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Improve E0308 error message wording #70242

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Mar 24, 2020
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Improve E0308 error message wording #70242

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8 changes: 4 additions & 4 deletions src/librustc_error_codes/error_codes/E0308.md
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Expand Up @@ -13,7 +13,7 @@ let x: i32 = "I am not a number!";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
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Is this a typo? on the one hand

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Just my two cents, on one hand and on the other hand may not be easily understood by non-native english speakers. Maybe it's just me but I learned that late, and is easier to be understood.

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@DeeDeeG DeeDeeG Apr 13, 2020 •
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I meant to contribute a better message with no "on the other hand." But I got side-tracked and honestly a bit discouraged, since there's nobody to workshop good messages with, and no standards of what a good error message looks like.

I'd like there to be a working group to bounce ideas off of, and/or a style guide.

Here's what I had, roughly:

This error occurs when the compiler was unable to infer the concrete type of a variable.
It can occur for several cases; The most common case is a mismatch between
two types: the type explicitly assigned to a variable by the [code's] author,
and the type the compiler infers for an expression assigned to that variable.

Possibly a "hint" line about dynamic typing versus static typing:

In Rust each declared variable is assigned a static type. Dynamic typing is not supported.

See prior discussion here if you want: https://users.rust-lang.org/t/looking-for-feedback-on-an-improved-error-message-for-e0308/40004

I actually think the phrasing can be much simplified.

Any two mis-matching types assigned to the same variable cause this error. Regardless of explicit or inferred, at or after initialization, etc... Just... two non-matching types, one variable.

(Apparently there is a thing called shadowing, but if I read correctly, I believe it just spins up a new variable with a different compiled identity, but re-using the same name in source code. New instance of the variable can have a new type. It's still static typing under the hood, though. I am afraid of over-simplifying, and ending up with a message that is inaccurate regarding shadowing.)

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This error occurs when the compiler was unable to infer the concrete type of a variable.
It can occur for several cases; The most common case is a mismatch between
two types: the type explicitly assigned to a variable by the [code's] author,
and the type the compiler infers for an expression assigned to that variable.

That is better than the merged one I think but still kinda long-winded, I think documentation should be short, concise and understandable. What I would say a good example is IETF RFC docs, rust docs is good enough for me.

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Thanks for the reply.

I have some things to do at the moment, but I will post that version as a second PR when I get the chance. Hopefully before a new stable release.

It could probably be shortened, mostly by removing the "common example" of when this error occurs. (It is kind of arbitrary, and I think it might be overly specific when the error is more general than that.) But I don't want to be too drastic while I am still getting used to contributing to Rust.

Posting this for reference since you mentioned IETF RFCs as being a good example: https://tools.ietf.org/html/rfc7322 The IETF RFC Style Guide.

Maybe that can be an influence for a Rust docs "style guide" or best practices guide. No time now, but planning to look that over later.

assigned to the variable, on the other hand.
32 changes: 16 additions & 16 deletions src/test/ui/json-bom-plus-crlf-multifile.stderr
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -13,10 +13,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":621,"byte_end":622,"line_start":17,"line_end":17,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1; // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":"expected struct `std::string::String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":612,"byte_end":618,"line_start":17,"line_end":17,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String = 1; // Error in the middle of line.","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":621,"byte_end":622,"line_start":17,"line_end":17,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1; // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:17:22: error[E0308]: mismatched types
"}
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
Expand All @@ -34,10 +34,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":681,"byte_end":682,"line_start":19,"line_end":19,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1","highlight_start":22,"highlight_end":23}],"label":"expected struct `std::string::String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":672,"byte_end":678,"line_start":19,"line_end":19,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String = 1","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":681,"byte_end":682,"line_start":19,"line_end":19,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1","highlight_start":22,"highlight_end":23}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:19:22: error[E0308]: mismatched types
"}
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
Expand All @@ -55,10 +55,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":745,"byte_end":746,"line_start":23,"line_end":23,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1; // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":"expected struct `std::string::String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":735,"byte_end":741,"line_start":22,"line_end":22,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String =","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":745,"byte_end":746,"line_start":23,"line_end":23,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1; // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:23:1: error[E0308]: mismatched types
"}
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
Expand All @@ -76,10 +76,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":801,"byte_end":809,"line_start":25,"line_end":26,"column_start":22,"column_end":6,"is_primary":true,"text":[{"text":" let s : String = (","highlight_start":22,"highlight_end":23},{"text":" ); // Error spanning the newline.","highlight_start":1,"highlight_end":6}],"label":"expected struct `std::string::String`, found `()`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf-multifile-aux.rs","byte_start":792,"byte_end":798,"line_start":25,"line_end":25,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String = (","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"$DIR/json-bom-plus-crlf-multifile-aux.rs:25:22: error[E0308]: mismatched types
"}
{"message":"aborting due to 4 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 4 previous errors
Expand Down
32 changes: 16 additions & 16 deletions src/test/ui/json-bom-plus-crlf.stderr
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -13,10 +13,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":606,"byte_end":607,"line_start":16,"line_end":16,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1; // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":"expected struct `std::string::String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":597,"byte_end":603,"line_start":16,"line_end":16,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String = 1; // Error in the middle of line.","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":606,"byte_end":607,"line_start":16,"line_end":16,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1; // Error in the middle of line.","highlight_start":22,"highlight_end":23}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf.rs:16:22: error[E0308]: mismatched types
"}
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
Expand All @@ -34,10 +34,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":666,"byte_end":667,"line_start":18,"line_end":18,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1","highlight_start":22,"highlight_end":23}],"label":"expected struct `std::string::String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":657,"byte_end":663,"line_start":18,"line_end":18,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String = 1","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":666,"byte_end":667,"line_start":18,"line_end":18,"column_start":22,"column_end":23,"is_primary":true,"text":[{"text":" let s : String = 1","highlight_start":22,"highlight_end":23}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf.rs:18:22: error[E0308]: mismatched types
"}
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
Expand All @@ -55,10 +55,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":730,"byte_end":731,"line_start":22,"line_end":22,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1; // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":"expected struct `std::string::String`, found integer","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":720,"byte_end":726,"line_start":21,"line_end":21,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String =","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"try using a conversion method","code":null,"level":"help","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":730,"byte_end":731,"line_start":22,"line_end":22,"column_start":1,"column_end":2,"is_primary":true,"text":[{"text":"1; // Error after the newline.","highlight_start":1,"highlight_end":2}],"label":null,"suggested_replacement":"1.to_string()","suggestion_applicability":"MaybeIncorrect","expansion":null}],"children":[],"rendered":null}],"rendered":"$DIR/json-bom-plus-crlf.rs:22:1: error[E0308]: mismatched types
"}
{"message":"mismatched types","code":{"code":"E0308","explanation":"Expected type did not match the received type.
Expand All @@ -76,10 +76,10 @@ let x: i32 = \"I am not a number!\";
```

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.
variable. It can happen in several cases, the most common being a mismatch
between the type that the compiler inferred for a variable based on its
initializing expression, on the one hand, and the type the author explicitly
assigned to the variable, on the other hand.
"},"level":"error","spans":[{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":786,"byte_end":794,"line_start":24,"line_end":25,"column_start":22,"column_end":6,"is_primary":true,"text":[{"text":" let s : String = (","highlight_start":22,"highlight_end":23},{"text":" ); // Error spanning the newline.","highlight_start":1,"highlight_end":6}],"label":"expected struct `std::string::String`, found `()`","suggested_replacement":null,"suggestion_applicability":null,"expansion":null},{"file_name":"$DIR/json-bom-plus-crlf.rs","byte_start":777,"byte_end":783,"line_start":24,"line_end":24,"column_start":13,"column_end":19,"is_primary":false,"text":[{"text":" let s : String = (","highlight_start":13,"highlight_end":19}],"label":"expected due to this","suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[],"rendered":"$DIR/json-bom-plus-crlf.rs:24:22: error[E0308]: mismatched types
"}
{"message":"aborting due to 4 previous errors","code":null,"level":"error","spans":[],"children":[],"rendered":"error: aborting due to 4 previous errors
Expand Down