compiler: Hint at multiple crate versions if trait impl is for wrong ADT #146874

Enselic · 2025-09-22T06:03:14Z

If a user does e.g.

impl From<Bar> for foo::Foo

and get a compilation error about that From<Bar> is not implemented for Foo, check if multiple versions of the crate with Foo is present in the dependency graph. If so, give a hint about it.

Note that a test is added as a separate commit so it is easy to see what effect the fix has on the emitted error message.

This can be seen as a continuation of #124944.

I think this closes RUST-71693 but I haven't checked since it lacks a minimal reproducer. If this gets merged I'll ask that reporter if this fix works for them.

Real world example

I encountered this case in the wild and didn't realize I had multiple versions of a crate in my dependency graph. So I was a bit confused at first. For reference, here is what that looked like.

Click to expand

Before fix

error[E0277]: the trait bound `lambda_http::lambda_runtime::Diagnostic: From<Error>` is not satisfied
   --> src/main.rs:73:5
    |
73  |     lambda_http::run(service_fn(handle_event)).await
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Error>` is not implemented for `lambda_http::lambda_runtime::Diagnostic`
    |
    = help: the following other types implement trait `From<T>`:
              `lambda_http::lambda_runtime::Diagnostic` implements `From<&str>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError + Send + Sync>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Infallible>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<lambda_runtime::deserializer::DeserializeError>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::io::Error>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::string::String>`
    = note: required for `Error` to implement `Into<lambda_http::lambda_runtime::Diagnostic>`
note: required by a bound in `lambda_http::run`
   --> /home/martin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lambda_http-0.17.0/src/lib.rs:199:26
    |
194 | pub async fn run<'a, R, S, E>(handler: S) -> Result<(), Error>
    |              --- required by a bound in this function
...
199 |     E: std::fmt::Debug + Into<Diagnostic>,
    |                          ^^^^^^^^^^^^^^^^ required by this bound in `run`

error[E0277]: the trait bound `lambda_http::lambda_runtime::Diagnostic: From<Error>` is not satisfied
   --> src/main.rs:73:48
    |
73  |     lambda_http::run(service_fn(handle_event)).await
    |                                                ^^^^^ the trait `From<Error>` is not implemented for `lambda_http::lambda_runtime::Diagnostic`
    |
    = help: the following other types implement trait `From<T>`:
              `lambda_http::lambda_runtime::Diagnostic` implements `From<&str>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError + Send + Sync>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Infallible>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<lambda_runtime::deserializer::DeserializeError>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::io::Error>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::string::String>`
    = note: required for `Error` to implement `Into<lambda_http::lambda_runtime::Diagnostic>`
note: required by a bound in `lambda_http::run`
   --> /home/martin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lambda_http-0.17.0/src/lib.rs:199:26
    |
194 | pub async fn run<'a, R, S, E>(handler: S) -> Result<(), Error>
    |              --- required by a bound in this function
...
199 |     E: std::fmt::Debug + Into<Diagnostic>,
    |                          ^^^^^^^^^^^^^^^^ required by this bound in `run`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `auto-merge-dependabot-pull-requests-webhook` (bin "auto-merge-dependabot-pull-requests-webhook") due to 2 previous errors

After fix

   Compiling auto-merge-dependabot-pull-requests-webhook v0.1.0 (/home/martin/src/auto-merge-dependabot-prs/rust-webhook)
error[E0277]: the trait bound `lambda_http::lambda_runtime::Diagnostic: From<Error>` is not satisfied
   --> src/main.rs:73:5
    |
 73 |     lambda_http::run(service_fn(handle_event)).await
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<Error>` is not implemented for `lambda_http::lambda_runtime::Diagnostic`
    |
help: item with same name found
   --> /home/martin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lambda_runtime-0.13.0/src/diagnostic.rs:43:1
    |
 43 | pub struct Diagnostic {
    | ^^^^^^^^^^^^^^^^^^^^^
    = note: perhaps two different versions of crate `lambda_runtime` are being used?
    = help: the following other types implement trait `From<T>`:
              `lambda_http::lambda_runtime::Diagnostic` implements `From<&str>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError + Send + Sync>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Infallible>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<lambda_runtime::deserializer::DeserializeError>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::io::Error>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::string::String>`
    = note: required for `Error` to implement `Into<lambda_http::lambda_runtime::Diagnostic>`
note: required by a bound in `lambda_http::run`
   --> /home/martin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lambda_http-0.17.0/src/lib.rs:199:26
    |
194 | pub async fn run<'a, R, S, E>(handler: S) -> Result<(), Error>
    |              --- required by a bound in this function
...
199 |     E: std::fmt::Debug + Into<Diagnostic>,
    |                          ^^^^^^^^^^^^^^^^ required by this bound in `run`

error[E0277]: the trait bound `lambda_http::lambda_runtime::Diagnostic: From<Error>` is not satisfied
   --> src/main.rs:73:48
    |
 73 |     lambda_http::run(service_fn(handle_event)).await
    |                                                ^^^^^ the trait `From<Error>` is not implemented for `lambda_http::lambda_runtime::Diagnostic`
    |
help: item with same name found
   --> /home/martin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lambda_runtime-0.13.0/src/diagnostic.rs:43:1
    |
 43 | pub struct Diagnostic {
    | ^^^^^^^^^^^^^^^^^^^^^
    = note: perhaps two different versions of crate `lambda_runtime` are being used?
    = help: the following other types implement trait `From<T>`:
              `lambda_http::lambda_runtime::Diagnostic` implements `From<&str>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError + Send + Sync>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Box<dyn StdError>>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<Infallible>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<lambda_runtime::deserializer::DeserializeError>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::io::Error>`
              `lambda_http::lambda_runtime::Diagnostic` implements `From<std::string::String>`
    = note: required for `Error` to implement `Into<lambda_http::lambda_runtime::Diagnostic>`
note: required by a bound in `lambda_http::run`
   --> /home/martin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/lambda_http-0.17.0/src/lib.rs:199:26
    |
194 | pub async fn run<'a, R, S, E>(handler: S) -> Result<(), Error>
    |              --- required by a bound in this function
...
199 |     E: std::fmt::Debug + Into<Diagnostic>,
    |                          ^^^^^^^^^^^^^^^^ required by this bound in `run`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `auto-merge-dependabot-pull-requests-webhook` (bin "auto-merge-dependabot-pull-requests-webhook") due to 2 previous errors

If a user does e.g. impl From<Bar> for foo::Foo and get a compilation error about that `From<Bar>` is not implemented for `Foo`, check if multiple versions of the crate with `Foo` is present in the dependency graph. If so, give a hint about it. I encountered this case in the wild and didn't realize I had multiple versions of a crate in my dependency graph. So I was a bit confused at first. This fix will make life easier for others.

rustbot · 2025-09-22T06:03:20Z

r? @jieyouxu

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fmease · 2025-09-22T06:24:23Z

I haven't compared the two PRs but CC open PR #144674 which also touches this function.

Enselic · 2025-09-22T14:24:19Z

I haven't compared the two PRs but CC open PR #144674 which also touches this function.

👍 Here I and the author of that PR agreed that the PRs are orthogonal feature-wise and can be merged independently. Any merge conflicts are expected to be easy to resolve.

tests/run-make/crate-loading: Rename source files for clarity For rust-lang#146874 I originally tried to extend the existing test **tests/run-make/crate-loading**. That didn't work out since adding a re-export of the entire crate significantly changes the emitted error messsage. I did put some effort into making that test easier to understand however, by renaming its files. (Since I was confused myself at first.) Let's save some time for future devs by doing just the renames. Further cleanups are possible, but that will change the blessed output, so let's not do that right now. r? `@jieyouxu` since you have the context of rust-lang#146874

tests/run-make/crate-loading: Rename source files for clarity For rust-lang#146874 I originally tried to extend the existing test **tests/run-make/crate-loading**. That didn't work out since adding a re-export of the entire crate significantly changes the emitted error messsage. I did put some effort into making that test easier to understand however, by renaming its files. (Since I was confused myself at first.) Let's save some time for future devs by doing just the renames. Further cleanups are possible, but that will change the blessed output, so let's not do that right now. r? ``@jieyouxu`` since you have the context of rust-lang#146874

tests/run-make/crate-loading: Rename source files for clarity For rust-lang#146874 I originally tried to extend the existing test **tests/run-make/crate-loading**. That didn't work out since adding a re-export of the entire crate significantly changes the emitted error messsage. I did put some effort into making that test easier to understand however, by renaming its files. (Since I was confused myself at first.) Let's save some time for future devs by doing just the renames. Further cleanups are possible, but that will change the blessed output, so let's not do that right now. r? ```@jieyouxu``` since you have the context of rust-lang#146874

Rollup merge of #146875 - Enselic:better-file-names, r=jieyouxu tests/run-make/crate-loading: Rename source files for clarity For #146874 I originally tried to extend the existing test **tests/run-make/crate-loading**. That didn't work out since adding a re-export of the entire crate significantly changes the emitted error messsage. I did put some effort into making that test easier to understand however, by renaming its files. (Since I was confused myself at first.) Let's save some time for future devs by doing just the renames. Further cleanups are possible, but that will change the blessed output, so let's not do that right now. r? ```@jieyouxu``` since you have the context of #146874

tests/run-make/crate-loading: Rename source files for clarity For rust-lang#146874 I originally tried to extend the existing test **tests/run-make/crate-loading**. That didn't work out since adding a re-export of the entire crate significantly changes the emitted error messsage. I did put some effort into making that test easier to understand however, by renaming its files. (Since I was confused myself at first.) Let's save some time for future devs by doing just the renames. Further cleanups are possible, but that will change the blessed output, so let's not do that right now. r? ```@jieyouxu``` since you have the context of rust-lang#146874

Enselic added 2 commits September 22, 2025 07:25

tests: Check error when struct from wrong crate version is used for impl

e1ae7b7

rustbot assigned jieyouxu Sep 22, 2025

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 22, 2025

Enselic mentioned this pull request Sep 22, 2025

tests/run-make/crate-loading: Rename source files for clarity #146875

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compiler: Hint at multiple crate versions if trait impl is for wrong ADT #146874

compiler: Hint at multiple crate versions if trait impl is for wrong ADT #146874

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Enselic commented Sep 22, 2025 •

edited by jieyouxu

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Real world example

Before fix

After fix

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