 @@ LL |     fn bar();
 ...
 LL |     fn bar(s: _) {}
    |               ^ expected 0 parameters, found 1
+   |
+help: modify the signature to match the trait definition
+   |
+LL -     fn bar(s: _) {}
+LL +     fn bar() {}
+   |
 
 error: aborting due to 2 previous errors
 

tests/ui/traits/impl-different-num-params.stderr

@@ -6,6 +6,11 @@ LL |     fn bar(&self, x: usize) -> Self;
 ...
 LL |     fn bar(&self) -> isize {
    |            ^^^^^ expected 2 parameters, found 1
+   |
+help: modify the signature to match the trait definition
+   |
+LL |     fn bar(&self, x: usize) -> isize {
+   |                 ++++++++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/trait-method-signature-mismatch.stderr

@@ -7,6 +7,15 @@ LL |       fn foo(&mut self, x: i32, y: i32) -> i32;
 LL | /         &mut self,
 LL | |         x: i32,
    | |______________^ expected 3 parameters, found 2
+   |
+help: modify the signature to match the trait definition
+   |
+LL -     fn foo(
+LL -         &mut self,
+LL -         x: i32,
+LL -     ) {
+LL +     fn foo(&mut self, x: i32, y: i32) {
+   |
 
 error: aborting due to 1 previous error